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README.md
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README.md
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optional
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========
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A library for representing optional (nullable) objects in C++.
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```cpp
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optional<int> readInt(); // this function may return either an int or a not-an-int
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if (optional<int> oi = readInt()) // did I get a real int
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cout << "my int is: " << *oi; // use my int
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else
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cout << "I have no int";
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```
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For more information refer to the documentation provided with this library.
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@ -1,106 +0,0 @@
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[library Boost.Optional
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[quickbook 1.4]
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[authors [Cacciola Carballal, Fernando Luis]]
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[copyright 2003-2007 Fernando Luis Cacciola Carballal]
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[copyright 2014-2015 Andrzej Krzemieński]
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[category miscellaneous]
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[id optional]
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[dirname optional]
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[purpose
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Discriminated-union wrapper for optional values
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]
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[source-mode c++]
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[license
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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[@http://www.boost.org/LICENSE_1_0.txt])
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]
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]
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[/ Macros will be used for links so we have a central place to change them ]
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[/ Cited Boost resources ]
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||||
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[def __BOOST_VARIANT__ [@../../../variant/index.html Boost.Variant]]
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[def __BOOST_TUPLE__ [@../../../tuple/index.html Boost.Tuple]]
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[def __BOOST_TRIBOOL__ [@../../../../doc/html/tribool.html boost::tribool]]
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[def __OPTIONAL_POINTEE__ [@../../../utility/OptionalPointee.html `OptionalPointee`]]
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[def __COPY_CONSTRUCTIBLE__ [@../../../utility/CopyConstructible.html `CopyConstructible`]]
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[def __MOVE_CONSTRUCTIBLE__ `MoveConstructible`]
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[def __FUNCTION_EQUAL_POINTEES__ [@../../../utility/OptionalPointee.html#equal `equal_pointees()`]]
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[def __FUNCTION_LESS_POINTEES__ [@../../../utility/OptionalPointee.html#less `less_pointees()`]]
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[def __IN_PLACE_FACTORY_HPP__ [@../../../../boost/utility/in_place_factory.hpp in_place_factory.hpp]]
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[def __TYPED_IN_PLACE_FACTORY_HPP__ [@../../../../boost/utility/typed_in_place_factory.hpp typed_in_place_factory.hpp]]
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[/ Other web resources ]
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|
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[def __HASKELL__ [@http://www.haskell.org/ Haskell]]
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[def __SGI_DEFAULT_CONSTRUCTIBLE__ [@http://www.sgi.com/tech/stl/DefaultConstructible.html `DefaultConstructible`]]
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[def __SGI_LESS_THAN_COMPARABLE__ [@http://www.sgi.com/tech/stl/LessThanComparable.html `LessThanComparable`]]
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[def __SGI_EQUALITY_COMPARABLE__ [@http://www.sgi.com/tech/stl/EqualityComparable.html `EqualityComparable`]]
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[def __SGI_GENERATOR__ [@http://www.sgi.com/tech/stl/Generator.html `Generator`]]
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[/ Icons ]
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[def __SPACE__ [$images/space.png]]
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[def __GO_TO__ [$images/callouts/R.png]]
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[section Introduction]
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Class template `optional` is a wrapper for representing 'optional' (or 'nullable') objects who may not (yet) contain a valid value. Optional objects offer full value semantics; they are good for passing by value and usage inside STL containers. This is a header-only library.
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[heading Problem]
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Suppose we want to read a parameter form a config file which represents some integral value, let's call it `"MaxValue"`. It is possible that this parameter is not specified; such situation is no error. It is valid to not specify the parameter and in that case the program is supposed to behave slightly differently. Also, suppose that any possible value of type `int` is a valid value for `"MaxValue"`, so we cannot just use `-1` to represent the absence of the parameter in the config file.
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[heading Solution]
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This is how you solve it with `boost::optional`:
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#include <boost/optional.hpp>
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boost::optional<int> getConfigParam(std::string name); // return either an int or a `not-an-int`
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int main()
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{
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if (boost::optional<int> oi = getConfigParam("MaxValue")) // did I get a real int?
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runWithMax(*oi); // use my int
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else
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runWithNoMax();
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}
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[endsect]
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[include 01_quick_start.qbk]
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[section Tutorial]
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[include 10_motivation.qbk]
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[include 11_development.qbk]
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[include 12_when_to_use.qbk]
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[include 13_relational_operators.qbk]
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[include 14_io.qbk]
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[include 15_optional_references.qbk]
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[include 16_in_place_factories.qbk]
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[include 17_optional_bool.qbk]
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[include 18_exception_safety.qbk]
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[include 19_type_requirements.qbk]
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[include 1A_on_performance.qbk]
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[endsect]
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[section:reference Reference]
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[include 21_ref_none.qbk]
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[include 22_ref_bad_optional_access.qbk]
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[include 23_ref_optional_io.qbk]
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[include 24_ref_optional_fwd.qbk]
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[section Header <boost/optional/optional.hpp>]
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[include 27_ref_optional_synopsis.qbk]
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[include 28_ref_optional_semantics.qbk]
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[endsect]
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[include 29_ref_optional_convenience.qbk]
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[endsect]
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[include 90_dependencies.qbk]
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[include 91_relnotes.qbk]
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[include 92_acknowledgments.qbk]
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@ -1,158 +0,0 @@
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[/
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Boost.Optional
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|
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Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
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Copyright (c) 2014 Andrzej Krzemienski
|
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|
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)
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]
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[section Quick Start]
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[section Optional return values]
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Let's write and use a converter function that converts a `std::string` to an `int`. It is possible that for a given string (e.g. `"cat"`) there exists no value of type `int` capable of representing the conversion result. We do not consider such situation an error. We expect that the converter can be used only to check if the conversion is possible. A natural signature for this function can be:
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#include <boost/optional.hpp>
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boost::optional<int> convert(const std::string& text);
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All necessary functionality can be included with one header `<boost/optional.hpp>`. The above function signature means that the function can either return a value of type `int` or a flag indicating that no value of `int` is available. This does not indicate an error. It is like one additional value of `int`. This is how we can use our function:
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|
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const std::string& text = /*... */;
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boost::optional<int> oi = convert(text); // move-construct
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if (oi) // contextual conversion to bool
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int i = *oi; // operator*
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In order to test if `optional` contains a value, we use the contextual conversion to type `bool`. Because of this we can combine the initialization of the optional object and the test into one instruction:
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|
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if (boost::optional<int> oi = convert(text))
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int i = *oi;
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|
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We extract the contained value with `operator*` (and with `operator->` where it makes sense). An attempt to extract the contained value of an uninitialized optional object is an ['undefined behaviour] (UB). This implementation guards the call with `BOOST_ASSERT`. Therefore you should be sure that the contained value is there before extracting. For instance, the following code is reasonably UB-safe:
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|
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int i = *convert("100");
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|
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This is because we know that string value `"100"` converts to a valid value of `int`. If you do not like this potential UB, you can use an alternative way of extracting the contained value:
|
||||
|
||||
try {
|
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int j = convert(text).value();
|
||||
}
|
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catch (const boost::bad_optional_access&) {
|
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// deal with it
|
||||
}
|
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|
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This version throws an exception upon an attempt to access a non-existent contained value. If your way of dealing with the missing value is to use some default, like `0`, there exists a yet another alternative:
|
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|
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int k = convert(text).value_or(0);
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||||
|
||||
This uses the `atoi`-like approach to conversions: if `text` does not represent an integral number just return `0`. Finally, you can provide a callback to be called when trying to access the contained value fails:
|
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|
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int fallback_to_default()
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||||
{
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cerr << "could not convert; using -1 instead" << endl;
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return -1;
|
||||
}
|
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|
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int l = convert(text).value_or_eval(fallback_to_default);
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|
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This will call the provided callback and return whatever the callback returns. The callback can have side effects: they will only be observed when the optional object does not contain a value.
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|
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Now, let's consider how function `convert` can be implemented.
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|
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boost::optional<int> convert(const std::string& text)
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{
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std::stringstream s(text);
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int i;
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if ((s >> i) && s.get() == std::char_traits<char>::eof())
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return i;
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else
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return boost::none;
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}
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Observe the two return statements. `return i` uses the converting constructor that can create `optional<T>` from `T`. Thus constructed optional object is initialized and its value is a copy of `i`. The other return statement uses another converting constructor from a special tag `boost::none`. It is used to indicate that we want to create an uninitialized optional object.
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[endsect]
|
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|
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[section Optional automatic variables]
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We could write function `convert` in a slightly different manner, so that it has a single `return`-statement:
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boost::optional<int> convert(const std::string& text)
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{
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boost::optional<int> ans;
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std::stringstream s(text);
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int i;
|
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if ((s >> i) && s.get() == std::char_traits<char>::eof())
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ans = i;
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return ans;
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}
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The default constructor of `optional` creates an unitialized optional object. Unlike with `int`s you cannot have an `optional<int>` in an indeterminate state. Its state is always well defined. Instruction `ans = i` initializes the optional object. It uses the 'mixed' assignment from `int`. In general, for `optional<T>`, when an assignment from `T` is invoked, it can do two things. If the optional object is not initialized (our case here), it initializes the contained value using `T`'s copy constructor. If the optional object is already initialized, it assigns the new value to it using `T`'s copy assignment.
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[endsect]
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[section Optional data members]
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Suppose we want to implement a ['lazy load] optimization. This is because we do not want to perform an expensive initialization of our `Resource` until (if at all) it is really used. We can do it this way:
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class Widget
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{
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||||
mutable boost::optional<const Resource> resource_;
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||||
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public:
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||||
Widget() {}
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||||
const Resource& getResource() const // not thread-safe
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||||
{
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||||
if (resource_ == boost::none)
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resource_.emplace("resource", "arguments");
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||||
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return *resource_;
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||||
}
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||||
};
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`optional`'s default constructor creates an uninitialized optional. No call to `Resource`'s default constructor is attempted. `Resource` doesn't have to be __SGI_DEFAULT_CONSTRUCTIBLE__. In function `getResource` we first check if `resource_` is initialized. This time we do not use the contextual conversion to `bool`, but a comparison with `boost::none`. These two ways are equivalent. Function `emplace` initializes the optional in-place by perfect-forwarding the arguments to the constructor of `Resource`. No copy- or move-construction is involved here. `Resource` doesn't even have to be `MoveConstructible`.
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[note Function `emplace` is only available on compilers that support rvalue references and variadic templates. If your compiler does not support these features and you still need to avoid any move-constructions, use [link boost_optional.tutorial.in_place_factories In-Place Factories].]
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[endsect]
|
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[section Bypassing unnecessary default construction]
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Suppose we have class `Date`, which does not have a default constructor: there is no good candidate for a default date. We have a function that returns two dates in form of a `boost::tuple`:
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boost::tuple<Date, Date> getPeriod();
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|
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In other place we want to use the result of `getPeriod`, but want the two dates to be named: `begin` and `end`. We want to implement something like 'multiple return values':
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Date begin, end; // Error: no default ctor!
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boost::tie(begin, end) = getPeriod();
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The second line works already, this is the capability of __BOOST_TUPLE__ library, but the first line won't work. We could set some invented initial dates, but it is confusing and may be an unacceptable cost, given that these values will be overwritten in the next line anyway. This is where `optional` can help:
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boost::optional<Date> begin, end;
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boost::tie(begin, end) = getPeriod();
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|
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It works because inside `boost::tie` a move-assignment from `T` is invoked on `optional<T>`, which internally calls a move-constructor of `T`.
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[endsect]
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[section Storage in containers]
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Suppose you want to ask users to choose some number (an `int`). One of the valid responses is to choose nothing, which is represented by an uninitialized `optional<int>`. You want to make a histogram showing how many times each choice was made. You can use an `std::map`:
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std::map<boost::optional<int>, int> choices;
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|
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for (int i = 0; i < LIMIT; ++i) {
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boost::optional<int> choice = readChoice();
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++choices[choice];
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}
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This works because `optional<T>` is __SGI_LESS_THAN_COMPARABLE__ whenever `T` is __SGI_LESS_THAN_COMPARABLE__. In this case the state of being uninitialized is treated as a yet another value of `T`, which is compared less than any value of `T`.
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[endsect]
|
||||
|
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[endsect]
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@ -1,37 +0,0 @@
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[section When to use Optional]
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|
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It is recommended to use `optional<T>` in situations where there is exactly one, clear (to all parties) reason for having no value of type `T`, and where the lack of value is as natural as having any regular value of `T`. One example of such situation is asking the user in some GUI form to optionally specify some limit on an `int` value, but the user is allowed to say 'I want the number not to be constrained by the maximum'.
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For another example, consider a config parameter specifying how many threads the application should launch. Leaving this parameter unspecified means that the application should decide itself. For yet another example, consider a function returning the index of the smallest element in a `vector`. We need to be prepared for the situation, where the `vector` is empty. Therefore a natural signature for such function would be:
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|
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template <typename T>
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optional<size_t> find_smallest_elem(const std::vector<T>& vec);
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|
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Here, having received an empty `vec` and having no `size_t` to return is not a ['failure] but a ['normal], albeit irregular, situation.
|
||||
|
||||
Another typical situation is to indicate that we do not have a value yet, but we expect to have it later. This notion can be used in implementing solutions like lazy initialization or a two-phase initialization.
|
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`optional` can be used to take a non-__SGI_DEFAULT_CONSTRUCTIBLE__ type `T` and create a sibling type with a default constructor. This is a way to add a ['null-state] to any type that doesn't have it already.
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|
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Sometimes type `T` already provides a built-in null-state, but it may still be useful to wrap it into `optional`. Consider `std::string`. When you read a piece of text from a GUI form or a DB table, it is hardly ever that the empty string indicates anything else but a missing text. And some data bases do not even distinguish between a null string entry and a non-null string of length 0. Still, it may be practical to use `optional<string>` to indicate in the returned type that we want to treat the empty string in a special dedicated program path:
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|
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if(boost::optional<std::string> name = ask_user_name()) {
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assert(*name != "");
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||||
logon_as(*name);
|
||||
}
|
||||
else {
|
||||
skip_logon();
|
||||
}
|
||||
|
||||
In the example above, the assertion indicates that if we choose to use this technique, we must translate the empty string state to an optional object with no contained value (inside function `ask_user_name`).
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||||
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[heading Not recommended usages]
|
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|
||||
It is not recommended to use `optional` to indicate that we were not able to compute a value because of a ['failure]. It is difficult to define what a failure is, but it usually has one common characteristic: an associated information on the cause of the failure. This can be the type and member data of an exception object, or an error code. It is a bad design to signal a failure and not inform about the cause. If you do not want to use exceptions, and do not like the fact that by returning error codes you cannot return the computed value, you can use [@https://github.com/ptal/Boost.Expected Expected] library. It is sort of __BOOST_VARIANT__ that contains either a computed value or a reason why the computation failed.
|
||||
|
||||
Sometimes the distinction into what is a failure and what is a valid but irregular result is blurry and depends on a particular usage and personal preference. Consider a function that converts a `string` to an `int`. Is it a failure that you cannot convert? It might in some cases, but in other you may call it exactly for the purpose of figuring out if a given `string` is convertible, and you are not even interested in the resulting value. Sometimes when a conversion fails you may not consider it a failure, but you need to know why it cannot be converted; for instance at which character it is determined that the conversion is impossible. In this case returning `optional<T>` will not suffice. Finally, there is a use case where an input string that does not represent an `int` is not a failure condition, but during the conversion we use resources whose acquisition may fail. In that case the natural representation is to both return `optional<int>` and signal failure:
|
||||
|
||||
optional<int> convert1(const string& str); // throws
|
||||
expected<ErrorT, optional<int>> convert2(const string& str); // return either optional or error
|
||||
|
||||
[endsect]
|
||||
@ -1,37 +0,0 @@
|
||||
|
||||
[section Relational operators]
|
||||
|
||||
Type `optional<T>` is __SGI_EQUALITY_COMPARABLE__ whenever `T` is __SGI_EQUALITY_COMPARABLE__. Two optional objects containing a value compare in the same way as their contained values. The uninitialized state of `optional<T>` is treated as a distinct value, equal to itself, and unequal to any value of type `T`:
|
||||
|
||||
boost::optional<int> oN = boost::none;
|
||||
boost::optional<int> o0 = 0;
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||||
boost::optional<int> o1 = 1;
|
||||
|
||||
assert(oN != o0);
|
||||
assert(o1 != oN);
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||||
assert(o0 != o1);
|
||||
assert(oN == oN);
|
||||
assert(o0 == o0);
|
||||
|
||||
The converting constructor from `T` as well as from `boost::none` implies the existence and semantics of the mixed comparison between `T` and `optional<T>` as well as between `none_t` and `optionl<T>`:
|
||||
|
||||
assert(oN != 0);
|
||||
assert(o1 != boost::none);
|
||||
assert(o0 != 1);
|
||||
assert(oN == boost::none);
|
||||
assert(o0 == 0);
|
||||
|
||||
This mixed comparison has a practical interpretation, which is occasionally useful:
|
||||
|
||||
boost::optional<int> choice = ask_user();
|
||||
if (choice == 2)
|
||||
start_procedure_2();
|
||||
|
||||
In the above example, the meaning of the comparison is 'user chose number 2'. If user chose nothing, he didn't choose number 2.
|
||||
|
||||
In case where `optional<T>` is compared to `none`, it is not required that `T` be __SGI_EQUALITY_COMPARABLE__.
|
||||
|
||||
In a similar manner, type `optional<T>` is __SGI_LESS_THAN_COMPARABLE__ whenever `T` is __SGI_LESS_THAN_COMPARABLE__. The optional object containing no value is compared less than any value of `T`. To illustrate this, if the default ordering of `size_t` is {`0`, `1`, `2`, ...}, the default ordering of `optional<size_t>` is {`boost::none`, `0`, `1`, `2`, ...}. This order does not have a practical interpretation. The goal is to have any semantically correct default ordering in order for `optional<T>` to be usable in ordered associative containers (wherever `T` is usable).
|
||||
|
||||
Mixed relational operators are the only case where the contained value of an optional object can be inspected without the usage of value accessing function (`operator*`, `value`, `value_or`).
|
||||
[endsect]
|
||||
@ -1,37 +0,0 @@
|
||||
|
||||
[section IO operators]
|
||||
|
||||
It is possible to use `optional<T>` with IO streams, provided that `T` can be used with streams. IOStream operators are defined in a separate header.
|
||||
|
||||
``
|
||||
#include <iostream>
|
||||
#include <boost/optional/optional_io.hpp>
|
||||
|
||||
int main()
|
||||
{
|
||||
boost::optional<int> o1 = 1, oN = boost::none;
|
||||
std::cout << o1;
|
||||
std::cin >> oN;
|
||||
}
|
||||
``
|
||||
|
||||
The current implementation does not guarantee any particular output. What it guarantees is that if streaming out and then back in `T` gives the same value, then streaming out and then back in `optional<T>` will also give back the same result:
|
||||
|
||||
``
|
||||
#include <cassert>
|
||||
#include <sstream>
|
||||
#include <boost/optional/optional_io.hpp>
|
||||
|
||||
int main()
|
||||
{
|
||||
boost::optional<int> o1 = 1, oN = boost::none;
|
||||
boost::optional<int> x1, x2;
|
||||
std::stringstream s;
|
||||
s << o1 << oN;
|
||||
s >> x1 >> x2;
|
||||
assert (o1 == x1);
|
||||
assert (oN == x2);
|
||||
}
|
||||
``
|
||||
|
||||
[endsect]
|
||||
@ -1,120 +0,0 @@
|
||||
|
||||
[section Optional references]
|
||||
|
||||
This library allows the template parameter `T` to be of reference type:
|
||||
`T&`, and to some extent, `T const&`.
|
||||
|
||||
However, since references are not real objects some restrictions apply and
|
||||
some operations are not available in this case:
|
||||
|
||||
* Converting constructors
|
||||
* Converting assignment
|
||||
* InPlace construction
|
||||
* InPlace assignment
|
||||
* Value-access via pointer
|
||||
|
||||
Also, even though `optional<T&>` treats it wrapped pseudo-object much as
|
||||
a real value, a true real reference is stored so aliasing will ocurr:
|
||||
|
||||
* Copies of `optional<T&>` will copy the references but all these references
|
||||
will nonetheless refer to the same object.
|
||||
* Value-access will actually provide access to the referenced object
|
||||
rather than the reference itself.
|
||||
|
||||
[warning On compilers that do not conform to Standard C++ rules of reference binding, operations on optional references might give adverse results: rather than binding a reference to a designated object they may create an unexpected temporary and bind to it. For more details see [link boost_optional.dependencies_and_portability.optional_reference_binding Dependencies and Portability section].]
|
||||
|
||||
[heading Rvalue references]
|
||||
|
||||
Rvalue references and lvalue references to const have the ability in C++ to extend the life time of a temporary they bind to. Optional references do not have this capability, therefore to avoid surprising effects it is not possible to initialize an optional references from a temporary. Optional rvalue references are disabled altogether. Also, the initialization and assignment of an optional reference to const from rvalue reference is disabled.
|
||||
|
||||
const int& i = 1; // legal
|
||||
optional<const int&> oi = 1; // illegal
|
||||
|
||||
[endsect]
|
||||
|
||||
[section Rebinding semantics for assignment of optional references]
|
||||
|
||||
If you assign to an ['uninitialized ] `optional<T&>` the effect is to bind (for
|
||||
the first time) to the object. Clearly, there is no other choice.
|
||||
|
||||
int x = 1 ;
|
||||
int& rx = x ;
|
||||
optional<int&> ora ;
|
||||
optional<int&> orb(x) ;
|
||||
ora = orb ; // now 'ora' is bound to 'x' through 'rx'
|
||||
*ora = 2 ; // Changes value of 'x' through 'ora'
|
||||
assert(x==2);
|
||||
|
||||
If you assign to a bare C++ reference, the assignment is forwarded to the
|
||||
referenced object; its value changes but the reference is never rebound.
|
||||
|
||||
int a = 1 ;
|
||||
int& ra = a ;
|
||||
int b = 2 ;
|
||||
int& rb = b ;
|
||||
ra = rb ; // Changes the value of 'a' to 'b'
|
||||
assert(a==b);
|
||||
b = 3 ;
|
||||
assert(ra!=b); // 'ra' is not rebound to 'b'
|
||||
|
||||
Now, if you assign to an ['initialized ] `optional<T&>`, the effect is to
|
||||
[*rebind] to the new object instead of assigning the referee. This is unlike
|
||||
bare C++ references.
|
||||
|
||||
int a = 1 ;
|
||||
int b = 2 ;
|
||||
int& ra = a ;
|
||||
int& rb = b ;
|
||||
optional<int&> ora(ra) ;
|
||||
optional<int&> orb(rb) ;
|
||||
ora = orb ; // 'ora' is rebound to 'b'
|
||||
*ora = 3 ; // Changes value of 'b' (not 'a')
|
||||
assert(a==1);
|
||||
assert(b==3);
|
||||
|
||||
[heading Rationale]
|
||||
|
||||
Rebinding semantics for the assignment of ['initialized ] `optional` references has
|
||||
been chosen to provide [*consistency among initialization states] even at the
|
||||
expense of lack of consistency with the semantics of bare C++ references.
|
||||
It is true that `optional<U>` strives to behave as much as possible as `U`
|
||||
does whenever it is initialized; but in the case when `U` is `T&`, doing so would
|
||||
result in inconsistent behavior w.r.t to the lvalue initialization state.
|
||||
|
||||
Imagine `optional<T&>` forwarding assignment to the referenced object (thus
|
||||
changing the referenced object value but not rebinding), and consider the
|
||||
following code:
|
||||
|
||||
optional<int&> a = get();
|
||||
int x = 1 ;
|
||||
int& rx = x ;
|
||||
optional<int&> b(rx);
|
||||
a = b ;
|
||||
|
||||
What does the assignment do?
|
||||
|
||||
If `a` is ['uninitialized], the answer is clear: it binds to `x` (we now have
|
||||
another reference to `x`).
|
||||
But what if `a` is already ['initialized]? it would change the value of the
|
||||
referenced object (whatever that is); which is inconsistent with the other
|
||||
possible case.
|
||||
|
||||
If `optional<T&>` would assign just like `T&` does, you would never be able to
|
||||
use Optional's assignment without explicitly handling the previous
|
||||
initialization state unless your code is capable of functioning whether
|
||||
after the assignment, `a` aliases the same object as `b` or not.
|
||||
|
||||
That is, you would have to discriminate in order to be consistent.
|
||||
|
||||
If in your code rebinding to another object is not an option, then it is very
|
||||
likely that binding for the first time isn't either. In such case, assignment
|
||||
to an ['uninitialized ] `optional<T&>` shall be prohibited. It is quite possible
|
||||
that in such a scenario it is a precondition that the lvalue must be already
|
||||
initialized. If it isn't, then binding for the first time is OK
|
||||
while rebinding is not which is IMO very unlikely.
|
||||
In such a scenario, you can assign the value itself directly, as in:
|
||||
|
||||
assert(!!opt);
|
||||
*opt=value;
|
||||
|
||||
[endsect]
|
||||
@ -1,139 +0,0 @@
|
||||
|
||||
[section In-Place Factories]
|
||||
|
||||
One of the typical problems with wrappers and containers is that their
|
||||
interfaces usually provide an operation to initialize or assign the
|
||||
contained object as a copy of some other object. This not only requires the
|
||||
underlying type to be __COPY_CONSTRUCTIBLE__, but also requires the existence of
|
||||
a fully constructed object, often temporary, just to follow the copy from:
|
||||
|
||||
struct X
|
||||
{
|
||||
X ( int, std::string ) ;
|
||||
} ;
|
||||
|
||||
class W
|
||||
{
|
||||
X wrapped_ ;
|
||||
|
||||
public:
|
||||
|
||||
W ( X const& x ) : wrapped_(x) {}
|
||||
} ;
|
||||
|
||||
void foo()
|
||||
{
|
||||
// Temporary object created.
|
||||
W ( X(123,"hello") ) ;
|
||||
}
|
||||
|
||||
A solution to this problem is to support direct construction of the
|
||||
contained object right in the container's storage.
|
||||
In this scheme, the user only needs to supply the arguments to the
|
||||
constructor to use in the wrapped object construction.
|
||||
|
||||
class W
|
||||
{
|
||||
X wrapped_ ;
|
||||
|
||||
public:
|
||||
|
||||
W ( X const& x ) : wrapped_(x) {}
|
||||
W ( int a0, std::string a1) : wrapped_(a0,a1) {}
|
||||
} ;
|
||||
|
||||
void foo()
|
||||
{
|
||||
// Wrapped object constructed in-place
|
||||
// No temporary created.
|
||||
W (123,"hello") ;
|
||||
}
|
||||
|
||||
A limitation of this method is that it doesn't scale well to wrapped
|
||||
objects with multiple constructors nor to generic code were the constructor
|
||||
overloads are unknown.
|
||||
|
||||
The solution presented in this library is the family of [*InPlaceFactories]
|
||||
and [*TypedInPlaceFactories].
|
||||
These factories are a family of classes which encapsulate an increasing
|
||||
number of arbitrary constructor parameters and supply a method to construct
|
||||
an object of a given type using those parameters at an address specified by
|
||||
the user via placement new.
|
||||
|
||||
For example, one member of this family looks like:
|
||||
|
||||
template<class T,class A0, class A1>
|
||||
class TypedInPlaceFactory2
|
||||
{
|
||||
A0 m_a0 ; A1 m_a1 ;
|
||||
|
||||
public:
|
||||
|
||||
TypedInPlaceFactory2( A0 const& a0, A1 const& a1 ) : m_a0(a0), m_a1(a1) {}
|
||||
|
||||
void construct ( void* p ) { new (p) T(m_a0,m_a1) ; }
|
||||
} ;
|
||||
|
||||
A wrapper class aware of this can use it as:
|
||||
|
||||
class W
|
||||
{
|
||||
X wrapped_ ;
|
||||
|
||||
public:
|
||||
|
||||
W ( X const& x ) : wrapped_(x) {}
|
||||
W ( TypedInPlaceFactory2 const& fac ) { fac.construct(&wrapped_) ; }
|
||||
} ;
|
||||
|
||||
void foo()
|
||||
{
|
||||
// Wrapped object constructed in-place via a TypedInPlaceFactory.
|
||||
// No temporary created.
|
||||
W ( TypedInPlaceFactory2<X,int,std::string>(123,"hello")) ;
|
||||
}
|
||||
|
||||
The factories are divided in two groups:
|
||||
|
||||
* [_TypedInPlaceFactories]: those which take the target type as a primary
|
||||
template parameter.
|
||||
* [_InPlaceFactories]: those with a template `construct(void*)` member
|
||||
function taking the target type.
|
||||
|
||||
Within each group, all the family members differ only in the number of
|
||||
parameters allowed.
|
||||
|
||||
This library provides an overloaded set of helper template functions to
|
||||
construct these factories without requiring unnecessary template parameters:
|
||||
|
||||
template<class A0,...,class AN>
|
||||
InPlaceFactoryN <A0,...,AN> in_place ( A0 const& a0, ..., AN const& aN) ;
|
||||
|
||||
template<class T,class A0,...,class AN>
|
||||
TypedInPlaceFactoryN <T,A0,...,AN> in_place ( T const& a0, A0 const& a0, ..., AN const& aN) ;
|
||||
|
||||
In-place factories can be used generically by the wrapper and user as follows:
|
||||
|
||||
class W
|
||||
{
|
||||
X wrapped_ ;
|
||||
|
||||
public:
|
||||
|
||||
W ( X const& x ) : wrapped_(x) {}
|
||||
|
||||
template< class InPlaceFactory >
|
||||
W ( InPlaceFactory const& fac ) { fac.template <X>construct(&wrapped_) ; }
|
||||
|
||||
} ;
|
||||
|
||||
void foo()
|
||||
{
|
||||
// Wrapped object constructed in-place via a InPlaceFactory.
|
||||
// No temporary created.
|
||||
W ( in_place(123,"hello") ) ;
|
||||
}
|
||||
|
||||
The factories are implemented in the headers: __IN_PLACE_FACTORY_HPP__ and __TYPED_IN_PLACE_FACTORY_HPP__
|
||||
|
||||
[endsect]
|
||||
@ -1,52 +0,0 @@
|
||||
|
||||
[section A note about optional<bool>]
|
||||
|
||||
`optional<bool>` should be used with special caution and consideration.
|
||||
|
||||
First, it is functionally similar to a tristate boolean (false, maybe, true)
|
||||
—such as __BOOST_TRIBOOL__— except that in a tristate boolean, the maybe state
|
||||
[_represents a valid value], unlike the corresponding state of an uninitialized
|
||||
`optional<bool>`.
|
||||
It should be carefully considered if an `optional<bool>` instead of a `tribool`
|
||||
is really needed.
|
||||
|
||||
Second, although `optional<>` provides a contextual conversion to `bool` in C++11,
|
||||
this falls back to an implicit conversion on older compilers. This conversion refers
|
||||
to the initialization state and not to the contained value. Using `optional<bool>`
|
||||
can lead to subtle errors due to the implicit `bool` conversion:
|
||||
|
||||
void foo ( bool v ) ;
|
||||
void bar()
|
||||
{
|
||||
optional<bool> v = try();
|
||||
|
||||
// The following intended to pass the value of 'v' to foo():
|
||||
foo(v);
|
||||
// But instead, the initialization state is passed
|
||||
// due to a typo: it should have been foo(*v).
|
||||
}
|
||||
|
||||
The only implicit conversion is to `bool`, and it is safe in the sense that
|
||||
typical integral promotions don't apply (i.e. if `foo()` takes an `int`
|
||||
instead, it won't compile).
|
||||
|
||||
Third, mixed comparisons with `bool` work differently than similar mixed comparisons between pointers and `bool`, so the results might surprise you:
|
||||
|
||||
optional<bool> oEmpty(none), oTrue(true), oFalse(false);
|
||||
|
||||
if (oEmpty == none); // renders true
|
||||
if (oEmpty == false); // renders false!
|
||||
if (oEmpty == true); // renders false!
|
||||
|
||||
if (oFalse == none); // renders false
|
||||
if (oFalse == false); // renders true!
|
||||
if (oFalse == true); // renders false
|
||||
|
||||
if (oTrue == none); // renders false
|
||||
if (oTrue == false); // renders false
|
||||
if (oTrue == true); // renders true
|
||||
|
||||
In other words, for `optional<>`, the following assertion does not hold:
|
||||
|
||||
assert((opt == false) == (!opt));
|
||||
[endsect]
|
||||
@ -1,57 +0,0 @@
|
||||
|
||||
[section Exception Safety Guarantees]
|
||||
|
||||
This library assumes that `T`'s destructor does not throw exceptions. If it does, the behaviour of many operations on `optional<T>` is undefined.
|
||||
|
||||
The following mutating operations never throw exceptions:
|
||||
|
||||
* `optional<T>::operator= ( none_t ) noexcept`
|
||||
* `optional<T>::reset() noexcept`
|
||||
|
||||
In addition, the following constructors and the destructor never throw exceptions:
|
||||
|
||||
* `optional<T>::optional() noexcept`
|
||||
* `optional<T>::optional( none_t ) noexcept`
|
||||
|
||||
|
||||
Regarding the following assignment functions:
|
||||
|
||||
* `optional<T>::operator= ( optional<T> const& )`
|
||||
* `optional<T>::operator= ( T const& )`
|
||||
* `template<class U> optional<T>::operator= ( optional<U> const& )`
|
||||
* `template<class InPlaceFactory> optional<T>::operator= ( InPlaceFactory const& )`
|
||||
* `template<class TypedInPlaceFactory> optional<T>::operator= ( TypedInPlaceFactory const& ) `
|
||||
* `optional<T>::reset( T const& )`
|
||||
|
||||
They forward calls to the corresponding `T`'s constructors or assignments (depending on whether the optional object is initialized or not); so if both `T`'s constructor and the assignment provide strong exception safety guarantee, `optional<T>`'s assignment also provides strong exception safety guarantee; otherwise we only get the basic guarantee. Additionally, if both involved `T`'s constructor and the assignment never throw, `optional<T>`'s assignment also never throws.
|
||||
|
||||
Unless `T`'s constructor or assignment throws, assignments to `optional<T>` do not throw anything else on its own. A throw during assignment never changes the initialization state of any optional object involved:
|
||||
|
||||
|
||||
optional<T> opt1(val1);
|
||||
optional<T> opt2(val2);
|
||||
assert(opt1);
|
||||
assert(opt2);
|
||||
|
||||
try
|
||||
{
|
||||
opt1 = opt2; // throws
|
||||
}
|
||||
catch(...)
|
||||
{
|
||||
assert(opt1);
|
||||
assert(opt2);
|
||||
}
|
||||
|
||||
This also applies to move assignments/constructors. However, move operations are made no-throw more often.
|
||||
|
||||
Operation `emplace` provides basic exception safety guarantee. If it throws, the optional object becomes uninitialized regardless of its initial state, and its previous contained value (if any) is destroyed. It doesn't call any assignment or move/copy constructor on `T`.
|
||||
|
||||
[heading Swap]
|
||||
|
||||
Unless `swap` on optional is customized, its primary implementation forwards calls to `T`'s `swap` or move constructor (depending on the initialization state of the optional objects). Thus, if both `T`'s `swap` and move constructor never throw, `swap` on `optional<T>` never throws. similarly, if both `T`'s `swap` and move constructor offer strong guarantee, `swap` on `optional<T>` also offers a strong guarantee.
|
||||
|
||||
In case `swap` on optional is customized, the call to `T`'s move constructor are replaced with the calls to `T`'s default constructor followed by `swap`. (This is more useful on older compilers that do not support move semantics, when one wants to acheive stronger exception safety guarantees.) In this case the exception safety guarantees for `swap` are reliant on the guarantees of `T`'s `swap` and default constructor
|
||||
[endsect]
|
||||
|
||||
|
||||
@ -1,38 +0,0 @@
|
||||
|
||||
[section Type requirements]
|
||||
|
||||
The very minimum requirement of `optional<T>` is that `T` is a complete type and that it has a publicly accessible destructor. `T` doesn't even need to be constructible. You can use a very minimum interface:
|
||||
|
||||
optional<T> o; // uninitialized
|
||||
assert(o == none); // check if initialized
|
||||
assert(!o); //
|
||||
o.value(); // always throws
|
||||
|
||||
But this is practically useless. In order for `optional<T>` to be able to do anything useful and offer all the spectrum of ways of accessing the contained value, `T` needs to have at least one accessible constructor. In that case you need to initialize the optional object with function `emplace()`, or if your compiler does not support it, resort to [link boost_optional.tutorial.in_place_factories In-Place Factories]:
|
||||
|
||||
optional<T> o;
|
||||
o.emplace("T", "ctor", "params");
|
||||
|
||||
If `T` is __MOVE_CONSTRUCTIBLE__, `optional<T>` is also __MOVE_CONSTRUCTIBLE__ and can be easily initialized from an rvalue of type `T` and be passed by value:
|
||||
|
||||
optional<T> o = make_T();
|
||||
optional<T> p = optional<T>();
|
||||
|
||||
If `T` is __COPY_CONSTRUCTIBLE__, `optional<T>` is also __COPY_CONSTRUCTIBLE__ and can be easily initialized from an lvalue of type `T`:
|
||||
|
||||
T v = make_T();
|
||||
optional<T> o = v;
|
||||
optional<T> p = o;
|
||||
|
||||
If `T` is not `MoveAssignable`, it is still possible to reset the value of `optional<T>` using function `emplace()`:
|
||||
|
||||
optional<const T> o = make_T();
|
||||
o.emplace(make_another_T());
|
||||
|
||||
If `T` is `Moveable` (both __MOVE_CONSTRUCTIBLE__ and `MoveAssignable`) then `optional<T>` is also `Moveable` and additionally can be constructed and assigned from an rvalue of type `T`.
|
||||
|
||||
Similarly, if `T` is `Copyable` (both __COPY_CONSTRUCTIBLE__ and `CopyAssignable`) then `optional<T>` is also `Copyable` and additionally can be constructed and assigned from an lvalue of type `T`.
|
||||
|
||||
`T` ['is not] required to be __SGI_DEFAULT_CONSTRUCTIBLE__.
|
||||
|
||||
[endsect]
|
||||
@ -1,131 +0,0 @@
|
||||
|
||||
[section Performance considerations]
|
||||
|
||||
Technical details aside, the memory layout of `optional<T>` is more-less this:
|
||||
|
||||
template <typename T>
|
||||
class optional
|
||||
{
|
||||
bool _initialized;
|
||||
std::aligned_storage_t<sizeof(t), alignof(T)> _storage;
|
||||
};
|
||||
|
||||
But for the purpose of this analysis, considering memory layouts, we can think of it as:
|
||||
|
||||
template <typename T>
|
||||
class optional
|
||||
{
|
||||
bool _initialized;
|
||||
T _storage;
|
||||
};
|
||||
|
||||
Given type `optional<int>`, and assuming that `sizeof(int) == 4`, we will get `sizeof(optional<int>) == 8`. This is so because of the alignment rules, for our two members we get the following alignment:
|
||||
|
||||
[$images/opt_align1.png]
|
||||
|
||||
This means you can fit twice as many `int`s as `optional<int>`s into the same space of memory. Therefore, if the size of the objects is critical for your application (e.g., because you want to utilize your CPU cache in order to gain performance) and you have determined you are willing to trade the code clarity, it is recommended that you simply go with type `int` and use some 'magic value' to represent ['not-an-int].
|
||||
|
||||
Even if you cannot spare any value of `int` to represent ['not-an-int] (e.g., because every value is useful, or you do want to signal ['not-an-int] explicitly), at least for `Trivial` types you should consider storing the value and the `bool` flag representing the ['null-state] separately. Consider the following class:
|
||||
|
||||
struct Record
|
||||
{
|
||||
optional<int> _min;
|
||||
optional<int> _max;
|
||||
};
|
||||
|
||||
Its memory layout can be depicted as follows:
|
||||
|
||||
[$images/opt_align2.png]
|
||||
|
||||
This is exactly the same as if we had the following members:
|
||||
|
||||
struct Record
|
||||
{
|
||||
bool _has_min;
|
||||
int _min;
|
||||
bool _has_max;
|
||||
int _max;
|
||||
};
|
||||
|
||||
But when they are stored separately, we at least have an option to reorder them like this:
|
||||
|
||||
struct Record
|
||||
{
|
||||
bool _has_min;
|
||||
bool _has_max;
|
||||
int _min;
|
||||
int _max;
|
||||
};
|
||||
|
||||
Which gives us the following layout (and smaller total size):
|
||||
|
||||
[$images/opt_align3.png]
|
||||
|
||||
Sometimes it requires detailed consideration what data we make optional. In our case above, if we determine that both minimum and maximum value can be provided or not provided together, but one is never provided without the other, we can make only one optional memebr:
|
||||
|
||||
struct Limits
|
||||
{
|
||||
int _min;
|
||||
int _max;
|
||||
};
|
||||
|
||||
struct Record
|
||||
{
|
||||
optional<Limits> _limits;
|
||||
};
|
||||
|
||||
This would give us the following layout:
|
||||
|
||||
[$images/opt_align4.png]
|
||||
|
||||
[heading Optional function parameters]
|
||||
|
||||
Having function parameters of type `const optional<T>&` may incur certain unexpected run-time cost connected to copy construction of `T`. Consider the following code.
|
||||
|
||||
void fun(const optional<Big>& v)
|
||||
{
|
||||
if (v) doSomethingWith(*v);
|
||||
else doSomethingElse();
|
||||
}
|
||||
|
||||
int main()
|
||||
{
|
||||
optional<Big> ov;
|
||||
Big v;
|
||||
fun(none);
|
||||
fun(ov); // no copy
|
||||
fun(v); // copy constructor of Big
|
||||
}
|
||||
|
||||
No copy elision or move semantics can save us from copying type `Big` here. Not that we need any copy, but this is how `optional` works. In order to avoid copying in this case, one could provide second overload of `fun`:
|
||||
|
||||
void fun(const Big& v)
|
||||
{
|
||||
doSomethingWith(v);
|
||||
}
|
||||
|
||||
int main()
|
||||
{
|
||||
optional<Big> ov;
|
||||
Big v;
|
||||
fun(ov); // no copy
|
||||
fun(v); // no copy: second overload selected
|
||||
}
|
||||
|
||||
Alternatively, you could consider using an optional reference instead:
|
||||
|
||||
void fun(optional<const Big&> v) // note where the reference is
|
||||
{
|
||||
if (v) doSomethingWith(*v);
|
||||
else doSomethingElse();
|
||||
}
|
||||
|
||||
int main()
|
||||
{
|
||||
optional<Big> ov;
|
||||
Big v;
|
||||
fun(none);
|
||||
fun(ov); // doesn't compile
|
||||
fun(v); // no copy
|
||||
}
|
||||
[endsect]
|
||||
@ -1,31 +0,0 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
Copyright (c) 2015 Andrzej Krzemienski
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
[section Header <boost/none.hpp>]
|
||||
|
||||
[section Synopsis]
|
||||
```
|
||||
namespace boost {
|
||||
|
||||
class none_t {/* see below */};
|
||||
|
||||
const none_t none (/* see below */);
|
||||
|
||||
} // namespace boost
|
||||
```
|
||||
|
||||
Class `none_t` is meant to serve as a tag for selecting appropriate overloads of from `optional`'s interface. It is an empty, trivially copyable class with disabled default constructor.
|
||||
|
||||
Constant `none` is used to indicate an optional object that does not contain a value in initialization, assignment and relational operations of `optional`.
|
||||
|
||||
[endsect]
|
||||
|
||||
[endsect]
|
||||
@ -1,40 +0,0 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
Copyright (c) 2015 Andrzej Krzemienski
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
[section Header <boost/optional/bad_optional_access.hpp>]
|
||||
|
||||
[section Synopsis]
|
||||
```
|
||||
namespace boost {
|
||||
|
||||
class bad_optional_access : public std::logic_error
|
||||
{
|
||||
public:
|
||||
bad_optional_access(); ``[link reference_bad_optional_access_constructor __GO_TO__]``
|
||||
};
|
||||
|
||||
} // namespace boost
|
||||
```
|
||||
[endsect]
|
||||
|
||||
[section Detailed semantics]
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_bad_optional_access_constructor]
|
||||
|
||||
`bad_optional_access();`
|
||||
|
||||
* [*Effect:] Constructs an object of class `bad_optional_access`.
|
||||
* [*Postconditions:] `what()` returns an implementation-defined NTBS.
|
||||
|
||||
[endsect]
|
||||
[endsect]
|
||||
@ -1,74 +0,0 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
Copyright (c) 2015 Andrzej Krzemienski
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
[section:io_header Header <boost/optional/optional_io.hpp>]
|
||||
|
||||
[section:io_synop Synopsis]
|
||||
```
|
||||
#include <istream>
|
||||
#include <ostream>
|
||||
#include <boost/optional/optional.hpp>
|
||||
|
||||
namespace boost {
|
||||
|
||||
template <class CharType, class CharTrait, class T>
|
||||
std::basic_ostream<CharType, CharTrait>&
|
||||
operator<<(std::basic_ostream<CharType, CharTrait>& out, optional<T> const& v); ``[link reference_operator_ostream __GO_TO__]``
|
||||
|
||||
template <class CharType, class CharTrait>
|
||||
std::basic_ostream<CharType, CharTrait>&
|
||||
operator<<(std::basic_ostream<CharType, CharTrait>& out, none_t const&); ``[link reference_operator_ostream_none __GO_TO__]``
|
||||
|
||||
template<class CharType, class CharTrait, class T>
|
||||
std::basic_istream<CharType, CharTrait>&
|
||||
operator>>(std::basic_istream<CharType, CharTrait>& in, optional<T>& v); ``[link reference_operator_istream __GO_TO__]``
|
||||
|
||||
} // namespace boost
|
||||
```
|
||||
|
||||
[endsect]
|
||||
|
||||
[section:io_semantics Detailed semantics]
|
||||
|
||||
|
||||
[#reference_operator_ostream]
|
||||
|
||||
|
||||
`template <class CharType, class CharTrait, class T>` [br]
|
||||
\u00A0\u00A0\u00A0\u00A0`std::basic_ostream<CharType, CharTrait>&` [br]
|
||||
\u00A0\u00A0\u00A0\u00A0`operator<<(std::basic_ostream<CharType, CharTrait>& out, optional<T> const& v);`
|
||||
|
||||
* [*Effect:] Outputs an implementation-defined string. The output contains the information about whether the optional object contains a value or not. If `v` contains a value, the output contains result of calling `out << *v`.
|
||||
* [*Returns:] `out`.
|
||||
|
||||
__SPACE__
|
||||
[#reference_operator_ostream_none]
|
||||
|
||||
`template <class CharType, class CharTrait, class T>` [br]
|
||||
\u00A0\u00A0\u00A0\u00A0`std::basic_ostream<CharType, CharTrait>&` [br]
|
||||
\u00A0\u00A0\u00A0\u00A0`operator<<(std::basic_ostream<CharType, CharTrait>& out, none_t);`
|
||||
|
||||
* [*Effect:] Outputs an implementation-defined string.
|
||||
* [*Returns:] `out`.
|
||||
|
||||
__SPACE__
|
||||
[#reference_operator_istream]
|
||||
|
||||
`template <class CharType, class CharTrait, class T>` [br]
|
||||
\u00A0\u00A0\u00A0\u00A0`std::basic_ostream<CharType, CharTrait>&` [br]
|
||||
\u00A0\u00A0\u00A0\u00A0`operator>>(std::basic_istream<CharType, CharTrait>& in, optional<T>& v);`
|
||||
|
||||
* [*Requires:] `T` is __SGI_DEFAULT_CONSTRUCTIBLE__ and __MOVE_CONSTRUCTIBLE__.
|
||||
* [*Effect:] Reads the value of optional object from `in`. If the string representation indicates that the optional object should contain a value, `v` contains a value and its contained value is obtained as if by default-constructing an object `o` of type `T` and then calling `in >> o`; otherwise `v` does not contain a value, and the previously contained value (if any) has been destroyed.
|
||||
* [*Returns:] `out`.
|
||||
|
||||
[endsect]
|
||||
[endsect]
|
||||
@ -1,32 +0,0 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
Copyright (c) 2015 Andrzej Krzemienski
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
[section Header <boost/optional/optional_fwd.hpp>]
|
||||
|
||||
[section Synopsis]
|
||||
```
|
||||
namespace boost {
|
||||
|
||||
template <class T> class optional ;
|
||||
|
||||
template <class T> void swap ( optional<T>& , optional<T>& );
|
||||
|
||||
template <class T> struct optional_swap_should_use_default_constructor ;
|
||||
|
||||
} // namespace boost
|
||||
```
|
||||
|
||||
This header only contains declarations.
|
||||
|
||||
[endsect]
|
||||
|
||||
|
||||
[endsect]
|
||||
@ -1,151 +0,0 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
|
||||
[#ref_header_optional_optional_hpp][section:header_optional_optional Synopsis]
|
||||
|
||||
```// In Header: <`[@boost:/boost/optional/optional.hpp boost/optional/optional.hpp]'''<phrase role="comment">></phrase>'''``
|
||||
|
||||
namespace boost {
|
||||
|
||||
template <class T>
|
||||
class optional
|
||||
{
|
||||
public :
|
||||
|
||||
typedef T value_type;
|
||||
|
||||
// (If T is of reference type, the parameters and results by reference are by value)
|
||||
|
||||
optional () noexcept ; ``[link reference_optional_constructor __GO_TO__]``
|
||||
|
||||
optional ( none_t ) noexcept ; ``[link reference_optional_constructor_none_t __GO_TO__]``
|
||||
|
||||
optional ( T const& v ) ; ``[link reference_optional_constructor_value __GO_TO__]``
|
||||
|
||||
optional ( T&& v ) ; ``[link reference_optional_constructor_move_value __GO_TO__]``
|
||||
|
||||
// [new in 1.34]
|
||||
optional ( bool condition, T const& v ) ; ``[link reference_optional_constructor_bool_value __GO_TO__]``
|
||||
|
||||
optional ( optional const& rhs ) ; ``[link reference_optional_constructor_optional __GO_TO__]``
|
||||
|
||||
optional ( optional&& rhs ) noexcept(``['see below]``) ; ``[link reference_optional_move_constructor_optional __GO_TO__]``
|
||||
|
||||
template<class U> explicit optional ( optional<U> const& rhs ) ; ``[link reference_optional_constructor_other_optional __GO_TO__]``
|
||||
|
||||
template<class U> explicit optional ( optional<U>&& rhs ) ; ``[link reference_optional_move_constructor_other_optional __GO_TO__]``
|
||||
|
||||
template<class InPlaceFactory> explicit optional ( InPlaceFactory const& f ) ; ``[link reference_optional_constructor_factory __GO_TO__]``
|
||||
|
||||
template<class TypedInPlaceFactory> explicit optional ( TypedInPlaceFactory const& f ) ; ``[link reference_optional_constructor_factory __GO_TO__]``
|
||||
|
||||
optional& operator = ( none_t ) noexcept ; ``[link reference_optional_operator_equal_none_t __GO_TO__]``
|
||||
|
||||
optional& operator = ( T const& v ) ; ``[link reference_optional_operator_equal_value __GO_TO__]``
|
||||
|
||||
optional& operator = ( T&& v ) ; ``[link reference_optional_operator_move_equal_value __GO_TO__]``
|
||||
|
||||
optional& operator = ( optional const& rhs ) ; ``[link reference_optional_operator_equal_optional __GO_TO__]``
|
||||
|
||||
optional& operator = ( optional&& rhs ) noexcept(``['see below]``) ; ``[link reference_optional_operator_move_equal_optional __GO_TO__]``
|
||||
|
||||
template<class U> optional& operator = ( optional<U> const& rhs ) ; ``[link reference_optional_operator_equal_other_optional __GO_TO__]``
|
||||
|
||||
template<class U> optional& operator = ( optional<U>&& rhs ) ; ``[link reference_optional_operator_move_equal_other_optional __GO_TO__]``
|
||||
|
||||
template<class... Args> void emplace ( Args...&& args ) ; ``[link reference_optional_emplace __GO_TO__]``
|
||||
|
||||
template<class InPlaceFactory> optional& operator = ( InPlaceFactory const& f ) ; ``[link reference_optional_operator_equal_factory __GO_TO__]``
|
||||
|
||||
template<class TypedInPlaceFactory> optional& operator = ( TypedInPlaceFactory const& f ) ; ``[link reference_optional_operator_equal_factory __GO_TO__]``
|
||||
|
||||
T const& get() const ; ``[link reference_optional_get __GO_TO__]``
|
||||
T& get() ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
T const* operator ->() const ; ``[link reference_optional_operator_arrow __GO_TO__]``
|
||||
T* operator ->() ; ``[link reference_optional_operator_arrow __GO_TO__]``
|
||||
|
||||
T const& operator *() const& ; ``[link reference_optional_operator_asterisk __GO_TO__]``
|
||||
T& operator *() & ; ``[link reference_optional_operator_asterisk __GO_TO__]``
|
||||
T&& operator *() && ; ``[link reference_optional_operator_asterisk_move __GO_TO__]``
|
||||
|
||||
T const& value() const& ; ``[link reference_optional_value __GO_TO__]``
|
||||
T& value() & ; ``[link reference_optional_value __GO_TO__]``
|
||||
T&& value() && ; ``[link reference_optional_value_move __GO_TO__]``
|
||||
|
||||
template<class U> T value_or( U && v ) const& ; ``[link reference_optional_value_or __GO_TO__]``
|
||||
template<class U> T value_or( U && v ) && ; ``[link reference_optional_value_or_move __GO_TO__]``
|
||||
|
||||
template<class F> T value_or_eval( F f ) const& ; ``[link reference_optional_value_or_call __GO_TO__]``
|
||||
template<class F> T value_or_eval( F f ) && ; ``[link reference_optional_value_or_call_move __GO_TO__]``
|
||||
|
||||
T const* get_ptr() const ; ``[link reference_optional_get_ptr __GO_TO__]``
|
||||
T* get_ptr() ; ``[link reference_optional_get_ptr __GO_TO__]``
|
||||
|
||||
explicit operator bool() const noexcept ; ``[link reference_optional_operator_bool __GO_TO__]``
|
||||
|
||||
bool operator!() const noexcept ; ``[link reference_optional_operator_not __GO_TO__]``
|
||||
|
||||
// deprecated methods
|
||||
|
||||
// (deprecated)
|
||||
void reset() noexcept ; ``[link reference_optional_reset __GO_TO__]``
|
||||
|
||||
// (deprecated)
|
||||
void reset ( T const& ) ; ``[link reference_optional_reset_value __GO_TO__]``
|
||||
|
||||
// (deprecated)
|
||||
bool is_initialized() const ; ``[link reference_optional_is_initialized __GO_TO__]``
|
||||
|
||||
// (deprecated)
|
||||
T const& get_value_or( T const& default ) const ; ``[link reference_optional_get_value_or_value __GO_TO__]``
|
||||
};
|
||||
|
||||
template<class T> inline bool operator == ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_equal_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator != ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_not_equal_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator < ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_less_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator > ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_greater_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator <= ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_less_or_equal_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator >= ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_greater_or_equal_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator == ( optional<T> const& x, none_t ) noexcept ; ``[link reference_operator_compare_equal_optional_none __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator != ( optional<T> const& x, none_t ) noexcept ; ``[link reference_operator_compare_not_equal_optional_none __GO_TO__]``
|
||||
|
||||
template<class T> inline optional<T> make_optional ( T const& v ) ; ``[link reference_make_optional_value __GO_TO__]``
|
||||
|
||||
template<class T> inline optional<T> make_optional ( bool condition, T const& v ) ; ``[link reference_make_optional_bool_value __GO_TO__]``
|
||||
|
||||
template<class T> inline T const& get_optional_value_or ( optional<T> const& opt, T const& default ) ; ``[link reference_optional_get_value_or_value __GO_TO__]``
|
||||
|
||||
template<class T> inline T const& get ( optional<T> const& opt ) ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
template<class T> inline T& get ( optional<T> & opt ) ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
template<class T> inline T const* get ( optional<T> const* opt ) ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
template<class T> inline T* get ( optional<T>* opt ) ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
template<class T> inline T const* get_pointer ( optional<T> const& opt ) ; ``[link reference_optional_get_ptr __GO_TO__]``
|
||||
|
||||
template<class T> inline T* get_pointer ( optional<T> & opt ) ; ``[link reference_optional_get_ptr __GO_TO__]``
|
||||
|
||||
template<class T> inline void swap( optional<T>& x, optional<T>& y ) ; ``[link reference_swap_optional_optional __GO_TO__]``
|
||||
|
||||
} // namespace boost
|
||||
|
||||
|
||||
[endsect]
|
||||
File diff suppressed because it is too large
Load Diff
@ -1,81 +0,0 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
|
||||
[section Dependencies and Portability]
|
||||
|
||||
[section Dependencies]
|
||||
The implementation uses the following other Boost modules:
|
||||
|
||||
# assert
|
||||
# config
|
||||
# core
|
||||
# detail
|
||||
# move
|
||||
# mpl
|
||||
# static_assert
|
||||
# throw_exception
|
||||
# type_traits
|
||||
# utility
|
||||
|
||||
[endsect]
|
||||
|
||||
|
||||
[section Optional Reference Binding]
|
||||
|
||||
On compilers that do not conform to Standard C++ rules of reference binding, operations on optional references might give adverse results: rather than binding a reference to a designated object they may create an unexpected temporary and bind to it. Compilers known to have these deficiencies include GCC versions 4.2, 4.3, 4.4, 4.5; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0, 11.0, 12.0. On these compilers prefer using direct-initialization and copy assignment of optional references to copy-initialization and assignment from `T&`:
|
||||
|
||||
const int i = 0;
|
||||
optional<const int&> or1;
|
||||
optional<const int&> or2 = i; // not portable
|
||||
or1 = i; // not portable
|
||||
|
||||
optional<const int&> or3(i); // portable
|
||||
or1 = optional<const int&>(i); // portable
|
||||
|
||||
In order to check if your compiler correctly implements reference binding use this test program.
|
||||
|
||||
#include <cassert>
|
||||
|
||||
const int global_i = 0;
|
||||
|
||||
struct TestingReferenceBinding
|
||||
{
|
||||
TestingReferenceBinding(const int& ii)
|
||||
{
|
||||
assert(&ii == &global_i);
|
||||
}
|
||||
|
||||
void operator=(const int& ii)
|
||||
{
|
||||
assert(&ii == &global_i);
|
||||
}
|
||||
|
||||
void operator=(int&&) // remove this if your compiler doesn't have rvalue refs
|
||||
{
|
||||
assert(false);
|
||||
}
|
||||
};
|
||||
|
||||
int main()
|
||||
{
|
||||
const int& iref = global_i;
|
||||
assert(&iref == &global_i);
|
||||
|
||||
TestingReferenceBinding ttt = global_i;
|
||||
ttt = global_i;
|
||||
|
||||
TestingReferenceBinding ttt2 = iref;
|
||||
ttt2 = iref;
|
||||
}
|
||||
|
||||
[endsect]
|
||||
|
||||
[endsect]
|
||||
@ -1,55 +0,0 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2015 Andrzej Krzemienski
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
|
||||
[section:relnotes Release Notes]
|
||||
|
||||
[heading Boost Release 1.60]
|
||||
|
||||
* Changed the implementation of `boost::none` again. Now it is a const object with internal linkage (as any other tag). This fixes [@https://svn.boost.org/trac/boost/ticket/11203 Trac #11203].
|
||||
|
||||
[heading Boost Release 1.59]
|
||||
|
||||
* For C++03 compilers, added 0-argument overload for member function `emplace()`, and therewith removed the dependency on `<boost/utility/in_place_factory.hpp>`.
|
||||
* Fixed [@https://svn.boost.org/trac/boost/ticket/11241 Trac #11241].
|
||||
|
||||
[heading Boost Release 1.58]
|
||||
|
||||
* `boost::none_t` is no longer convertible from literal `0`. This avoids a bug where `optional<rational<int>> oi = 0;` would initialize an optional object with no contained value.
|
||||
* Improved the trick that prevents streaming out `optional` without header `optional_io.hpp` by using safe-bool idiom. This addresses [@https://svn.boost.org/trac/boost/ticket/10825 Trac #10825].
|
||||
* IOStream operators are now mentioned in documentation.
|
||||
* Added a way to manually disable move semantics: just define macro `BOOST_OPTIONAL_CONFIG_NO_RVALUE_REFERENCES`. This can be used to work around [@https://svn.boost.org/trac/boost/ticket/10399 Trac #10399].
|
||||
* It is no longer possible to assign `optional<U>` to `optional<T>` when `U` is not assignable or convertible to `T` ([@https://svn.boost.org/trac/boost/ticket/11087 Trac #11087]).
|
||||
* Value accessors now work correctly on rvalues of `optional<T&>` ([@https://svn.boost.org/trac/boost/ticket/10839 Trac #10839]).
|
||||
|
||||
|
||||
[heading Boost Release 1.57]
|
||||
|
||||
* [@https://github.com/boostorg/optional/pull/9 Git pull #9]: ['"Supply `<string>` to fix C++03 compile error on `logic_error("...")`"].
|
||||
|
||||
|
||||
[heading Boost Release 1.56]
|
||||
|
||||
* Added support for rvalue references. Now `optional<T>` works with moveable but non-copyable `T`'s,
|
||||
* Improved `swap` (now uses move operations),
|
||||
* Added function `emplace()`. This is the last of the requests from [@https://svn.boost.org/trac/boost/ticket/1841 Trac #1841],
|
||||
* `optional` is moveable, including conditional `noexcept` specifications, which make it `move_if_noexcept`-friendly,
|
||||
* Using explicit operator bool() on platforms that support it ([@https://svn.boost.org/trac/boost/ticket/4227 Trac #4227]) (breaking change),
|
||||
* Forward declaration of `operator<<(ostream&, optional const&)` to prevent inadvertent incorrect serialization of optional objects,
|
||||
* Removed deprecated function `reset()` from examples ([@https://svn.boost.org/trac/boost/ticket/9005 Trac #9005]),
|
||||
* Equality comparison with `boost::none` does not require that `T` be EqualityComparable,
|
||||
* Optional rvalue references are explicitly disallowed,
|
||||
* Binding temporaries to optional references is explicitly disallowed (breaking change),
|
||||
* More ways to access the contained value, functions `value()`, `value_or()`, `value_or_eval()`,
|
||||
* Updated and reorganized documentation, added tutorial and quick guide sections.
|
||||
|
||||
|
||||
|
||||
[endsect][/ relnotes]
|
||||
@ -10,40 +10,26 @@
|
||||
# Quickbook
|
||||
# -----------------------------------------------------------------------------
|
||||
|
||||
using boostbook ;
|
||||
import quickbook ;
|
||||
|
||||
path-constant images : html ;
|
||||
|
||||
xml optional
|
||||
:
|
||||
00_optional.qbk
|
||||
optional.qbk
|
||||
;
|
||||
|
||||
install images
|
||||
:
|
||||
images/opt_align1.png
|
||||
images/opt_align2.png
|
||||
images/opt_align3.png
|
||||
images/opt_align4.png
|
||||
:
|
||||
<location>html/images
|
||||
;
|
||||
|
||||
boostbook standalone
|
||||
:
|
||||
optional
|
||||
:
|
||||
<format>html:<xsl:param>boost.root=../../../..
|
||||
<format>html:<xsl:param>boost.libraries=../../../../libs/libraries.htm
|
||||
<xsl:param>chapter.autolabel=0
|
||||
<xsl:param>chunk.section.depth=8
|
||||
<xsl:param>toc.section.depth=2
|
||||
<xsl:param>boost.root=../../../..
|
||||
<xsl:param>boost.libraries=../../../libraries.htm
|
||||
<xsl:param>toc.max.depth=2
|
||||
<xsl:param>generate.section.toc.level=1
|
||||
<xsl:param>toc.section.depth=2
|
||||
<xsl:param>chunk.section.depth=1
|
||||
<format>pdf:<xsl:param>img.src.path=$(images)/
|
||||
<format>pdf:<xsl:param>boost.url.prefix=http://www.boost.org/doc/libs/release/libs/optional/doc/html
|
||||
<format>docbook:<auto-index-internal>on
|
||||
;
|
||||
|
||||
|
||||
|
||||
@ -9,7 +9,7 @@
|
||||
]
|
||||
|
||||
|
||||
[section:acknowledgements Acknowledgements]
|
||||
[section Acknowledgments]
|
||||
|
||||
[heading Pre-formal review]
|
||||
|
||||
@ -55,8 +55,6 @@ Rob Stewart, and others.
|
||||
with the proper semantics.
|
||||
* Mat Marcus shown the virtues of a value-oriented interface, influencing
|
||||
the current design, and contributed the idea of "none".
|
||||
* Vladimir Batov's design of Boost.Convert library motivated the development
|
||||
of value accessors for `optional`: functions `value`, `value_or`, `value_or_eval`.
|
||||
|
||||
[endsect]
|
||||
|
||||
@ -2,16 +2,16 @@
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
Copyright (c) 2015 Andrzej Krzemienski
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
[section Header <boost/optional.hpp>]
|
||||
|
||||
This is an alias for header [link boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional `<boost/optional/optional.hpp>`].
|
||||
[section Dependencies and Portability]
|
||||
|
||||
The implementation uses `type_traits/alignment_of.hpp` and
|
||||
`type_traits/type_with_alignment.hpp`
|
||||
|
||||
[endsect]
|
||||
[endsect]
|
||||
@ -1,4 +1,4 @@
|
||||
[/
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
@ -8,18 +8,18 @@
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
[section Design Overview]
|
||||
[section Development]
|
||||
|
||||
[section The models]
|
||||
|
||||
In C++, we can ['declare] an object (a variable) of type `T`, and we can give this
|
||||
variable an ['initial value] (through an ['initializer]. (cf. 8.5)).
|
||||
variable an ['initial value] (through an ['initializer]. (c.f. 8.5)).
|
||||
When a declaration includes a non-empty initializer (an initial value is given),
|
||||
it is said that the object has been initialized.
|
||||
If the declaration uses an empty initializer (no initial value is given), and
|
||||
neither default nor value initialization applies, it is said that the object is
|
||||
[*uninitialized]. Its actual value exist but has an ['indeterminate initial value]
|
||||
(cf. 8.5/11).
|
||||
(c.f. 8.5.9).
|
||||
`optional<T>` intends to formalize the notion of initialization (or lack of it)
|
||||
allowing a program to test whether an object has been initialized and stating
|
||||
that access to the value of an uninitialized object is undefined behavior. That
|
||||
@ -38,7 +38,7 @@ additional information to tell if an object has been effectively initialized.
|
||||
One of the typical ways in which this has been historically dealt with is via
|
||||
a special value: `EOF`, `npos`, -1, etc... This is equivalent to adding the
|
||||
special value to the set of possible values of a given type. This super set of
|
||||
`T` plus some ['nil_t]—where `nil_t` is some stateless POD—can be modeled in modern
|
||||
`T` plus some ['nil_t]—were `nil_t` is some stateless POD-can be modeled in modern
|
||||
languages as a [*discriminated union] of T and nil_t. Discriminated unions are
|
||||
often called ['variants]. A variant has a ['current type], which in our case is either
|
||||
`T` or `nil_t`.
|
||||
@ -58,7 +58,7 @@ The observation made in the last paragraph about the irrelevant nature of the
|
||||
additional `nil_t` with respect to [_purpose] of `optional<T>` suggests an
|
||||
alternative model: a ['container] that either has a value of `T` or nothing.
|
||||
|
||||
As of this writing I don't know of any precedent for a variable-size
|
||||
As of this writing I don't know of any precedence for a variable-size
|
||||
fixed-capacity (of 1) stack-based container model for optional values, yet I
|
||||
believe this is the consequence of the lack of practical implementations of
|
||||
such a container rather than an inherent shortcoming of the container model.
|
||||
@ -171,19 +171,19 @@ Also, the presence of the possibly uninitialized state requires additional
|
||||
operations not provided by `T` itself which are supported by a special interface.
|
||||
|
||||
[heading Lexically-hinted Value Access in the presence of possibly
|
||||
uninitialized optional objects: The operators * and ->]
|
||||
untitialized optional objects: The operators * and ->]
|
||||
|
||||
A relevant feature of a pointer is that it can have a [*null pointer value].
|
||||
This is a ['special] value which is used to indicate that the pointer is not
|
||||
referring to any object at all. In other words, null pointer values convey
|
||||
the notion of nonexistent objects.
|
||||
the notion of inexistent objects.
|
||||
|
||||
This meaning of the null pointer value allowed pointers to became a ['de
|
||||
facto] standard for handling optional objects because all you have to do
|
||||
to refer to a value which you don't really have is to use a null pointer
|
||||
value of the appropriate type. Pointers have been used for decades—from
|
||||
the days of C APIs to modern C++ libraries—to ['refer] to optional (that is,
|
||||
possibly nonexistent) objects; particularly as optional arguments to a
|
||||
possibly inexistent) objects; particularly as optional arguments to a
|
||||
function, but also quite often as optional data members.
|
||||
|
||||
The possible presence of a null pointer value makes the operations that
|
||||
@ -197,7 +197,7 @@ be undefined unless the implied pointee actually exist.
|
||||
Such a ['de facto] idiom for referring to optional objects can be formalized
|
||||
in the form of a concept: the __OPTIONAL_POINTEE__ concept.
|
||||
This concept captures the syntactic usage of operators `*`, `->` and
|
||||
contextual conversion to `bool` to convey the notion of optionality.
|
||||
conversion to `bool` to convey the notion of optionality.
|
||||
|
||||
However, pointers are good to [_refer] to optional objects, but not particularly
|
||||
good to handle the optional objects in all other respects, such as initializing
|
||||
102
doc/examples.qbk
Normal file
102
doc/examples.qbk
Normal file
@ -0,0 +1,102 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
|
||||
[section Examples]
|
||||
|
||||
[section Optional return values]
|
||||
|
||||
optional<char> get_async_input()
|
||||
{
|
||||
if ( !queue.empty() )
|
||||
return optional<char>(queue.top());
|
||||
else return optional<char>(); // uninitialized
|
||||
}
|
||||
|
||||
void receive_async_message()
|
||||
{
|
||||
optional<char> rcv ;
|
||||
// The safe boolean conversion from 'rcv' is used here.
|
||||
while ( (rcv = get_async_input()) && !timeout() )
|
||||
output(*rcv);
|
||||
}
|
||||
|
||||
[endsect]
|
||||
|
||||
[section Optional local variables]
|
||||
|
||||
optional<string> name ;
|
||||
if ( database.open() )
|
||||
{
|
||||
name.reset ( database.lookup(employer_name) ) ;
|
||||
}
|
||||
else
|
||||
{
|
||||
if ( can_ask_user )
|
||||
name.reset ( user.ask(employer_name) ) ;
|
||||
}
|
||||
|
||||
if ( name )
|
||||
print(*name);
|
||||
else print("employer's name not found!");
|
||||
|
||||
[endsect]
|
||||
|
||||
[section Optional data members]
|
||||
|
||||
class figure
|
||||
{
|
||||
public:
|
||||
|
||||
figure()
|
||||
{
|
||||
// data member 'm_clipping_rect' is uninitialized at this point.
|
||||
}
|
||||
|
||||
void clip_in_rect ( rect const& rect )
|
||||
{
|
||||
....
|
||||
m_clipping_rect.reset ( rect ) ; // initialized here.
|
||||
}
|
||||
|
||||
void draw ( canvas& cvs )
|
||||
{
|
||||
if ( m_clipping_rect )
|
||||
do_clipping(*m_clipping_rect);
|
||||
|
||||
cvs.drawXXX(..);
|
||||
}
|
||||
|
||||
// this can return NULL.
|
||||
rect const* get_clipping_rect() { return get_pointer(m_clipping_rect); }
|
||||
|
||||
private :
|
||||
|
||||
optional<rect> m_clipping_rect ;
|
||||
|
||||
};
|
||||
|
||||
[endsect]
|
||||
|
||||
[section Bypassing expensive unnecessary default construction]
|
||||
|
||||
class ExpensiveCtor { ... } ;
|
||||
class Fred
|
||||
{
|
||||
Fred() : mLargeVector(10000) {}
|
||||
|
||||
std::vector< optional<ExpensiveCtor> > mLargeVector ;
|
||||
} ;
|
||||
|
||||
[endsect]
|
||||
|
||||
[endsect]
|
||||
|
||||
|
||||
14
doc/html/HTML.manifest
Normal file
14
doc/html/HTML.manifest
Normal file
@ -0,0 +1,14 @@
|
||||
index.html
|
||||
boost_optional/development.html
|
||||
boost_optional/synopsis.html
|
||||
boost_optional/detailed_semantics.html
|
||||
boost_optional/examples.html
|
||||
boost_optional/optional_references.html
|
||||
boost_optional/rebinding_semantics_for_assignment_of_optional_references.html
|
||||
boost_optional/in_place_factories.html
|
||||
boost_optional/a_note_about_optional_bool_.html
|
||||
boost_optional/exception_safety_guarantees.html
|
||||
boost_optional/type_requirements.html
|
||||
boost_optional/implementation_notes.html
|
||||
boost_optional/dependencies_and_portability.html
|
||||
boost_optional/acknowledgments.html
|
||||
@ -1,28 +1,28 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
|
||||
<title>A note about optional<bool></title>
|
||||
<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
|
||||
<link rel="home" href="../index.html" title="Chapter 1. Boost.Optional">
|
||||
<link rel="up" href="../index.html" title="Chapter 1. Boost.Optional">
|
||||
<link rel="prev" href="detailed_semantics.html" title="Detailed Semantics">
|
||||
<link rel="stylesheet" href="../boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
|
||||
<link rel="home" href="../index.html" title="Chapter<EFBFBD>1.<2E>Boost.Optional">
|
||||
<link rel="up" href="../index.html" title="Chapter<EFBFBD>1.<2E>Boost.Optional">
|
||||
<link rel="prev" href="in_place_factories.html" title="In-Place Factories">
|
||||
<link rel="next" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">
|
||||
</head>
|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
|
||||
<table cellpadding="2" width="100%"><tr>
|
||||
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
|
||||
<td align="center"><a href="../../../../../index.html">Home</a></td>
|
||||
<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="../../../../libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../more/index.htm">More</a></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="exception_safety_guarantees.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
<a accesskey="p" href="in_place_factories.html"><img src="../../../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="exception_safety_guarantees.html"><img src="../../../../../doc/html/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="section" title="A note about optional<bool>">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.a_note_about_optional_bool_"></a><a class="link" href="a_note_about_optional_bool_.html" title="A note about optional<bool>">A note about
|
||||
optional<bool></a>
|
||||
@ -32,7 +32,7 @@
|
||||
be used with special caution and consideration.
|
||||
</p>
|
||||
<p>
|
||||
First, it is functionally similar to a tristate boolean (false, maybe, true)
|
||||
First, it is functionally similar to a tristate boolean (false,maybe,true)
|
||||
—such as <a href="../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>—
|
||||
except that in a tristate boolean, the maybe state <span class="underline">represents
|
||||
a valid value</span>, unlike the corresponding state of an uninitialized
|
||||
@ -42,11 +42,10 @@
|
||||
needed.
|
||||
</p>
|
||||
<p>
|
||||
Second, although <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>
|
||||
provides a contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
in C++11, this falls back to an implicit conversion on older compilers. This
|
||||
conversion refers to the initialization state and not to the contained value.
|
||||
Using <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> can
|
||||
Second, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>
|
||||
provides an implicit conversion to <code class="computeroutput"><span class="keyword">bool</span></code>.
|
||||
This conversion refers to the initialization state and not to the contained
|
||||
value. Using <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> can
|
||||
lead to subtle errors due to the implicit <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
conversion:
|
||||
</p>
|
||||
@ -55,11 +54,11 @@
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="keyword">try</span><span class="special">();</span>
|
||||
|
||||
<span class="comment">// The following intended to pass the value of 'v' to foo():</span>
|
||||
<span class="identifier">foo</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="comment">// But instead, the initialization state is passed</span>
|
||||
<span class="comment">// due to a typo: it should have been foo(*v).</span>
|
||||
<span class="special">}</span>
|
||||
<span class="comment">// The following intended to pass the value of 'v' to foo():
|
||||
</span> <span class="identifier">foo</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="comment">// But instead, the initialization state is passed
|
||||
</span> <span class="comment">// due to a typo: it should have been foo(*v).
|
||||
</span><span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The only implicit conversion is to <code class="computeroutput"><span class="keyword">bool</span></code>,
|
||||
@ -68,33 +67,10 @@
|
||||
takes an <code class="computeroutput"><span class="keyword">int</span></code> instead, it won't
|
||||
compile).
|
||||
</p>
|
||||
<p>
|
||||
Third, mixed comparisons with <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
work differently than similar mixed comparisons between pointers and <code class="computeroutput"><span class="keyword">bool</span></code>, so the results might surprise you:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">oEmpty</span><span class="special">(</span><span class="identifier">none</span><span class="special">),</span> <span class="identifier">oTrue</span><span class="special">(</span><span class="keyword">true</span><span class="special">),</span> <span class="identifier">oFalse</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders true</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false!</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false!</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders true!</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders true</span>
|
||||
</pre>
|
||||
<p>
|
||||
In other words, for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>, the following assertion does not hold:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">((</span><span class="identifier">opt</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">)</span> <span class="special">==</span> <span class="special">(!</span><span class="identifier">opt</span><span class="special">));</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -102,7 +78,7 @@
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="exception_safety_guarantees.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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|
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@ -1,130 +0,0 @@
|
||||
<html>
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|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.acknowledgements"></a><a class="link" href="acknowledgements.html" title="Acknowledgements">Acknowledgements</a>
|
||||
</h2></div></div></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgements.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.acknowledgements.pre_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.acknowledgements.pre_formal_review">Pre-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Peter Dimov suggested the name 'optional', and was the first to point out
|
||||
the need for aligned storage.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman
|
||||
coded 'aligned_storage', which are the core of the optional class implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
|
||||
and their work influenced the current implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Gennadiy Rozental made extensive and important comments which shaped the
|
||||
design.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vesa Karvonen and Douglas Gregor made quite useful comparisons between
|
||||
optional, variant and any; and made other relevant comments.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor and Peter Dimov commented on comparisons and evaluation
|
||||
in boolean contexts.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Many others have participated with useful comments: Aleksey Gurotov, Kevlin
|
||||
Henney, David Abrahams, and others I can't recall.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgements.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.acknowledgements.post_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.acknowledgements.post_formal_review">Post-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
William Kempf carefully considered the originally proposed interface and
|
||||
suggested the new interface which is currently used. He also started and
|
||||
fueled the discussion about the analogy optional<>/smart pointer
|
||||
and about relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
|
||||
focused on the relational semantics of optional (originally undefined);
|
||||
concluding with the fact that the pointer-like interface doesn't make it
|
||||
a pointer so it shall have deep relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Augustus Saunders also explored the different relational semantics between
|
||||
optional<> and a pointer and developed the OptionalPointee concept
|
||||
as an aid against potential conflicts on generic code.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman noticed that optional<> can be seen as an API on top
|
||||
of variant<T,nil_t>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
|
||||
the Maybe type constructor (analogy originally pointed out by David Sankel).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey,
|
||||
Rob Stewart, and others.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman made the case for the support of references and helped with
|
||||
the proper semantics.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Mat Marcus shown the virtues of a value-oriented interface, influencing
|
||||
the current design, and contributed the idea of "none".
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vladimir Batov's design of Boost.Convert library motivated the development
|
||||
of value accessors for <code class="computeroutput"><span class="identifier">optional</span></code>:
|
||||
functions <code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
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|
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<div class="spirit-nav">
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|
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<a name="boost_optional.acknowledgments"></a><a class="link" href="acknowledgments.html" title="Acknowledgments">Acknowledgments</a>
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<h4>
|
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<a name="boost_optional.acknowledgments.h0"></a>
|
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<span class="phrase"><a name="boost_optional.acknowledgments.pre_formal_review"></a></span><a class="link" href="acknowledgments.html#boost_optional.acknowledgments.pre_formal_review">Pre-formal
|
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<a name="boost_optional.acknowledgments.pre_formal_review"></a><h4>
|
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<a name="id583998"></a>
|
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<a class="link" href="acknowledgments.html#boost_optional.acknowledgments.pre_formal_review">Pre-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem">
|
||||
Peter Dimov suggested the name 'optional', and was the first to point out
|
||||
the need for aligned storage.
|
||||
</li>
|
||||
Peter Dimov suggested the name 'optional', and was the first to point out
|
||||
the need for aligned storage.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman
|
||||
coded 'aligned_storage', which are the core of the optional class implementation.
|
||||
</li>
|
||||
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman coded
|
||||
'aligned_storage', which are the core of the optional class implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
|
||||
and their work influenced the current implementation.
|
||||
</li>
|
||||
Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
|
||||
and their work influenced the current implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Gennadiy Rozental made extensive and important comments which shaped the
|
||||
design.
|
||||
</li>
|
||||
Gennadiy Rozental made extensive and important comments which shaped the
|
||||
design.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vesa Karvonen and Douglas Gregor made quite useful comparisons between
|
||||
optional, variant and any; and made other relevant comments.
|
||||
</li>
|
||||
Vesa Karvonen and Douglas Gregor made quite useful comparisons between optional,
|
||||
variant and any; and made other relevant comments.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor and Peter Dimov commented on comparisons and evaluation
|
||||
in boolean contexts.
|
||||
</li>
|
||||
Douglas Gregor and Peter Dimov commented on comparisons and evaluation in
|
||||
boolean contexts.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
</li>
|
||||
Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Many others have participated with useful comments: Aleksey Gurotov, Kevlin
|
||||
Henney, David Abrahams, and others I can't recall.
|
||||
</li>
|
||||
Many others have participated with useful comments: Aleksey Gurotov, Kevlin
|
||||
Henney, David Abrahams, and others I can't recall.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgments.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.acknowledgments.post_formal_review"></a></span><a class="link" href="acknowledgments.html#boost_optional.acknowledgments.post_formal_review">Post-formal
|
||||
<a name="boost_optional.acknowledgments.post_formal_review"></a><h4>
|
||||
<a name="id584068"></a>
|
||||
<a class="link" href="acknowledgments.html#boost_optional.acknowledgments.post_formal_review">Post-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem">
|
||||
William Kempf carefully considered the originally proposed interface and
|
||||
suggested the new interface which is currently used. He also started and
|
||||
fueled the discussion about the analogy optional<>/smart pointer
|
||||
and about relational operators.
|
||||
</li>
|
||||
William Kempf carefully considered the originally proposed interface and
|
||||
suggested the new interface which is currently used. He also started and
|
||||
fueled the discussion about the analogy optional<>/smart pointer and
|
||||
about relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
|
||||
focused on the relational semantics of optional (originally undefined);
|
||||
concluding with the fact that the pointer-like interface doesn't make it
|
||||
a pointer so it shall have deep relational operators.
|
||||
</li>
|
||||
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
|
||||
focused on the relational semantics of optional (originally undefined); concluding
|
||||
with the fact that the pointer-like interface doesn't make it a pointer so
|
||||
it shall have deep relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Augustus Saunders also explored the different relational semantics between
|
||||
optional<> and a pointer and developed the OptionalPointee concept
|
||||
as an aid against potential conflicts on generic code.
|
||||
</li>
|
||||
Augustus Saunders also explored the different relational semantics between
|
||||
optional<> and a pointer and developed the OptionalPointee concept
|
||||
as an aid against potential conflicts on generic code.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman noticed that optional<> can be seen as an API on top
|
||||
of variant<T,nil_t>.
|
||||
</li>
|
||||
Joel de Guzman noticed that optional<> can be seen as an API on top
|
||||
of variant<T,nil_t>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
|
||||
the Maybe type constructor (analogy originally pointed out by David Sankel).
|
||||
</li>
|
||||
Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
|
||||
the Maybe type constructor (analogy originally pointed out by David Sankel).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey,
|
||||
Rob Stewart, and others.
|
||||
</li>
|
||||
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey, Rob
|
||||
Stewart, and others.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman made the case for the support of references and helped with
|
||||
the proper semantics.
|
||||
</li>
|
||||
Joel de Guzman made the case for the support of references and helped with
|
||||
the proper semantics.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Mat Marcus shown the virtues of a value-oriented interface, influencing
|
||||
the current design, and contributed the idea of "none".
|
||||
</li>
|
||||
Mat Marcus shown the virtues of a value-oriented interface, influencing the
|
||||
current design, and contributed the idea of "none".
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -119,7 +119,7 @@
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="dependencies_and_portability/optional_reference_binding.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a>
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|
||||
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|
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies">Dependencies</a></span></dt>
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<dt><span class="section"><a href="dependencies_and_portability/optional_reference_binding.html">Optional
|
||||
Reference Binding</a></span></dt>
|
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</dl></div>
|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.dependencies_and_portability.dependencies"></a><a class="link" href="dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies" title="Dependencies">Dependencies</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
The implementation uses the following other Boost modules:
|
||||
</p>
|
||||
<div class="orderedlist"><ol class="orderedlist" type="1">
|
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<li class="listitem">
|
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assert
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</li>
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<li class="listitem">
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config
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core
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<li class="listitem">
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detail
|
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<li class="listitem">
|
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move
|
||||
</li>
|
||||
<li class="listitem">
|
||||
mpl
|
||||
</li>
|
||||
<li class="listitem">
|
||||
static_assert
|
||||
</li>
|
||||
<li class="listitem">
|
||||
throw_exception
|
||||
</li>
|
||||
<li class="listitem">
|
||||
type_traits
|
||||
</li>
|
||||
<li class="listitem">
|
||||
utility
|
||||
</li>
|
||||
</ol></div>
|
||||
</div>
|
||||
The implementation uses <code class="computeroutput"><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">alignment_of</span><span class="special">.</span><span class="identifier">hpp</span></code> and
|
||||
<code class="computeroutput"><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">type_with_alignment</span><span class="special">.</span><span class="identifier">hpp</span></code>
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
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@ -83,7 +42,7 @@
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|
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<a name="boost_optional.dependencies_and_portability.optional_reference_binding"></a><a class="link" href="optional_reference_binding.html" title="Optional Reference Binding">Optional
|
||||
Reference Binding</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
On compilers that do not conform to Standard C++ rules of reference binding,
|
||||
operations on optional references might give adverse results: rather than
|
||||
binding a reference to a designated object they may create an unexpected
|
||||
temporary and bind to it. Compilers known to have these deficiencies include
|
||||
GCC versions 4.2, 4.3, 4.4, 4.5; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0,
|
||||
11.0, 12.0. On these compilers prefer using direct-initialization and copy
|
||||
assignment of optional references to copy-initialization and assignment from
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or1</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or2</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// not portable</span>
|
||||
<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// not portable</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or3</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
|
||||
<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&>(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
|
||||
</pre>
|
||||
<p>
|
||||
In order to check if your compiler correctly implements reference binding
|
||||
use this test program.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">global_i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">struct</span> <span class="identifier">TestingReferenceBinding</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">TestingReferenceBinding</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">int</span><span class="special">&&)</span> <span class="comment">// remove this if your compiler doesn't have rvalue refs</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">iref</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">iref</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
<span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
|
||||
<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
|
||||
<span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
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|
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<a name="boost_optional.development"></a><a class="link" href="development.html" title="Development">Development</a>
|
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</h2></div></div></div>
|
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<div class="toc"><dl class="toc">
|
||||
<div class="toc"><dl>
|
||||
<dt><span class="section"><a href="development.html#boost_optional.development.the_models">The models</a></span></dt>
|
||||
<dt><span class="section"><a href="development.html#boost_optional.development.the_semantics">The semantics</a></span></dt>
|
||||
<dt><span class="section"><a href="development.html#boost_optional.development.the_interface">The Interface</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="section" title="The models">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_models"></a><a class="link" href="development.html#boost_optional.development.the_models" title="The models">The models</a>
|
||||
</h3></div></div></div>
|
||||
@ -39,12 +39,12 @@
|
||||
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this variable
|
||||
an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
|
||||
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
|
||||
(c.f. 8.5)). When a declaration includes a non-empty initializer (an initial
|
||||
value is given), it is said that the object has been initialized. If the
|
||||
declaration uses an empty initializer (no initial value is given), and neither
|
||||
default nor value initialization applies, it is said that the object is
|
||||
<span class="bold"><strong>uninitialized</strong></span>. Its actual value exist but
|
||||
has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11). <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> intends
|
||||
has an <span class="emphasis"><em>indeterminate initial value</em></span> (c.f. 8.5.9). <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> intends
|
||||
to formalize the notion of initialization (or lack of it) allowing a program
|
||||
to test whether an object has been initialized and stating that access to
|
||||
the value of an uninitialized object is undefined behavior. That is, when
|
||||
@ -67,12 +67,13 @@
|
||||
special value: <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">npos</span></code>, -1, etc... This is equivalent to
|
||||
adding the special value to the set of possible values of a given type. This
|
||||
super set of <code class="computeroutput"><span class="identifier">T</span></code> plus some
|
||||
<span class="emphasis"><em>nil_t</em></span>—where <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD—can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
|
||||
unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a <span class="emphasis"><em>current
|
||||
type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
or <code class="computeroutput"><span class="identifier">nil_t</span></code>. Using the <a href="../../../../variant/index.html" target="_top">Boost.Variant</a> library, this model
|
||||
can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>.
|
||||
<span class="emphasis"><em>nil_t</em></span>—were <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD-can be modeled in modern languages as a <span class="bold"><strong>discriminated
|
||||
union</strong></span> of T and nil_t. Discriminated unions are often called <span class="emphasis"><em>variants</em></span>.
|
||||
A variant has a <span class="emphasis"><em>current type</em></span>, which in our case is either
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> or <code class="computeroutput"><span class="identifier">nil_t</span></code>.
|
||||
Using the <a href="../../../../variant/index.html" target="_top">Boost.Variant</a>
|
||||
library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>.
|
||||
There is precedent for a discriminated union as a model for an optional value:
|
||||
the <a href="http://www.haskell.org/" target="_top">Haskell</a> <span class="bold"><strong>Maybe</strong></span>
|
||||
built-in type constructor. Thus, a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
|
||||
@ -95,7 +96,7 @@
|
||||
value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
|
||||
</p>
|
||||
<p>
|
||||
As of this writing I don't know of any precedent for a variable-size fixed-capacity
|
||||
As of this writing I don't know of any precedence for a variable-size fixed-capacity
|
||||
(of 1) stack-based container model for optional values, yet I believe this
|
||||
is the consequence of the lack of practical implementations of such a container
|
||||
rather than an inherent shortcoming of the container model.
|
||||
@ -109,65 +110,65 @@
|
||||
<p>
|
||||
Discriminated-union:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the variant
|
||||
implies copies of the value.
|
||||
</li>
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the variant
|
||||
implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code> from
|
||||
a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Single-element container:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the container
|
||||
implies copies of the value.
|
||||
</li>
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the container
|
||||
implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the container is empty models testing if the optional is initialized
|
||||
</li>
|
||||
Testing if the container is empty models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from an empty container models the undefined behavior of trying to access
|
||||
the value of an uninitialized optional
|
||||
</li>
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code> from
|
||||
an empty container models the undefined behavior of trying to access the
|
||||
value of an uninitialized optional
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="section" title="The semantics">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_semantics"></a><a class="link" href="development.html#boost_optional.development.the_semantics" title="The semantics">The semantics</a>
|
||||
</h3></div></div></div>
|
||||
@ -192,59 +193,58 @@
|
||||
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
the required basic semantics:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce a
|
||||
formally uninitialized wrapped object.
|
||||
</li>
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce a formally
|
||||
uninitialized wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
|
||||
To introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span> To
|
||||
introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain a
|
||||
new yet equivalent wrapped object.
|
||||
</li>
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain a new
|
||||
yet equivalent wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of some
|
||||
object.
|
||||
</li>
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of some
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To assign
|
||||
to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To assign
|
||||
to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
|
||||
initialize the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To initialize
|
||||
the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by the
|
||||
type T):</strong></span> To compare wrapped object values taking into account
|
||||
the presence of uninitialized states.
|
||||
</li>
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by the
|
||||
type T):</strong></span> To compare wrapped object values taking into account
|
||||
the presence of uninitialized states.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
|
||||
object.
|
||||
</li>
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects. (with
|
||||
whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
swap).
|
||||
</li>
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects. (with
|
||||
whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
swap).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the wrapped
|
||||
object (if any) and leave the wrapper in the uninitialized state.
|
||||
</li>
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the wrapped
|
||||
object (if any) and leave the wrapper in the uninitialized state.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Additional operations are useful, such as converting constructors and converting
|
||||
@ -252,7 +252,7 @@
|
||||
via a pointer to the wrapped object or null.
|
||||
</p>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="section" title="The Interface">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_interface"></a><a class="link" href="development.html#boost_optional.development.the_interface" title="The Interface">The Interface</a>
|
||||
</h3></div></div></div>
|
||||
@ -291,9 +291,9 @@
|
||||
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself which are supported by a special interface.
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.development.the_interface.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="development.html#boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
<a name="boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_"></a><h5>
|
||||
<a name="id555569"></a>
|
||||
<a class="link" href="development.html#boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
Value Access in the presence of possibly untitialized optional objects: The
|
||||
operators * and -></a>
|
||||
</h5>
|
||||
@ -301,7 +301,7 @@
|
||||
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
|
||||
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value which
|
||||
is used to indicate that the pointer is not referring to any object at all.
|
||||
In other words, null pointer values convey the notion of nonexistent objects.
|
||||
In other words, null pointer values convey the notion of inexistent objects.
|
||||
</p>
|
||||
<p>
|
||||
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
|
||||
@ -309,7 +309,7 @@
|
||||
to do to refer to a value which you don't really have is to use a null pointer
|
||||
value of the appropriate type. Pointers have been used for decades—from
|
||||
the days of C APIs to modern C++ libraries—to <span class="emphasis"><em>refer</em></span>
|
||||
to optional (that is, possibly nonexistent) objects; particularly as optional
|
||||
to optional (that is, possibly inexistent) objects; particularly as optional
|
||||
arguments to a function, but also quite often as optional data members.
|
||||
</p>
|
||||
<p>
|
||||
@ -328,8 +328,8 @@
|
||||
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
|
||||
can be formalized in the form of a concept: the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
|
||||
concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-></span></code>
|
||||
and contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
to convey the notion of optionality.
|
||||
and conversion to <code class="computeroutput"><span class="keyword">bool</span></code> to convey
|
||||
the notion of optionality.
|
||||
</p>
|
||||
<p>
|
||||
However, pointers are good to <span class="underline">refer</span>
|
||||
@ -356,9 +356,9 @@
|
||||
possible undefined value because of the idiomatic aid present in the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a> concept
|
||||
incarnated by pointers.
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.development.the_interface.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="development.html#boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
<a name="boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a><h5>
|
||||
<a name="id555831"></a>
|
||||
<a class="link" href="development.html#boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
as a model of OptionalPointee</a>
|
||||
</h5>
|
||||
<p>
|
||||
@ -367,9 +367,9 @@
|
||||
about the possibly uninitialized state appealing to the familiar pointer
|
||||
semantics w.r.t. to null pointers.
|
||||
</p>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<div class="warning" title="Warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
|
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/html/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
@ -398,7 +398,7 @@
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -406,7 +406,7 @@
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="tutorial.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="synopsis.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.discussion"></a><a class="link" href="discussion.html" title="Discussion">Discussion</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not have
|
||||
a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or uses
|
||||
a runtime check and throws an exception if the postcondition is violated. This
|
||||
is a reasonable choice for example, for function (A), because the lack of a
|
||||
proper return value is directly related to an invalid parameter (out of domain
|
||||
argument), so it is appropriate to require the callee to supply only parameters
|
||||
in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail just
|
||||
because it can't find a value to return; so it is incorrect to consider such
|
||||
a situation an error and assert or throw an exception. This function must return,
|
||||
and somehow, must tell the callee that it is not returning a meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error to
|
||||
ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside itself,
|
||||
but in many applications, it is just impractical for performance reasons to
|
||||
treat this as an error (because detecting that the polygon has no area might
|
||||
be too expensive to be required to be tested previously), and either an arbitrary
|
||||
point (typically at infinity) is returned, or some efficient way to tell the
|
||||
callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved to
|
||||
communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>, points
|
||||
at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when such
|
||||
values do not exist. In these cases, the usual alternative is either to use
|
||||
a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> in place
|
||||
of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound type, such
|
||||
as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the result
|
||||
which indicates if the result is meaningful, has the advantage that can be
|
||||
turned into a consistent idiom since the first element of the pair can be whatever
|
||||
the function would conceptually return. For example, the last two functions
|
||||
could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error prone
|
||||
since the user can easily use the function result (first element of the pair)
|
||||
without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
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|
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<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_return_values">Optional
|
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return values</a></span></dt>
|
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<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_local_variables">Optional
|
||||
@ -36,7 +36,7 @@
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction">Bypassing
|
||||
expensive unnecessary default construction</a></span></dt>
|
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</dl></div>
|
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<div class="section">
|
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<div class="section" title="Optional return values">
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|
||||
<a name="boost_optional.examples.optional_return_values"></a><a class="link" href="examples.html#boost_optional.examples.optional_return_values" title="Optional return values">Optional
|
||||
return values</a>
|
||||
@ -45,19 +45,19 @@
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">queue</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()</span> <span class="special">)</span>
|
||||
<span class="keyword">return</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">>(</span><span class="identifier">queue</span><span class="special">.</span><span class="identifier">top</span><span class="special">());</span>
|
||||
<span class="keyword">else</span> <span class="keyword">return</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">>();</span> <span class="comment">// uninitialized</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">else</span> <span class="keyword">return</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">>();</span> <span class="comment">// uninitialized
|
||||
</span><span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">receive_async_message</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">rcv</span> <span class="special">;</span>
|
||||
<span class="comment">// The safe boolean conversion from 'rcv' is used here.</span>
|
||||
<span class="keyword">while</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">rcv</span> <span class="special">=</span> <span class="identifier">get_async_input</span><span class="special">())</span> <span class="special">&&</span> <span class="special">!</span><span class="identifier">timeout</span><span class="special">()</span> <span class="special">)</span>
|
||||
<span class="comment">// The safe boolean conversion from 'rcv' is used here.
|
||||
</span> <span class="keyword">while</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">rcv</span> <span class="special">=</span> <span class="identifier">get_async_input</span><span class="special">())</span> <span class="special">&&</span> <span class="special">!</span><span class="identifier">timeout</span><span class="special">()</span> <span class="special">)</span>
|
||||
<span class="identifier">output</span><span class="special">(*</span><span class="identifier">rcv</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="section" title="Optional local variables">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.optional_local_variables"></a><a class="link" href="examples.html#boost_optional.examples.optional_local_variables" title="Optional local variables">Optional
|
||||
local variables</a>
|
||||
@ -65,12 +65,12 @@
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">name</span> <span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">database</span><span class="special">.</span><span class="identifier">open</span><span class="special">()</span> <span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">name</span> <span class="special">=</span> <span class="identifier">database</span><span class="special">.</span><span class="identifier">lookup</span><span class="special">(</span><span class="identifier">employer_name</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">name</span><span class="special">.</span><span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">database</span><span class="special">.</span><span class="identifier">lookup</span><span class="special">(</span><span class="identifier">employer_name</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">can_ask_user</span> <span class="special">)</span>
|
||||
<span class="identifier">name</span> <span class="special">=</span> <span class="identifier">user</span><span class="special">.</span><span class="identifier">ask</span><span class="special">(</span><span class="identifier">employer_name</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">name</span><span class="special">.</span><span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">user</span><span class="special">.</span><span class="identifier">ask</span><span class="special">(</span><span class="identifier">employer_name</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">name</span> <span class="special">)</span>
|
||||
@ -78,7 +78,7 @@
|
||||
<span class="keyword">else</span> <span class="identifier">print</span><span class="special">(</span><span class="string">"employer's name not found!"</span><span class="special">);</span>
|
||||
</pre>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="section" title="Optional data members">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.optional_data_members"></a><a class="link" href="examples.html#boost_optional.examples.optional_data_members" title="Optional data members">Optional
|
||||
data members</a>
|
||||
@ -89,14 +89,14 @@
|
||||
|
||||
<span class="identifier">figure</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// data member 'm_clipping_rect' is uninitialized at this point.</span>
|
||||
<span class="special">}</span>
|
||||
<span class="comment">// data member 'm_clipping_rect' is uninitialized at this point.
|
||||
</span> <span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">clip_in_rect</span> <span class="special">(</span> <span class="identifier">rect</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rect</span> <span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="special">....</span>
|
||||
<span class="identifier">m_clipping_rect</span> <span class="special">=</span> <span class="identifier">rect</span> <span class="special">;</span> <span class="comment">// initialized here.</span>
|
||||
<span class="special">}</span>
|
||||
<span class="identifier">m_clipping_rect</span><span class="special">.</span><span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">rect</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// initialized here.
|
||||
</span> <span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">draw</span> <span class="special">(</span> <span class="identifier">canvas</span><span class="special">&</span> <span class="identifier">cvs</span> <span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
@ -106,8 +106,8 @@
|
||||
<span class="identifier">cvs</span><span class="special">.</span><span class="identifier">drawXXX</span><span class="special">(..);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="comment">// this can return NULL.</span>
|
||||
<span class="identifier">rect</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_clipping_rect</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">get_pointer</span><span class="special">(</span><span class="identifier">m_clipping_rect</span><span class="special">);</span> <span class="special">}</span>
|
||||
<span class="comment">// this can return NULL.
|
||||
</span> <span class="identifier">rect</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_clipping_rect</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">get_pointer</span><span class="special">(</span><span class="identifier">m_clipping_rect</span><span class="special">);</span> <span class="special">}</span>
|
||||
|
||||
<span class="keyword">private</span> <span class="special">:</span>
|
||||
|
||||
@ -116,7 +116,7 @@
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="section" title="Bypassing expensive unnecessary default construction">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.bypassing_expensive_unnecessary_default_construction"></a><a class="link" href="examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction" title="Bypassing expensive unnecessary default construction">Bypassing
|
||||
expensive unnecessary default construction</a>
|
||||
@ -133,7 +133,7 @@
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -141,7 +141,7 @@
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="optional_references.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="optional_references.html"><img src="../../../../../doc/html/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
|
||||
@ -1,11 +1,11 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
|
||||
<title>Exception Safety Guarantees</title>
|
||||
<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
|
||||
<link rel="home" href="../index.html" title="Chapter 1. Boost.Optional">
|
||||
<link rel="up" href="../index.html" title="Chapter 1. Boost.Optional">
|
||||
<link rel="stylesheet" href="../boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
|
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<link rel="home" href="../index.html" title="Chapter<EFBFBD>1.<2E>Boost.Optional">
|
||||
<link rel="up" href="../index.html" title="Chapter<EFBFBD>1.<2E>Boost.Optional">
|
||||
<link rel="prev" href="a_note_about_optional_bool_.html" title="A note about optional<bool>">
|
||||
<link rel="next" href="type_requirements.html" title="Type requirements">
|
||||
</head>
|
||||
@ -13,150 +13,120 @@
|
||||
<table cellpadding="2" width="100%"><tr>
|
||||
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
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||||
<td align="center"><a href="../../../../../index.html">Home</a></td>
|
||||
<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
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<td align="center"><a href="../../../../libraries.htm">Libraries</a></td>
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../more/index.htm">More</a></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
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<a accesskey="p" href="a_note_about_optional_bool_.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="type_requirements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<a accesskey="p" href="a_note_about_optional_bool_.html"><img src="../../../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="type_requirements.html"><img src="../../../../../doc/html/images/next.png" alt="Next"></a>
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||||
</div>
|
||||
<div class="section">
|
||||
<div class="section" title="Exception Safety Guarantees">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.exception_safety_guarantees"></a><a class="link" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">Exception Safety
|
||||
Guarantees</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
This library assumes that <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
destructor does not throw exceptions. If it does, the behaviour of many operations
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
undefined.
|
||||
Because of the current implementation (see <a class="link" href="implementation_notes.html" title="Implementation Notes">Implementation
|
||||
Notes</a>), all of the assignment methods:
|
||||
</p>
|
||||
<p>
|
||||
The following mutating operations never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span></code></li>
|
||||
<li class="listitem"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code></li>
|
||||
<li class="listitem"><code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span></code></li>
|
||||
<li class="listitem"><code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span></code></li>
|
||||
<li class="listitem"><code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span> </code></li>
|
||||
<li class="listitem"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>:::</span><span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&)</span></code></li>
|
||||
</ul></div>
|
||||
<p>
|
||||
In addition, the following constructors and the destructor never throw exceptions:
|
||||
Can only <span class="emphasis"><em>guarantee</em></span> the <span class="underline">basic
|
||||
exception safety</span>: The lvalue optional is left <span class="underline">uninitialized</span>
|
||||
if an exception is thrown (any previous value is <span class="emphasis"><em>first</em></span>
|
||||
destroyed using <code class="computeroutput"><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>)
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">()</span>
|
||||
<span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<p>
|
||||
On the other hand, the <span class="emphasis"><em>uninitializing</em></span> methods:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">none_t</span> <span class="special">)</span></code></li>
|
||||
<li class="listitem"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">()</span></code></li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Regarding the following assignment functions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span> </code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
They forward calls to the corresponding <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructors or assignments (depending on whether the optional object is initialized
|
||||
or not); so if both <code class="computeroutput"><span class="identifier">T</span></code>'s constructor
|
||||
and the assignment provide strong exception safety guarantee, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s assignment
|
||||
also provides strong exception safety guarantee; otherwise we only get the
|
||||
basic guarantee. Additionally, if both involved <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructor and the assignment never throw, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
assignment also never throws.
|
||||
Provide the no-throw guarantee (assuming a no-throw <code class="computeroutput"><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>)
|
||||
</p>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">T</span></code>'s constructor or assignment
|
||||
throws, assignments to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
do not throw anything else on its own. A throw during assignment never changes
|
||||
the initialization state of any optional object involved:
|
||||
However, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>
|
||||
itself doesn't throw any exceptions, the only source for exceptions here are
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>'s constructor, so if you
|
||||
know the exception guarantees for <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span> <span class="special">(</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>, you
|
||||
know that <code class="computeroutput"><span class="identifier">optional</span></code>'s assignment
|
||||
and reset has the same guarantees.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt1</span><span class="special">(</span><span class="identifier">val1</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt2</span><span class="special">(</span><span class="identifier">val2</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
|
||||
<pre class="programlisting"><span class="comment">//
|
||||
</span><span class="comment">// Case 1: Exception thrown during assignment.
|
||||
</span><span class="comment">//
|
||||
</span><span class="identifier">T</span> <span class="identifier">v0</span><span class="special">(</span><span class="number">123</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt0</span><span class="special">(</span><span class="identifier">v0</span><span class="special">);</span>
|
||||
<span class="keyword">try</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt2</span><span class="special">;</span> <span class="comment">// throws</span>
|
||||
<span class="identifier">T</span> <span class="identifier">v1</span><span class="special">(</span><span class="number">456</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt1</span><span class="special">(</span><span class="identifier">v1</span><span class="special">);</span>
|
||||
<span class="identifier">opt0</span> <span class="special">=</span> <span class="identifier">opt1</span> <span class="special">;</span>
|
||||
|
||||
<span class="comment">// If no exception was thrown, assignment succeeded.
|
||||
</span> <span class="identifier">assert</span><span class="special">(</span> <span class="special">*</span><span class="identifier">opt0</span> <span class="special">==</span> <span class="identifier">v1</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span><span class="special">(...)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
<span class="comment">// If any exception was thrown, 'opt0' is reset to uninitialized.
|
||||
</span> <span class="identifier">assert</span><span class="special">(</span> <span class="special">!</span><span class="identifier">opt0</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="comment">//
|
||||
</span><span class="comment">// Case 2: Exception thrown during reset(v)
|
||||
</span><span class="comment">//
|
||||
</span><span class="identifier">T</span> <span class="identifier">v0</span><span class="special">(</span><span class="number">123</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">v0</span><span class="special">);</span>
|
||||
<span class="keyword">try</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">T</span> <span class="identifier">v1</span><span class="special">(</span><span class="number">456</span><span class="special">);</span>
|
||||
<span class="identifier">opt</span><span class="special">.</span><span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">v1</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="comment">// If no exception was thrown, reset succeeded.
|
||||
</span> <span class="identifier">assert</span><span class="special">(</span> <span class="special">*</span><span class="identifier">opt</span> <span class="special">==</span> <span class="identifier">v1</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span><span class="special">(...)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// If any exception was thrown, 'opt' is reset to uninitialized.
|
||||
</span> <span class="identifier">assert</span><span class="special">(</span> <span class="special">!</span><span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This also applies to move assignments/constructors. However, move operations
|
||||
are made no-throw more often.
|
||||
</p>
|
||||
<p>
|
||||
Operation <code class="computeroutput"><span class="identifier">emplace</span></code> provides
|
||||
basic exception safety guarantee. If it throws, the optional object becomes
|
||||
uninitialized regardless of its initial state, and its previous contained value
|
||||
(if any) is destroyed. It doesn't call any assignment or move/copy constructor
|
||||
on <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<h4>
|
||||
<a name="boost_optional.exception_safety_guarantees.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.exception_safety_guarantees.swap"></a></span><a class="link" href="exception_safety_guarantees.html#boost_optional.exception_safety_guarantees.swap">Swap</a>
|
||||
<a name="boost_optional.exception_safety_guarantees.swap"></a><h4>
|
||||
<a name="id583497"></a>
|
||||
<a class="link" href="exception_safety_guarantees.html#boost_optional.exception_safety_guarantees.swap">Swap</a>
|
||||
</h4>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
|
||||
customized, its primary implementation forwards calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> or move constructor (depending
|
||||
on the initialization state of the optional objects). Thus, if both <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
and move constructor never throw, <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> never
|
||||
throws. similarly, if both <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and move constructor offer
|
||||
strong guarantee, <code class="computeroutput"><span class="identifier">swap</span></code> on
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> also
|
||||
offers a strong guarantee.
|
||||
</p>
|
||||
<p>
|
||||
In case <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
|
||||
customized, the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
move constructor are replaced with the calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
default constructor followed by <code class="computeroutput"><span class="identifier">swap</span></code>.
|
||||
(This is more useful on older compilers that do not support move semantics,
|
||||
when one wants to acheive stronger exception safety guarantees.) In this case
|
||||
the exception safety guarantees for <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
are reliant on the guarantees of <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and default constructor
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="special">)</span></code> has the same exception guarantee as <code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&,</span><span class="identifier">T</span><span class="special">&)</span></code>
|
||||
when both optionals are initialized. If only one of the optionals is initialized,
|
||||
it gives the same <span class="emphasis"><em>basic</em></span> exception guarantee as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code> (since
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">()</span></code> doesn't throw). If none of the optionals
|
||||
is initialized, it has no-throw guarantee since it is a no-op.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -164,7 +134,7 @@
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="a_note_about_optional_bool_.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="type_requirements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<link rel="prev" href="type_requirements.html" title="Type requirements">
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@ -13,16 +13,16 @@
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||||
<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../index.html">Home</a></td>
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
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<a accesskey="p" href="type_requirements.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="dependencies_and_portability.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
|
||||
<div class="section" title="Implementation Notes">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.implementation_notes"></a><a class="link" href="implementation_notes.html" title="Implementation Notes">Implementation Notes</a>
|
||||
</h2></div></div></div>
|
||||
@ -31,17 +31,17 @@
|
||||
currently implemented using a custom aligned storage facility built from <code class="computeroutput"><span class="identifier">alignment_of</span></code> and <code class="computeroutput"><span class="identifier">type_with_alignment</span></code>
|
||||
(both from Type Traits). It uses a separate boolean flag to indicate the initialization
|
||||
state. Placement new with <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
copy/move constructor and <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
destructor are explicitly used to initialize, copy, move and destroy optional
|
||||
values. As a result, <code class="computeroutput"><span class="identifier">T</span></code>'s default
|
||||
constructor is effectively by-passed, but the exception guarantees are basic.
|
||||
It is planned to replace the current implementation with another with stronger
|
||||
exception safety, such as a future <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span></code>.
|
||||
copy constructor and <code class="computeroutput"><span class="identifier">T</span></code>'s destructor
|
||||
are explicitly used to initialize,copy and destroy optional values. As a result,
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>'s default constructor is
|
||||
effectively by-passed, but the exception guarantees are basic. It is planned
|
||||
to replace the current implementation with another with stronger exception
|
||||
safety, such as a future <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span></code>.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -49,7 +49,7 @@
|
||||
</tr></table>
|
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="type_requirements.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="dependencies_and_portability.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
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<div class="section" title="In-Place Factories">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.in_place_factories"></a><a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place Factories</a>
|
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</h2></div></div></div>
|
||||
@ -36,7 +36,7 @@
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="special">(</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">X</span> <span class="special">(</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">:::</span><span class="identifier">string</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">W</span>
|
||||
@ -50,8 +50,8 @@
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Temporary object created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="comment">// Temporary object created.
|
||||
</span> <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
@ -71,9 +71,9 @@
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="comment">// Wrapped object constructed in-place
|
||||
</span> <span class="comment">// No temporary created.
|
||||
</span> <span class="identifier">W</span> <span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
@ -119,24 +119,24 @@
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span><span class="special"><</span><span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">))</span> <span class="special">;</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.
|
||||
</span> <span class="comment">// No temporary created.
|
||||
</span> <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span><span class="special"><</span><span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">))</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are divided in two groups:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem">
|
||||
<span class="underline">TypedInPlaceFactories</span>: those which
|
||||
take the target type as a primary template parameter.
|
||||
</li>
|
||||
<span class="underline">TypedInPlaceFactories</span>: those which
|
||||
take the target type as a primary template parameter.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="underline">InPlaceFactories</span>: those with a template
|
||||
<code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code> member
|
||||
function taking the target type.
|
||||
</li>
|
||||
<span class="underline">InPlaceFactories</span>: those with a template
|
||||
<code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code> member
|
||||
function taking the target type.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Within each group, all the family members differ only in the number of parameters
|
||||
@ -170,9 +170,9 @@
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">in_place</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.
|
||||
</span> <span class="comment">// No temporary created.
|
||||
</span> <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">in_place</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
@ -182,7 +182,7 @@
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -190,7 +190,7 @@
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
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<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="a_note_about_optional_bool_.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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@ -1,128 +0,0 @@
|
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<html>
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<head>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.motivation"></a><a class="link" href="motivation.html" title="Motivation">Motivation</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not have
|
||||
a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or uses
|
||||
a runtime check and throws an exception if the postcondition is violated. This
|
||||
is a reasonable choice for example, for function (A), because the lack of a
|
||||
proper return value is directly related to an invalid parameter (out of domain
|
||||
argument), so it is appropriate to require the callee to supply only parameters
|
||||
in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail just
|
||||
because it can't find a value to return; so it is incorrect to consider such
|
||||
a situation an error and assert or throw an exception. This function must return,
|
||||
and somehow, must tell the callee that it is not returning a meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error to
|
||||
ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside itself,
|
||||
but in many applications, it is just impractical for performance reasons to
|
||||
treat this as an error (because detecting that the polygon has no area might
|
||||
be too expensive to be required to be tested previously), and either an arbitrary
|
||||
point (typically at infinity) is returned, or some efficient way to tell the
|
||||
callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved to
|
||||
communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>, points
|
||||
at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when such
|
||||
values do not exist. In these cases, the usual alternative is either to use
|
||||
a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> in place
|
||||
of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound type, such
|
||||
as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the result
|
||||
which indicates if the result is meaningful, has the advantage that can be
|
||||
turned into a consistent idiom since the first element of the pair can be whatever
|
||||
the function would conceptually return. For example, the last two functions
|
||||
could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error prone
|
||||
since the user can easily use the function result (first element of the pair)
|
||||
without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<div class="section">
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<div class="section" title="Optional references">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
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<a name="boost_optional.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional references</a>
|
||||
</h2></div></div></div>
|
||||
@ -35,72 +35,43 @@
|
||||
However, since references are not real objects some restrictions apply and
|
||||
some operations are not available in this case:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem">
|
||||
Converting constructors
|
||||
</li>
|
||||
Converting constructors
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Converting assignment
|
||||
</li>
|
||||
Converting assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace construction
|
||||
</li>
|
||||
InPlace construction
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace assignment
|
||||
</li>
|
||||
InPlace assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access via pointer
|
||||
</li>
|
||||
Value-access via pointer
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Also, even though <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
treats it wrapped pseudo-object much as a real value, a true real reference
|
||||
is stored so aliasing will ocurr:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem">
|
||||
Copies of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
will copy the references but all these references will nonetheless refer
|
||||
to the same object.
|
||||
</li>
|
||||
Copies of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
will copy the references but all these references will nonetheless reefer
|
||||
to the same object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access will actually provide access to the referenced object rather
|
||||
than the reference itself.
|
||||
</li>
|
||||
Value-access will actually provide access to the referenced object rather
|
||||
than the reference itself.
|
||||
</li>
|
||||
</ul></div>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
On compilers that do not conform to Standard C++ rules of reference binding,
|
||||
operations on optional references might give adverse results: rather than
|
||||
binding a reference to a designated object they may create an unexpected
|
||||
temporary and bind to it. For more details see <a class="link" href="dependencies_and_portability/optional_reference_binding.html" title="Optional Reference Binding">Dependencies
|
||||
and Portability section</a>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<h4>
|
||||
<a name="boost_optional.optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.optional_references.rvalue_references"></a></span><a class="link" href="optional_references.html#boost_optional.optional_references.rvalue_references">Rvalue
|
||||
references</a>
|
||||
</h4>
|
||||
<p>
|
||||
Rvalue references and lvalue references to const have the ability in C++ to
|
||||
extend the life time of a temporary they bind to. Optional references do not
|
||||
have this capability, therefore to avoid surprising effects it is not possible
|
||||
to initialize an optional references from a temporary. Optional rvalue references
|
||||
are disabled altogether. Also, the initialization and assignment of an optional
|
||||
reference to const from rvalue reference is disabled.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// legal</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// illegal</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -108,7 +79,7 @@
|
||||
</tr></table>
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<hr>
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|
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.quick_start"></a><a class="link" href="quick_start.html" title="Quick Start">Quick Start</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="quick_start.html#boost_optional.quick_start.optional_return_values">Optional
|
||||
return values</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/optional_automatic_variables.html">Optional
|
||||
automatic variables</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/optional_data_members.html">Optional
|
||||
data members</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/bypassing_unnecessary_default_construction.html">Bypassing
|
||||
unnecessary default construction</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/storage_in_containers.html">Storage
|
||||
in containers</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.quick_start.optional_return_values"></a><a class="link" href="quick_start.html#boost_optional.quick_start.optional_return_values" title="Optional return values">Optional
|
||||
return values</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Let's write and use a converter function that converts a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
|
||||
to an <code class="computeroutput"><span class="keyword">int</span></code>. It is possible that
|
||||
for a given string (e.g. <code class="computeroutput"><span class="string">"cat"</span></code>)
|
||||
there exists no value of type <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
capable of representing the conversion result. We do not consider such situation
|
||||
an error. We expect that the converter can be used only to check if the conversion
|
||||
is possible. A natural signature for this function can be:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
All necessary functionality can be included with one header <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
The above function signature means that the function can either return a
|
||||
value of type <code class="computeroutput"><span class="keyword">int</span></code> or a flag
|
||||
indicating that no value of <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
is available. This does not indicate an error. It is like one additional
|
||||
value of <code class="computeroutput"><span class="keyword">int</span></code>. This is how we
|
||||
can use our function:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span> <span class="special">=</span> <span class="comment">/*... */</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span> <span class="comment">// move-construct</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oi</span><span class="special">)</span> <span class="comment">// contextual conversion to bool</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span> <span class="comment">// operator*</span>
|
||||
</pre>
|
||||
<p>
|
||||
In order to test if <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
contains a value, we use the contextual conversion to type <code class="computeroutput"><span class="keyword">bool</span></code>. Because of this we can combine the initialization
|
||||
of the optional object and the test into one instruction:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">))</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
We extract the contained value with <code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code> (and with <code class="computeroutput"><span class="keyword">operator</span><span class="special">-></span></code> where it makes sense). An attempt to
|
||||
extract the contained value of an uninitialized optional object is an <span class="emphasis"><em>undefined
|
||||
behaviour</em></span> (UB). This implementation guards the call with <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span></code>. Therefore you should be sure
|
||||
that the contained value is there before extracting. For instance, the following
|
||||
code is reasonably UB-safe:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">convert</span><span class="special">(</span><span class="string">"100"</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This is because we know that string value <code class="computeroutput"><span class="string">"100"</span></code>
|
||||
converts to a valid value of <code class="computeroutput"><span class="keyword">int</span></code>.
|
||||
If you do not like this potential UB, you can use an alternative way of extracting
|
||||
the contained value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span>
|
||||
<span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_optional_access</span><span class="special">&)</span> <span class="special">{</span>
|
||||
<span class="comment">// deal with it</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This version throws an exception upon an attempt to access a non-existent
|
||||
contained value. If your way of dealing with the missing value is to use
|
||||
some default, like <code class="computeroutput"><span class="number">0</span></code>, there exists
|
||||
a yet another alternative:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This uses the <code class="computeroutput"><span class="identifier">atoi</span></code>-like approach
|
||||
to conversions: if <code class="computeroutput"><span class="identifier">text</span></code> does
|
||||
not represent an integral number just return <code class="computeroutput"><span class="number">0</span></code>.
|
||||
Finally, you can provide a callback to be called when trying to access the
|
||||
contained value fails:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">fallback_to_default</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">cerr</span> <span class="special"><<</span> <span class="string">"could not convert; using -1 instead"</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
|
||||
<span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">fallback_to_default</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This will call the provided callback and return whatever the callback returns.
|
||||
The callback can have side effects: they will only be observed when the optional
|
||||
object does not contain a value.
|
||||
</p>
|
||||
<p>
|
||||
Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code>
|
||||
can be implemented.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span>
|
||||
<span class="keyword">return</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Observe the two return statements. <code class="computeroutput"><span class="keyword">return</span>
|
||||
<span class="identifier">i</span></code> uses the converting constructor
|
||||
that can create <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
from <code class="computeroutput"><span class="identifier">T</span></code>. Thus constructed
|
||||
optional object is initialized and its value is a copy of <code class="computeroutput"><span class="identifier">i</span></code>.
|
||||
The other return statement uses another converting constructor from a special
|
||||
tag <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>. It is used to indicate that we want
|
||||
to create an uninitialized optional object.
|
||||
</p>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
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|
||||
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<a name="boost_optional.quick_start.bypassing_unnecessary_default_construction"></a><a class="link" href="bypassing_unnecessary_default_construction.html" title="Bypassing unnecessary default construction">Bypassing
|
||||
unnecessary default construction</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose we have class <code class="computeroutput"><span class="identifier">Date</span></code>,
|
||||
which does not have a default constructor: there is no good candidate for
|
||||
a default date. We have a function that returns two dates in form of a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">Date</span><span class="special">,</span> <span class="identifier">Date</span><span class="special">></span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
In other place we want to use the result of <code class="computeroutput"><span class="identifier">getPeriod</span></code>,
|
||||
but want the two dates to be named: <code class="computeroutput"><span class="identifier">begin</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">end</span></code>. We want to implement
|
||||
something like 'multiple return values':
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">Date</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">;</span> <span class="comment">// Error: no default ctor!</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
The second line works already, this is the capability of <a href="../../../../../tuple/index.html" target="_top">Boost.Tuple</a>
|
||||
library, but the first line won't work. We could set some invented initial
|
||||
dates, but it is confusing and may be an unacceptable cost, given that these
|
||||
values will be overwritten in the next line anyway. This is where <code class="computeroutput"><span class="identifier">optional</span></code> can help:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">Date</span><span class="special">></span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
It works because inside <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span></code> a
|
||||
move-assignment from <code class="computeroutput"><span class="identifier">T</span></code> is
|
||||
invoked on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>,
|
||||
which internally calls a move-constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
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</div></td>
|
||||
</tr></table>
|
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|
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<div class="titlepage"><div><div><h3 class="title">
|
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<a name="boost_optional.quick_start.optional_automatic_variables"></a><a class="link" href="optional_automatic_variables.html" title="Optional automatic variables">Optional
|
||||
automatic variables</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
We could write function <code class="computeroutput"><span class="identifier">convert</span></code>
|
||||
in a slightly different manner, so that it has a single <code class="computeroutput"><span class="keyword">return</span></code>-statement:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">ans</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span>
|
||||
<span class="identifier">ans</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">return</span> <span class="identifier">ans</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The default constructor of <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
creates an unitialized optional object. Unlike with <code class="computeroutput"><span class="keyword">int</span></code>s
|
||||
you cannot have an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>
|
||||
in an indeterminate state. Its state is always well defined. Instruction
|
||||
<code class="computeroutput"><span class="identifier">ans</span> <span class="special">=</span>
|
||||
<span class="identifier">i</span></code> initializes the optional object.
|
||||
It uses the 'mixed' assignment from <code class="computeroutput"><span class="keyword">int</span></code>.
|
||||
In general, for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>,
|
||||
when an assignment from <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is invoked, it can do two things. If the optional object is not initialized
|
||||
(our case here), it initializes the contained value using <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
copy constructor. If the optional object is already initialized, it assigns
|
||||
the new value to it using <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
copy assignment.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
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<div class="titlepage"><div><div><h3 class="title">
|
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<a name="boost_optional.quick_start.optional_data_members"></a><a class="link" href="optional_data_members.html" title="Optional data members">Optional
|
||||
data members</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose we want to implement a <span class="emphasis"><em>lazy load</em></span> optimization.
|
||||
This is because we do not want to perform an expensive initialization of
|
||||
our <code class="computeroutput"><span class="identifier">Resource</span></code> until (if at
|
||||
all) it is really used. We can do it this way:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Widget</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">mutable</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">Resource</span><span class="special">></span> <span class="identifier">resource_</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
<span class="identifier">Widget</span><span class="special">()</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">const</span> <span class="identifier">Resource</span><span class="special">&</span> <span class="identifier">getResource</span><span class="special">()</span> <span class="keyword">const</span> <span class="comment">// not thread-safe</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">resource_</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">)</span>
|
||||
<span class="identifier">resource_</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"resource"</span><span class="special">,</span> <span class="string">"arguments"</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">return</span> <span class="special">*</span><span class="identifier">resource_</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code>'s default constructor
|
||||
creates an uninitialized optional. No call to <code class="computeroutput"><span class="identifier">Resource</span></code>'s
|
||||
default constructor is attempted. <code class="computeroutput"><span class="identifier">Resource</span></code>
|
||||
doesn't have to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a>. In function
|
||||
<code class="computeroutput"><span class="identifier">getResource</span></code> we first check
|
||||
if <code class="computeroutput"><span class="identifier">resource_</span></code> is initialized.
|
||||
This time we do not use the contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>,
|
||||
but a comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>.
|
||||
These two ways are equivalent. Function <code class="computeroutput"><span class="identifier">emplace</span></code>
|
||||
initializes the optional in-place by perfect-forwarding the arguments to
|
||||
the constructor of <code class="computeroutput"><span class="identifier">Resource</span></code>.
|
||||
No copy- or move-construction is involved here. <code class="computeroutput"><span class="identifier">Resource</span></code>
|
||||
doesn't even have to be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</p>
|
||||
<div class="note"><table border="0" summary="Note">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
|
||||
<th align="left">Note</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
Function <code class="computeroutput"><span class="identifier">emplace</span></code> is only
|
||||
available on compilers that support rvalue references and variadic templates.
|
||||
If your compiler does not support these features and you still need to
|
||||
avoid any move-constructions, use <a class="link" href="../tutorial/in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="optional_automatic_variables.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="bypassing_unnecessary_default_construction.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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||||
@ -1,135 +0,0 @@
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||||
<html>
|
||||
<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<a accesskey="p" href="../quick_start.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="optional_automatic_variables.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.quick_start.optional_return_values"></a><a class="link" href="optional_return_values.html" title="Optional return values">Optional
|
||||
return values</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Let's write and use a converter function that converts an a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
|
||||
to an <code class="computeroutput"><span class="keyword">int</span></code>. It is possible that
|
||||
for a given string (e.g. <code class="computeroutput"><span class="string">"cat"</span></code>)
|
||||
there exist no value of type <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
capable of representing the conversion result. We do not consider such situation
|
||||
an error. We expect that the converter can be used only to check if the conversion
|
||||
is possible. A natural signature for this function can be:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
All necessary functionality can be included with one header <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
The above function signature means that the function can either return a
|
||||
value of type <code class="computeroutput"><span class="keyword">int</span></code> or a flag
|
||||
indicating that no value of <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
is available. This does not indicate an error. It is like one additional
|
||||
value of <code class="computeroutput"><span class="keyword">int</span></code>. This is how we
|
||||
can use our function:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span> <span class="special">=</span> <span class="comment">/*... */</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span> <span class="comment">// move-construct</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oi</span><span class="special">)</span> <span class="comment">// contextual conversion to bool</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span> <span class="comment">// operator*</span>
|
||||
</pre>
|
||||
<p>
|
||||
In order to test if <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
contains a value, we use the contextual conversion to type <code class="computeroutput"><span class="keyword">bool</span></code>. Because of this we can combine the initialization
|
||||
of the optional object and the test into one instruction:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">))</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
We extract the contained value with <code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code> (and with <code class="computeroutput"><span class="keyword">operator</span><span class="special">-></span></code> where it makes sense). An attempt to
|
||||
extract the contained value of an uninitialized optional object is an <span class="emphasis"><em>undefined
|
||||
behaviour</em></span> (UB). This implementation guards the call with <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span></code>. Therefore you should be sure
|
||||
that the contained value is there before extracting. For instance, the following
|
||||
code is reasonably UB-safe:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">convert</span><span class="special">(</span><span class="string">"100"</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This is because we know that string value <code class="computeroutput"><span class="string">"100"</span></code>
|
||||
converts to a valid value of <code class="computeroutput"><span class="keyword">int</span></code>.
|
||||
If you do not like this potential UB, you can use an alternative way of extracting
|
||||
the contained value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span>
|
||||
<span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_optional_access</span><span class="special">&)</span> <span class="special">{</span>
|
||||
<span class="comment">// deal with it</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This version throws an exception upon an attempt to access a non-existent
|
||||
contained value. If your way of dealing with the missing value is to use
|
||||
some default, like <code class="computeroutput"><span class="number">0</span></code>, there exists
|
||||
a yet another alternative:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This uses the <code class="computeroutput"><span class="identifier">atoi</span></code>-like approach
|
||||
to conversions: if <code class="computeroutput"><span class="identifier">text</span></code> does
|
||||
not represent an integral number just return <code class="computeroutput"><span class="number">0</span></code>.
|
||||
Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code>
|
||||
can be implemented.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span>
|
||||
<span class="keyword">return</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Observe the two return statements. <code class="computeroutput"><span class="keyword">return</span>
|
||||
<span class="identifier">i</span></code> uses the converting constructor
|
||||
that can create <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
from <code class="computeroutput"><span class="identifier">T</span></code>. Thus constructed
|
||||
optional object is initialized and its value is a copy of <code class="computeroutput"><span class="identifier">i</span></code>.
|
||||
The other return statement uses another converting constructor from a special
|
||||
tag <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>. It is used to indicate that we want
|
||||
to create an uninitialized optional object.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="../quick_start.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="optional_automatic_variables.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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@ -1,64 +0,0 @@
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||||
<html>
|
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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|
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<hr>
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|
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<a accesskey="p" href="bypassing_unnecessary_default_construction.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.quick_start.storage_in_containers"></a><a class="link" href="storage_in_containers.html" title="Storage in containers">Storage
|
||||
in containers</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose you want to ask users to choose some number (an <code class="computeroutput"><span class="keyword">int</span></code>).
|
||||
One of the valid responses is to choose nothing, which is represented by
|
||||
an uninitialized <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>.
|
||||
You want to make a histogram showing how many times each choice was made.
|
||||
You can use an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>,</span> <span class="keyword">int</span><span class="special">></span> <span class="identifier">choices</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">for</span> <span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">LIMIT</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">readChoice</span><span class="special">();</span>
|
||||
<span class="special">++</span><span class="identifier">choices</span><span class="special">[</span><span class="identifier">choice</span><span class="special">];</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This works because <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. In this case
|
||||
the state of being uninitialized is treated as a yet another value of <code class="computeroutput"><span class="identifier">T</span></code>, which is compared less than any value
|
||||
of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"></td>
|
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
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</p>
|
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</div></td>
|
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|
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<div class="section" title="Rebinding semantics for assignment of optional references">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references"></a><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">Rebinding
|
||||
semantics for assignment of optional references</a>
|
||||
@ -36,23 +36,23 @@
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'
|
||||
</span><span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'
|
||||
</span><span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If you assign to a bare C++ reference, the assignment is forwarded to the referenced
|
||||
object; its value changes but the reference is never rebound.
|
||||
object; it's value changes but the reference is never rebound.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span>
|
||||
<span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'
|
||||
</span><span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span>
|
||||
<span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'</span>
|
||||
</pre>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'
|
||||
</span></pre>
|
||||
<p>
|
||||
Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>,
|
||||
the effect is to <span class="bold"><strong>rebind</strong></span> to the new object
|
||||
@ -64,14 +64,14 @@
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'
|
||||
</span><span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')
|
||||
</span><span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span>
|
||||
</pre>
|
||||
<h4>
|
||||
<a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a>
|
||||
<a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale"></a><h4>
|
||||
<a name="id579234"></a>
|
||||
<a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a>
|
||||
</h4>
|
||||
<p>
|
||||
Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span>
|
||||
@ -117,16 +117,16 @@
|
||||
or not.
|
||||
</p>
|
||||
<p>
|
||||
That is, you would have to discriminate in order to be consistent.
|
||||
That is, you would have to discriminate in order to be consistency.
|
||||
</p>
|
||||
<p>
|
||||
If in your code rebinding to another object is not an option, then it is very
|
||||
likely that binding for the first time isn't either. In such case, assignment
|
||||
If in your code rebinding to another object is not an option, then is very
|
||||
likely that binding for the fist time isn't either. In such case, assignment
|
||||
to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
shall be prohibited. It is quite possible that in such a scenario it is a precondition
|
||||
that the lvalue must be already initialized. If it isn't, then binding for
|
||||
the first time is OK while rebinding is not which is IMO very unlikely. In
|
||||
such a scenario, you can assign the value itself directly, as in:
|
||||
shall be prohibited. It is quite possible that in such scenario the precondition
|
||||
that the lvalue must be already initialized exist. If it doesn't, then binding
|
||||
for the first time is OK while rebinding is not which is IMO very unlikely.
|
||||
In such scenario, you can assign the value itself directly, as in:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span>
|
||||
<span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span>
|
||||
@ -134,7 +134,7 @@
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -142,7 +142,7 @@
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|
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</h4></div></div></div>
|
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<p>
|
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</p>
|
||||
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">bad_optional_access</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">logic_error</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
<span class="identifier">bad_optional_access</span><span class="special">();</span> <a class="link" href="header__boost_optional_bad_optional_access_hpp_/detailed_semantics.html#reference_bad_optional_access_constructor"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
</div>
|
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
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</p>
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</div></td>
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<div class="spirit-nav">
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<a accesskey="p" href="../../optional/reference.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="header__boost_optional_bad_optional_access_hpp_/detailed_semantics.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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@ -1,60 +0,0 @@
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<html>
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<a name="reference_bad_optional_access_constructor"></a><code class="computeroutput"><span class="identifier">bad_optional_access</span><span class="special">();</span></code>
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<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="identifier">what</span><span class="special">()</span></code> returns an implementation-defined
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||||
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">optional</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">swap</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">optional_swap_should_use_default_constructor</span> <span class="special">;</span>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
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</p>
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This header only contains declarations.
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
|
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<a name="boost_optional.reference.io_header.io_synop"></a><a class="link" href="io_header.html#boost_optional.reference.io_header.io_synop" title="Synopsis">Synopsis</a>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">istream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">ostream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">out</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span><span class="special">);</span> <a class="link" href="io_header/io_semantics.html#reference_operator_ostream"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">></span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">out</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="keyword">const</span><span class="special">&);</span> <a class="link" href="io_header/io_semantics.html#reference_operator_ostream_none"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="keyword">operator</span><span class="special">>>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">in</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">v</span><span class="special">);</span> <a class="link" href="io_header/io_semantics.html#reference_operator_istream"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
</div>
|
||||
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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semantics</a>
|
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</h4></div></div></div>
|
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<p>
|
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<a name="reference_operator_ostream"></a><code class="computeroutput"><span class="keyword">template</span>
|
||||
<span class="special"><</span><span class="keyword">class</span>
|
||||
<span class="identifier">CharType</span><span class="special">,</span>
|
||||
<span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span></code>
|
||||
<br>     <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span></code> <br>     <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="identifier">out</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span><span class="special">);</span></code>
|
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</p>
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||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effect:</strong></span> Outputs an implementation-defined
|
||||
string. The output contains the information about whether the optional
|
||||
object contains a value or not. If <code class="computeroutput"><span class="identifier">v</span></code>
|
||||
contains a value, the output contains result of calling <code class="computeroutput"><span class="identifier">out</span> <span class="special"><<</span>
|
||||
<span class="special">*</span><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">out</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span> <a name="reference_operator_ostream_none"></a>
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span>
|
||||
<span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span></code> <br>     <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span></code>
|
||||
<br>     <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">out</span><span class="special">,</span> <span class="identifier">none_t</span><span class="special">);</span></code>
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effect:</strong></span> Outputs an implementation-defined
|
||||
string.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">out</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span> <a name="reference_operator_istream"></a>
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span>
|
||||
<span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span></code> <br>     <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span></code>
|
||||
<br>     <code class="computeroutput"><span class="keyword">operator</span><span class="special">>>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">in</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">v</span><span class="special">);</span></code>
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a> and
|
||||
<code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effect:</strong></span> Reads the value of optional
|
||||
object from <code class="computeroutput"><span class="identifier">in</span></code>. If
|
||||
the string representation indicates that the optional object should
|
||||
contain a value, <code class="computeroutput"><span class="identifier">v</span></code>
|
||||
contains a value and its contained value is obtained as if by default-constructing
|
||||
an object <code class="computeroutput"><span class="identifier">o</span></code> of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> and then calling
|
||||
<code class="computeroutput"><span class="identifier">in</span> <span class="special">>></span>
|
||||
<span class="identifier">o</span></code>; otherwise <code class="computeroutput"><span class="identifier">v</span></code> does not contain a value, and the
|
||||
previously contained value (if any) has been destroyed.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">out</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
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<a name="boost_optional.relnotes"></a><a class="link" href="relnotes.html" title="Release Notes">Release Notes</a>
|
||||
</h2></div></div></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_60"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_60">Boost
|
||||
Release 1.60</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Changed the implementation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
|
||||
again. Now it is a const object with internal linkage (as any other tag).
|
||||
This fixes <a href="https://svn.boost.org/trac/boost/ticket/11203" target="_top">Trac
|
||||
#11203</a>.
|
||||
</li></ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_59"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_59">Boost
|
||||
Release 1.59</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
For C++03 compilers, added 0-argument overload for member function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>,
|
||||
and therewith removed the dependency on <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">in_place_factory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/11241" target="_top">Trac #11241</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h2"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_58"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_58">Boost
|
||||
Release 1.58</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none_t</span></code> is no longer convertible from
|
||||
literal <code class="computeroutput"><span class="number">0</span></code>. This avoids a bug
|
||||
where <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">rational</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span></code> would
|
||||
initialize an optional object with no contained value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Improved the trick that prevents streaming out <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
without header <code class="computeroutput"><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span></code>
|
||||
by using safe-bool idiom. This addresses <a href="https://svn.boost.org/trac/boost/ticket/10825" target="_top">Trac
|
||||
#10825</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
IOStream operators are now mentioned in documentation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added a way to manually disable move semantics: just define macro <code class="computeroutput"><span class="identifier">BOOST_OPTIONAL_CONFIG_NO_RVALUE_REFERENCES</span></code>.
|
||||
This can be used to work around <a href="https://svn.boost.org/trac/boost/ticket/10399" target="_top">Trac
|
||||
#10399</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
It is no longer possible to assign <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is not assignable or convertible to <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
(<a href="https://svn.boost.org/trac/boost/ticket/11087" target="_top">Trac #11087</a>).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value accessors now work correctly on rvalues of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> (<a href="https://svn.boost.org/trac/boost/ticket/10839" target="_top">Trac
|
||||
#10839</a>).
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h3"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_57"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_57">Boost
|
||||
Release 1.57</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<a href="https://github.com/boostorg/optional/pull/9" target="_top">Git pull #9</a>:
|
||||
<span class="emphasis"><em>"Supply <code class="computeroutput"><span class="special"><</span><span class="identifier">string</span><span class="special">></span></code>
|
||||
to fix C++03 compile error on <code class="computeroutput"><span class="identifier">logic_error</span><span class="special">(</span><span class="string">"..."</span><span class="special">)</span></code>"</em></span>.
|
||||
</li></ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h4"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_56"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_56">Boost
|
||||
Release 1.56</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Added support for rvalue references. Now <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> works with moveable but non-copyable
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>'s,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Improved <code class="computeroutput"><span class="identifier">swap</span></code> (now uses
|
||||
move operations),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>. This is the last of the requests from
|
||||
<a href="https://svn.boost.org/trac/boost/ticket/1841" target="_top">Trac #1841</a>,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> is moveable, including
|
||||
conditional <code class="computeroutput"><span class="keyword">noexcept</span></code> specifications,
|
||||
which make it <code class="computeroutput"><span class="identifier">move_if_noexcept</span></code>-friendly,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Using explicit operator bool() on platforms that support it (<a href="https://svn.boost.org/trac/boost/ticket/4227" target="_top">Trac
|
||||
#4227</a>) (breaking change),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Forward declaration of <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">ostream</span><span class="special">&,</span> <span class="identifier">optional</span>
|
||||
<span class="keyword">const</span><span class="special">&)</span></code>
|
||||
to prevent inadvertent incorrect serialization of optional objects,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Removed deprecated function <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code> from examples (<a href="https://svn.boost.org/trac/boost/ticket/9005" target="_top">Trac
|
||||
#9005</a>),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Equality comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
|
||||
does not require that <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
be EqualityComparable,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Optional rvalue references are explicitly disallowed,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Binding temporaries to optional references is explicitly disallowed (breaking
|
||||
change),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
More ways to access the contained value, functions <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">value_or</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span><span class="special">()</span></code>,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Updated and reorganized documentation, added tutorial and quick guide sections.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="section" title="Synopsis">
|
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.synopsis"></a><a class="link" href="synopsis.html" title="Synopsis">Synopsis</a>
|
||||
</h2></div></div></div>
|
||||
<pre class="programlisting"><code class="computeroutput"><span class="comment">// In Header: <</span></code><a href="../../../../../boost/optional/optional.hpp" target="_top">boost/optional/optional.hpp</a><span class="comment">></span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span> <span class="special">:</span>
|
||||
|
||||
<span class="comment">// (If T is of reference type, the parameters and results by reference are by value)</span>
|
||||
<span class="comment">// (If T is of reference type, the parameters and results by reference are by value)
|
||||
</span>
|
||||
<span class="identifier">optional</span> <span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_none_t"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_none_t"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_move_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// [new in 1.34]</span>
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_bool_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="comment">// [new in 1.34]
|
||||
</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_bool_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_move_constructor_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_move_constructor_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_none_t"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_move_equal_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_move_equal_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_move_equal_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span> <span class="special">(</span> <span class="identifier">Args</span><span class="special">...&&</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_emplace"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// [new in 1.34]
|
||||
</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&&;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value_or"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value_or"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">explicit</span> <span class="keyword">operator</span> <span class="keyword">bool</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_bool"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">operator</span> <span class="identifier">unspecified</span><span class="special">-</span><span class="keyword">bool</span><span class="special">-</span><span class="identifier">type</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_bool"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_not"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_not"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// deprecated methods</span>
|
||||
<span class="comment">// deprecated methods
|
||||
</span>
|
||||
<span class="comment">// (deprecated)
|
||||
</span> <span class="keyword">void</span> <span class="identifier">reset</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_reset"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_reset"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="comment">// (deprecated)
|
||||
</span> <span class="keyword">void</span> <span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_reset_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_reset_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="comment">// (deprecated)
|
||||
</span> <span class="keyword">bool</span> <span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_is_initialized"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_is_initialized"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_equal_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
@ -131,15 +108,14 @@
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">>=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_greater_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_equal_optional_none"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="comment">// [new in 1.34]
|
||||
</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_make_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_not_equal_optional_none"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="comment">// [new in 1.34]
|
||||
</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_make_optional_bool_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_make_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_make_optional_bool_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_optional_value_or</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="comment">// [new in 1.34]
|
||||
</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_optional_value_or</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
@ -155,12 +131,12 @@
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_swap_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
<span class="special">}</span> <span class="comment">// namespace boost
|
||||
</span></pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -168,7 +144,7 @@
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
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<a accesskey="p" href="tutorial/type_requirements.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="detailed_semantics.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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</html>
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|
||||
@ -1,523 +0,0 @@
|
||||
<html>
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<td align="center"><a href="../../../../../more/index.htm">More</a></td>
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</tr></table>
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<hr>
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<a accesskey="p" href="quick_start.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="synopsis.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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||||
<a name="boost_optional.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview">Design Overview</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.motivation"></a><a class="link" href="tutorial.html#boost_optional.tutorial.motivation" title="Motivation">Motivation</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not
|
||||
have a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or
|
||||
uses a runtime check and throws an exception if the postcondition is violated.
|
||||
This is a reasonable choice for example, for function (A), because the lack
|
||||
of a proper return value is directly related to an invalid parameter (out
|
||||
of domain argument), so it is appropriate to require the callee to supply
|
||||
only parameters in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail
|
||||
just because it can't find a value to return; so it is incorrect to consider
|
||||
such a situation an error and assert or throw an exception. This function
|
||||
must return, and somehow, must tell the callee that it is not returning a
|
||||
meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error
|
||||
to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside
|
||||
itself, but in many applications, it is just impractical for performance
|
||||
reasons to treat this as an error (because detecting that the polygon has
|
||||
no area might be too expensive to be required to be tested previously), and
|
||||
either an arbitrary point (typically at infinity) is returned, or some efficient
|
||||
way to tell the callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved
|
||||
to communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>,
|
||||
points at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when
|
||||
such values do not exist. In these cases, the usual alternative is either
|
||||
to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound
|
||||
type, such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the
|
||||
result which indicates if the result is meaningful, has the advantage that
|
||||
can be turned into a consistent idiom since the first element of the pair
|
||||
can be whatever the function would conceptually return. For example, the
|
||||
last two functions could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error
|
||||
prone since the user can easily use the function result (first element of
|
||||
the pair) without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview" title="Design Overview">Design Overview</a>
|
||||
</h3></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview.the_models">The
|
||||
models</a></span></dt>
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview.the_semantics">The
|
||||
semantics</a></span></dt>
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview.the_interface">The
|
||||
Interface</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_models"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_models" title="The models">The
|
||||
models</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this
|
||||
variable an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
|
||||
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
|
||||
value is given), it is said that the object has been initialized. If the
|
||||
declaration uses an empty initializer (no initial value is given), and
|
||||
neither default nor value initialization applies, it is said that the object
|
||||
is <span class="bold"><strong>uninitialized</strong></span>. Its actual value exist
|
||||
but has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11).
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
intends to formalize the notion of initialization (or lack of it) allowing
|
||||
a program to test whether an object has been initialized and stating that
|
||||
access to the value of an uninitialized object is undefined behavior. That
|
||||
is, when a variable is declared as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and no initial value is given, the
|
||||
variable is <span class="emphasis"><em>formally</em></span> uninitialized. A formally uninitialized
|
||||
optional object has conceptually no value at all and this situation can
|
||||
be tested at runtime. It is formally <span class="emphasis"><em>undefined behavior</em></span>
|
||||
to try to access the value of an uninitialized optional. An uninitialized
|
||||
optional can be assigned a value, in which case its initialization state
|
||||
changes to initialized. Furthermore, given the formal treatment of initialization
|
||||
states in optional objects, it is even possible to reset an optional to
|
||||
<span class="emphasis"><em>uninitialized</em></span>.
|
||||
</p>
|
||||
<p>
|
||||
In C++ there is no formal notion of uninitialized objects, which means
|
||||
that objects always have an initial value even if indeterminate. As discussed
|
||||
on the previous section, this has a drawback because you need additional
|
||||
information to tell if an object has been effectively initialized. One
|
||||
of the typical ways in which this has been historically dealt with is via
|
||||
a special value: <code class="computeroutput"><span class="identifier">EOF</span></code>,
|
||||
<code class="computeroutput"><span class="identifier">npos</span></code>, -1, etc... This is
|
||||
equivalent to adding the special value to the set of possible values of
|
||||
a given type. This super set of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus some <span class="emphasis"><em>nil_t</em></span>—where <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD—can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
|
||||
unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a
|
||||
<span class="emphasis"><em>current type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code> or <code class="computeroutput"><span class="identifier">nil_t</span></code>.
|
||||
Using the <a href="../../../../variant/index.html" target="_top">Boost.Variant</a>
|
||||
library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>. There is precedent for a discriminated
|
||||
union as a model for an optional value: the <a href="http://www.haskell.org/" target="_top">Haskell</a>
|
||||
<span class="bold"><strong>Maybe</strong></span> built-in type constructor. Thus,
|
||||
a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
|
||||
serves as a conceptual foundation.
|
||||
</p>
|
||||
<p>
|
||||
A <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code> follows naturally from the traditional
|
||||
idiom of extending the range of possible values adding an additional sentinel
|
||||
value with the special meaning of <span class="emphasis"><em>Nothing</em></span>. However,
|
||||
this additional <span class="emphasis"><em>Nothing</em></span> value is largely irrelevant
|
||||
for our purpose since our goal is to formalize the notion of uninitialized
|
||||
objects and, while a special extended value can be used to convey that
|
||||
meaning, it is not strictly necessary in order to do so.
|
||||
</p>
|
||||
<p>
|
||||
The observation made in the last paragraph about the irrelevant nature
|
||||
of the additional <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
with respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
suggests an alternative model: a <span class="emphasis"><em>container</em></span> that either
|
||||
has a value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
|
||||
</p>
|
||||
<p>
|
||||
As of this writing I don't know of any precedent for a variable-size fixed-capacity
|
||||
(of 1) stack-based container model for optional values, yet I believe this
|
||||
is the consequence of the lack of practical implementations of such a container
|
||||
rather than an inherent shortcoming of the container model.
|
||||
</p>
|
||||
<p>
|
||||
In any event, both the discriminated-union or the single-element container
|
||||
models serve as a conceptual ground for a class representing optional—i.e.
|
||||
possibly uninitialized—objects. For instance, these models show the
|
||||
<span class="emphasis"><em>exact</em></span> semantics required for a wrapper of optional
|
||||
values:
|
||||
</p>
|
||||
<p>
|
||||
Discriminated-union:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
variant implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Single-element container:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
container implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the container is empty models testing if the optional is
|
||||
initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from an empty container models the undefined behavior of trying to
|
||||
access the value of an uninitialized optional
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_semantics"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_semantics" title="The semantics">The
|
||||
semantics</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> are intended to be used in places where
|
||||
objects of type <code class="computeroutput"><span class="identifier">T</span></code> would
|
||||
but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s purpose is to formalize the additional
|
||||
possibly uninitialized state. From the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus the additional semantics corresponding to this special state. As such,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
could be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>. Of course, we can't do that in C++,
|
||||
so we need to compose the desired semantics using a different mechanism.
|
||||
Doing it the other way around, that is, making <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a <span class="emphasis"><em>subtype</em></span> of
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> is not only conceptually
|
||||
wrong but also impractical: it is not allowed to derive from a non-class
|
||||
type, such as a built-in type.
|
||||
</p>
|
||||
<p>
|
||||
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> the required basic semantics:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce
|
||||
a formally uninitialized wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
|
||||
To introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain
|
||||
a new yet equivalent wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of
|
||||
some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To
|
||||
assign to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
|
||||
initialize the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by
|
||||
the type T):</strong></span> To compare wrapped object values taking into
|
||||
account the presence of uninitialized states.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects.
|
||||
(with whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s swap).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the
|
||||
wrapped object (if any) and leave the wrapper in the uninitialized
|
||||
state.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Additional operations are useful, such as converting constructors and converting
|
||||
assignments, in-place construction and assignment, and safe value access
|
||||
via a pointer to the wrapped object or null.
|
||||
</p>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_interface" title="The Interface">The
|
||||
Interface</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Since the purpose of optional is to allow us to use objects with a formal
|
||||
uninitialized additional state, the interface could try to follow the interface
|
||||
of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
|
||||
as much as possible. In order to choose the proper degree of adoption of
|
||||
the native <code class="computeroutput"><span class="identifier">T</span></code> interface,
|
||||
the following must be noted: Even if all the operations supported by an
|
||||
instance of type <code class="computeroutput"><span class="identifier">T</span></code> are
|
||||
defined for the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
extends such a set of values with a new value for which most (otherwise
|
||||
valid) operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> itself is merely a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
wrapper (modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
|
||||
any attempt to define such operations upon uninitialized optionals will
|
||||
be totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
interface only for those operations which are well defined (w.r.t the type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
|
||||
are uninitialized. These operations include: construction, copy-construction,
|
||||
assignment, swap and relational operations.
|
||||
</p>
|
||||
<p>
|
||||
For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized,
|
||||
a different interface is chosen (which will be explained next).
|
||||
</p>
|
||||
<p>
|
||||
Also, the presence of the possibly uninitialized state requires additional
|
||||
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself which are supported by a special interface.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
Value Access in the presence of possibly untitialized optional objects:
|
||||
The operators * and -></a>
|
||||
</h6>
|
||||
<p>
|
||||
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
|
||||
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value
|
||||
which is used to indicate that the pointer is not referring to any object
|
||||
at all. In other words, null pointer values convey the notion of nonexistent
|
||||
objects.
|
||||
</p>
|
||||
<p>
|
||||
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
|
||||
facto</em></span> standard for handling optional objects because all you
|
||||
have to do to refer to a value which you don't really have is to use a
|
||||
null pointer value of the appropriate type. Pointers have been used for
|
||||
decades—from the days of C APIs to modern C++ libraries—to <span class="emphasis"><em>refer</em></span>
|
||||
to optional (that is, possibly nonexistent) objects; particularly as optional
|
||||
arguments to a function, but also quite often as optional data members.
|
||||
</p>
|
||||
<p>
|
||||
The possible presence of a null pointer value makes the operations that
|
||||
access the pointee's value possibly undefined, therefore, expressions which
|
||||
use dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="special">*</span><span class="identifier">p</span>
|
||||
<span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code> and <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="identifier">p</span><span class="special">-></span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly convey the notion of optionality,
|
||||
and this information is tied to the <span class="emphasis"><em>syntax</em></span> of the
|
||||
expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> tell by themselves
|
||||
—without any additional context— that the expression will be undefined
|
||||
unless the implied pointee actually exist.
|
||||
</p>
|
||||
<p>
|
||||
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
|
||||
can be formalized in the form of a concept: the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
|
||||
concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-></span></code>
|
||||
and contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
to convey the notion of optionality.
|
||||
</p>
|
||||
<p>
|
||||
However, pointers are good to <span class="underline">refer</span>
|
||||
to optional objects, but not particularly good to handle the optional objects
|
||||
in all other respects, such as initializing or moving/copying them. The
|
||||
problem resides in the shallow-copy of pointer semantics: if you need to
|
||||
effectively move or copy the object, pointers alone are not enough. The
|
||||
problem is that copies of pointers do not imply copies of pointees. For
|
||||
example, as was discussed in the motivation, pointers alone cannot be used
|
||||
to return optional objects from a function because the object must move
|
||||
outside from the function and into the caller's context.
|
||||
</p>
|
||||
<p>
|
||||
A solution to the shallow-copy problem that is often used is to resort
|
||||
to dynamic allocation and use a smart pointer to automatically handle the
|
||||
details of this. For example, if a function is to optionally return an
|
||||
object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code>
|
||||
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
|
||||
is a built-in or small POD, this technique is very poor in terms of required
|
||||
resources. Optional objects are essentially values so it is very convenient
|
||||
to be able to use automatic storage and deep-copy semantics to manipulate
|
||||
optional values just as we do with ordinary values. Pointers do not have
|
||||
this semantics, so are inappropriate for the initialization and transport
|
||||
of optional values, yet are quite convenient for handling the access to
|
||||
the possible undefined value because of the idiomatic aid present in the
|
||||
<a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
|
||||
concept incarnated by pointers.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
as a model of OptionalPointee</a>
|
||||
</h6>
|
||||
<p>
|
||||
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> to lexically warn
|
||||
about the possibly uninitialized state appealing to the familiar pointer
|
||||
semantics w.r.t. to null pointers.
|
||||
</p>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> is not, and does not model, a
|
||||
pointer</span>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<p>
|
||||
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> does not have shallow-copy so does
|
||||
not alias: two different optionals never refer to the <span class="emphasis"><em>same</em></span>
|
||||
value unless <code class="computeroutput"><span class="identifier">T</span></code> itself is
|
||||
a reference (but may have <span class="emphasis"><em>equivalent</em></span> values). The
|
||||
difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and a pointer must be kept in mind,
|
||||
particularly because the semantics of relational operators are different:
|
||||
since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is a value-wrapper, relational operators are deep: they compare optional
|
||||
values; but relational operators for pointers are shallow: they do not
|
||||
compare pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
|
||||
on some situations but not always. Specifically, on generic code written
|
||||
for both, you cannot use relational operators directly, and must use the
|
||||
template functions <a href="../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
and <a href="../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead.
|
||||
</p>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="quick_start.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="synopsis.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -1,108 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>A note about optional<bool></title>
|
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<a accesskey="p" href="in_place_factories.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="exception_safety_guarantees.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
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||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.a_note_about_optional_bool_"></a><a class="link" href="a_note_about_optional_bool_.html" title="A note about optional<bool>">A
|
||||
note about optional<bool></a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> should
|
||||
be used with special caution and consideration.
|
||||
</p>
|
||||
<p>
|
||||
First, it is functionally similar to a tristate boolean (false, maybe, true)
|
||||
—such as <a href="../../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>—
|
||||
except that in a tristate boolean, the maybe state <span class="underline">represents
|
||||
a valid value</span>, unlike the corresponding state of an uninitialized
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
It should be carefully considered if an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>
|
||||
instead of a <code class="computeroutput"><span class="identifier">tribool</span></code> is really
|
||||
needed.
|
||||
</p>
|
||||
<p>
|
||||
Second, although <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> provides a contextual conversion
|
||||
to <code class="computeroutput"><span class="keyword">bool</span></code> in C++11, this falls
|
||||
back to an implicit conversion on older compilers. This conversion refers
|
||||
to the initialization state and not to the contained value. Using <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> can
|
||||
lead to subtle errors due to the implicit <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
conversion:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="keyword">void</span> <span class="identifier">bar</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="keyword">try</span><span class="special">();</span>
|
||||
|
||||
<span class="comment">// The following intended to pass the value of 'v' to foo():</span>
|
||||
<span class="identifier">foo</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="comment">// But instead, the initialization state is passed</span>
|
||||
<span class="comment">// due to a typo: it should have been foo(*v).</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The only implicit conversion is to <code class="computeroutput"><span class="keyword">bool</span></code>,
|
||||
and it is safe in the sense that typical integral promotions don't apply
|
||||
(i.e. if <code class="computeroutput"><span class="identifier">foo</span><span class="special">()</span></code>
|
||||
takes an <code class="computeroutput"><span class="keyword">int</span></code> instead, it won't
|
||||
compile).
|
||||
</p>
|
||||
<p>
|
||||
Third, mixed comparisons with <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
work differently than similar mixed comparisons between pointers and <code class="computeroutput"><span class="keyword">bool</span></code>, so the results might surprise you:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">oEmpty</span><span class="special">(</span><span class="identifier">none</span><span class="special">),</span> <span class="identifier">oTrue</span><span class="special">(</span><span class="keyword">true</span><span class="special">),</span> <span class="identifier">oFalse</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders true</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false!</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false!</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders true!</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders true</span>
|
||||
</pre>
|
||||
<p>
|
||||
In other words, for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>, the following assertion does not
|
||||
hold:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">((</span><span class="identifier">opt</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">)</span> <span class="special">==</span> <span class="special">(!</span><span class="identifier">opt</span><span class="special">));</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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|
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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</div>
|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview"></a><a class="link" href="design_overview.html" title="Design Overview">Design Overview</a>
|
||||
</h3></div></div></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_models"></a><a class="link" href="design_overview.html#boost_optional.tutorial.design_overview.the_models" title="The models">The
|
||||
models</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this
|
||||
variable an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
|
||||
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
|
||||
value is given), it is said that the object has been initialized. If the
|
||||
declaration uses an empty initializer (no initial value is given), and
|
||||
neither default nor value initialization applies, it is said that the object
|
||||
is <span class="bold"><strong>uninitialized</strong></span>. Its actual value exist
|
||||
but has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11).
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
intends to formalize the notion of initialization (or lack of it) allowing
|
||||
a program to test whether an object has been initialized and stating that
|
||||
access to the value of an uninitialized object is undefined behavior. That
|
||||
is, when a variable is declared as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and no initial value is given, the
|
||||
variable is <span class="emphasis"><em>formally</em></span> uninitialized. A formally uninitialized
|
||||
optional object has conceptually no value at all and this situation can
|
||||
be tested at runtime. It is formally <span class="emphasis"><em>undefined behavior</em></span>
|
||||
to try to access the value of an uninitialized optional. An uninitialized
|
||||
optional can be assigned a value, in which case its initialization state
|
||||
changes to initialized. Furthermore, given the formal treatment of initialization
|
||||
states in optional objects, it is even possible to reset an optional to
|
||||
<span class="emphasis"><em>uninitialized</em></span>.
|
||||
</p>
|
||||
<p>
|
||||
In C++ there is no formal notion of uninitialized objects, which means
|
||||
that objects always have an initial value even if indeterminate. As discussed
|
||||
on the previous section, this has a drawback because you need additional
|
||||
information to tell if an object has been effectively initialized. One
|
||||
of the typical ways in which this has been historically dealt with is via
|
||||
a special value: <code class="computeroutput"><span class="identifier">EOF</span></code>,
|
||||
<code class="computeroutput"><span class="identifier">npos</span></code>, -1, etc... This is
|
||||
equivalent to adding the special value to the set of possible values of
|
||||
a given type. This super set of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus some <span class="emphasis"><em>nil_t</em></span>—where <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD—can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
|
||||
unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a
|
||||
<span class="emphasis"><em>current type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code> or <code class="computeroutput"><span class="identifier">nil_t</span></code>.
|
||||
Using the <a href="../../../../../variant/index.html" target="_top">Boost.Variant</a>
|
||||
library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>. There is precedent for a discriminated
|
||||
union as a model for an optional value: the <a href="http://www.haskell.org/" target="_top">Haskell</a>
|
||||
<span class="bold"><strong>Maybe</strong></span> built-in type constructor. Thus,
|
||||
a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
|
||||
serves as a conceptual foundation.
|
||||
</p>
|
||||
<p>
|
||||
A <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code> follows naturally from the traditional
|
||||
idiom of extending the range of possible values adding an additional sentinel
|
||||
value with the special meaning of <span class="emphasis"><em>Nothing</em></span>. However,
|
||||
this additional <span class="emphasis"><em>Nothing</em></span> value is largely irrelevant
|
||||
for our purpose since our goal is to formalize the notion of uninitialized
|
||||
objects and, while a special extended value can be used to convey that
|
||||
meaning, it is not strictly necessary in order to do so.
|
||||
</p>
|
||||
<p>
|
||||
The observation made in the last paragraph about the irrelevant nature
|
||||
of the additional <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
with respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
suggests an alternative model: a <span class="emphasis"><em>container</em></span> that either
|
||||
has a value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
|
||||
</p>
|
||||
<p>
|
||||
As of this writing I don't know of any precedent for a variable-size fixed-capacity
|
||||
(of 1) stack-based container model for optional values, yet I believe this
|
||||
is the consequence of the lack of practical implementations of such a container
|
||||
rather than an inherent shortcoming of the container model.
|
||||
</p>
|
||||
<p>
|
||||
In any event, both the discriminated-union or the single-element container
|
||||
models serve as a conceptual ground for a class representing optional—i.e.
|
||||
possibly uninitialized—objects. For instance, these models show the
|
||||
<span class="emphasis"><em>exact</em></span> semantics required for a wrapper of optional
|
||||
values:
|
||||
</p>
|
||||
<p>
|
||||
Discriminated-union:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
variant implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Single-element container:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
container implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the container is empty models testing if the optional is
|
||||
initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from an empty container models the undefined behavior of trying to
|
||||
access the value of an uninitialized optional
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="design_overview/the_semantics.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
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|
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|
||||
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|
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@ -1,187 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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|
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|
||||
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|
||||
<link rel="next" href="../when_to_use_optional.html" title="When to use Optional">
|
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|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td align="center"><a href="../../../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td>
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|
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../../../more/index.htm">More</a></td>
|
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</tr></table>
|
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<hr>
|
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<div class="spirit-nav">
|
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<a accesskey="p" href="the_semantics.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../when_to_use_optional.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface"></a><a class="link" href="the_interface.html" title="The Interface">The
|
||||
Interface</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Since the purpose of optional is to allow us to use objects with a formal
|
||||
uninitialized additional state, the interface could try to follow the interface
|
||||
of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
|
||||
as much as possible. In order to choose the proper degree of adoption of
|
||||
the native <code class="computeroutput"><span class="identifier">T</span></code> interface,
|
||||
the following must be noted: Even if all the operations supported by an
|
||||
instance of type <code class="computeroutput"><span class="identifier">T</span></code> are
|
||||
defined for the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
extends such a set of values with a new value for which most (otherwise
|
||||
valid) operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> itself is merely a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
wrapper (modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
|
||||
any attempt to define such operations upon uninitialized optionals will
|
||||
be totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
interface only for those operations which are well defined (w.r.t the type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
|
||||
are uninitialized. These operations include: construction, copy-construction,
|
||||
assignment, swap and relational operations.
|
||||
</p>
|
||||
<p>
|
||||
For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized,
|
||||
a different interface is chosen (which will be explained next).
|
||||
</p>
|
||||
<p>
|
||||
Also, the presence of the possibly uninitialized state requires additional
|
||||
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself which are supported by a special interface.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="the_interface.html#boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
Value Access in the presence of possibly uninitialized optional objects:
|
||||
The operators * and -></a>
|
||||
</h6>
|
||||
<p>
|
||||
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
|
||||
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value
|
||||
which is used to indicate that the pointer is not referring to any object
|
||||
at all. In other words, null pointer values convey the notion of nonexistent
|
||||
objects.
|
||||
</p>
|
||||
<p>
|
||||
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
|
||||
facto</em></span> standard for handling optional objects because all you
|
||||
have to do to refer to a value which you don't really have is to use a
|
||||
null pointer value of the appropriate type. Pointers have been used for
|
||||
decades—from the days of C APIs to modern C++ libraries—to <span class="emphasis"><em>refer</em></span>
|
||||
to optional (that is, possibly nonexistent) objects; particularly as optional
|
||||
arguments to a function, but also quite often as optional data members.
|
||||
</p>
|
||||
<p>
|
||||
The possible presence of a null pointer value makes the operations that
|
||||
access the pointee's value possibly undefined, therefore, expressions which
|
||||
use dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="special">*</span><span class="identifier">p</span>
|
||||
<span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code> and <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="identifier">p</span><span class="special">-></span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly convey the notion of optionality,
|
||||
and this information is tied to the <span class="emphasis"><em>syntax</em></span> of the
|
||||
expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> tell by themselves
|
||||
—without any additional context— that the expression will be undefined
|
||||
unless the implied pointee actually exist.
|
||||
</p>
|
||||
<p>
|
||||
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
|
||||
can be formalized in the form of a concept: the <a href="../../../../../../utility/OptionalPointee.html" target="_top"><code class="computeroutput"><span class="identifier">OptionalPointee</span></code></a> concept. This
|
||||
concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>,
|
||||
<code class="computeroutput"><span class="special">-></span></code> and contextual conversion
|
||||
to <code class="computeroutput"><span class="keyword">bool</span></code> to convey the notion
|
||||
of optionality.
|
||||
</p>
|
||||
<p>
|
||||
However, pointers are good to <span class="underline">refer</span>
|
||||
to optional objects, but not particularly good to handle the optional objects
|
||||
in all other respects, such as initializing or moving/copying them. The
|
||||
problem resides in the shallow-copy of pointer semantics: if you need to
|
||||
effectively move or copy the object, pointers alone are not enough. The
|
||||
problem is that copies of pointers do not imply copies of pointees. For
|
||||
example, as was discussed in the motivation, pointers alone cannot be used
|
||||
to return optional objects from a function because the object must move
|
||||
outside from the function and into the caller's context.
|
||||
</p>
|
||||
<p>
|
||||
A solution to the shallow-copy problem that is often used is to resort
|
||||
to dynamic allocation and use a smart pointer to automatically handle the
|
||||
details of this. For example, if a function is to optionally return an
|
||||
object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code>
|
||||
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
|
||||
is a built-in or small POD, this technique is very poor in terms of required
|
||||
resources. Optional objects are essentially values so it is very convenient
|
||||
to be able to use automatic storage and deep-copy semantics to manipulate
|
||||
optional values just as we do with ordinary values. Pointers do not have
|
||||
this semantics, so are inappropriate for the initialization and transport
|
||||
of optional values, yet are quite convenient for handling the access to
|
||||
the possible undefined value because of the idiomatic aid present in the
|
||||
<a href="../../../../../../utility/OptionalPointee.html" target="_top"><code class="computeroutput"><span class="identifier">OptionalPointee</span></code></a>
|
||||
concept incarnated by pointers.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="the_interface.html#boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
as a model of OptionalPointee</a>
|
||||
</h6>
|
||||
<p>
|
||||
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> to lexically warn
|
||||
about the possibly uninitialized state appealing to the familiar pointer
|
||||
semantics w.r.t. to null pointers.
|
||||
</p>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../../doc/src/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> is not, and does not model, a
|
||||
pointer</span>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<p>
|
||||
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> does not have shallow-copy so does
|
||||
not alias: two different optionals never refer to the <span class="emphasis"><em>same</em></span>
|
||||
value unless <code class="computeroutput"><span class="identifier">T</span></code> itself is
|
||||
a reference (but may have <span class="emphasis"><em>equivalent</em></span> values). The
|
||||
difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and a pointer must be kept in mind,
|
||||
particularly because the semantics of relational operators are different:
|
||||
since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is a value-wrapper, relational operators are deep: they compare optional
|
||||
values; but relational operators for pointers are shallow: they do not
|
||||
compare pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
|
||||
on some situations but not always. Specifically, on generic code written
|
||||
for both, you cannot use relational operators directly, and must use the
|
||||
template functions <a href="../../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
and <a href="../../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="the_semantics.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../when_to_use_optional.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
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|
||||
</body>
|
||||
</html>
|
||||
@ -1,121 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
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<title>The semantics</title>
|
||||
<link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css">
|
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<link rel="next" href="the_interface.html" title="The Interface">
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<table cellpadding="2" width="100%"><tr>
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|
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
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<td align="center"><a href="../../../../../../../more/index.htm">More</a></td>
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|
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<a accesskey="p" href="../design_overview.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="the_interface.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_semantics"></a><a class="link" href="the_semantics.html" title="The semantics">The
|
||||
semantics</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> are intended to be used in places where
|
||||
objects of type <code class="computeroutput"><span class="identifier">T</span></code> would
|
||||
but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s purpose is to formalize the additional
|
||||
possibly uninitialized state. From the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus the additional semantics corresponding to this special state. As such,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
could be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>. Of course, we can't do that in C++,
|
||||
so we need to compose the desired semantics using a different mechanism.
|
||||
Doing it the other way around, that is, making <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a <span class="emphasis"><em>subtype</em></span> of
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> is not only conceptually
|
||||
wrong but also impractical: it is not allowed to derive from a non-class
|
||||
type, such as a built-in type.
|
||||
</p>
|
||||
<p>
|
||||
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> the required basic semantics:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce
|
||||
a formally uninitialized wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
|
||||
To introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain
|
||||
a new yet equivalent wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of
|
||||
some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To
|
||||
assign to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
|
||||
initialize the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by
|
||||
the type T):</strong></span> To compare wrapped object values taking into
|
||||
account the presence of uninitialized states.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects.
|
||||
(with whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s swap).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the
|
||||
wrapped object (if any) and leave the wrapper in the uninitialized
|
||||
state.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Additional operations are useful, such as converting constructors and converting
|
||||
assignments, in-place construction and assignment, and safe value access
|
||||
via a pointer to the wrapped object or null.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../design_overview.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="the_interface.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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|
||||
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||||
</html>
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||||
@ -1,175 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>Exception Safety Guarantees</title>
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||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.exception_safety_guarantees"></a><a class="link" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">Exception
|
||||
Safety Guarantees</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
This library assumes that <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
destructor does not throw exceptions. If it does, the behaviour of many operations
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
undefined.
|
||||
</p>
|
||||
<p>
|
||||
The following mutating operations never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
In addition, the following constructors and the destructor never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">()</span>
|
||||
<span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">(</span>
|
||||
<span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Regarding the following assignment functions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span> </code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
They forward calls to the corresponding <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructors or assignments (depending on whether the optional object is
|
||||
initialized or not); so if both <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructor and the assignment provide strong exception safety guarantee,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
assignment also provides strong exception safety guarantee; otherwise we
|
||||
only get the basic guarantee. Additionally, if both involved <code class="computeroutput"><span class="identifier">T</span></code>'s constructor and the assignment never
|
||||
throw, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
assignment also never throws.
|
||||
</p>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">T</span></code>'s constructor or
|
||||
assignment throws, assignments to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
do not throw anything else on its own. A throw during assignment never changes
|
||||
the initialization state of any optional object involved:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt1</span><span class="special">(</span><span class="identifier">val1</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt2</span><span class="special">(</span><span class="identifier">val2</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">try</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt2</span><span class="special">;</span> <span class="comment">// throws</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span><span class="special">(...)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This also applies to move assignments/constructors. However, move operations
|
||||
are made no-throw more often.
|
||||
</p>
|
||||
<p>
|
||||
Operation <code class="computeroutput"><span class="identifier">emplace</span></code> provides
|
||||
basic exception safety guarantee. If it throws, the optional object becomes
|
||||
uninitialized regardless of its initial state, and its previous contained
|
||||
value (if any) is destroyed. It doesn't call any assignment or move/copy
|
||||
constructor on <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.exception_safety_guarantees.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.exception_safety_guarantees.swap"></a></span><a class="link" href="exception_safety_guarantees.html#boost_optional.tutorial.exception_safety_guarantees.swap">Swap</a>
|
||||
</h5>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
|
||||
customized, its primary implementation forwards calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> or move constructor
|
||||
(depending on the initialization state of the optional objects). Thus, if
|
||||
both <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
and move constructor never throw, <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> never
|
||||
throws. similarly, if both <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and move constructor
|
||||
offer strong guarantee, <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> also
|
||||
offers a strong guarantee.
|
||||
</p>
|
||||
<p>
|
||||
In case <code class="computeroutput"><span class="identifier">swap</span></code> on optional
|
||||
is customized, the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
move constructor are replaced with the calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
default constructor followed by <code class="computeroutput"><span class="identifier">swap</span></code>.
|
||||
(This is more useful on older compilers that do not support move semantics,
|
||||
when one wants to acheive stronger exception safety guarantees.) In this
|
||||
case the exception safety guarantees for <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
are reliant on the guarantees of <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and default constructor
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="a_note_about_optional_bool_.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="type_requirements.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -1,197 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>In-Place Factories</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<link rel="home" href="../../index.html" title="Boost.Optional">
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<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
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<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
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||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="rebinding_semantics_for_assignment_of_optional_references.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="a_note_about_optional_bool_.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.in_place_factories"></a><a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
One of the typical problems with wrappers and containers is that their interfaces
|
||||
usually provide an operation to initialize or assign the contained object
|
||||
as a copy of some other object. This not only requires the underlying type
|
||||
to be <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>, but also requires
|
||||
the existence of a fully constructed object, often temporary, just to follow
|
||||
the copy from:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="special">(</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Temporary object created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
A solution to this problem is to support direct construction of the contained
|
||||
object right in the container's storage. In this scheme, the user only needs
|
||||
to supply the arguments to the constructor to use in the wrapped object construction.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="keyword">int</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">a1</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">a0</span><span class="special">,</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
A limitation of this method is that it doesn't scale well to wrapped objects
|
||||
with multiple constructors nor to generic code were the constructor overloads
|
||||
are unknown.
|
||||
</p>
|
||||
<p>
|
||||
The solution presented in this library is the family of <span class="bold"><strong>InPlaceFactories</strong></span>
|
||||
and <span class="bold"><strong>TypedInPlaceFactories</strong></span>. These factories
|
||||
are a family of classes which encapsulate an increasing number of arbitrary
|
||||
constructor parameters and supply a method to construct an object of a given
|
||||
type using those parameters at an address specified by the user via placement
|
||||
new.
|
||||
</p>
|
||||
<p>
|
||||
For example, one member of this family looks like:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">A1</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory2</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">A0</span> <span class="identifier">m_a0</span> <span class="special">;</span> <span class="identifier">A1</span> <span class="identifier">m_a1</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">TypedInPlaceFactory2</span><span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a1</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">m_a0</span><span class="special">(</span><span class="identifier">a0</span><span class="special">),</span> <span class="identifier">m_a1</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">construct</span> <span class="special">(</span> <span class="keyword">void</span><span class="special">*</span> <span class="identifier">p</span> <span class="special">)</span> <span class="special">{</span> <span class="keyword">new</span> <span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">m_a0</span><span class="special">,</span><span class="identifier">m_a1</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
A wrapper class aware of this can use it as:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span><span class="special"><</span><span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">))</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are divided in two groups:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="underline">TypedInPlaceFactories</span>: those which
|
||||
take the target type as a primary template parameter.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="underline">InPlaceFactories</span>: those with a
|
||||
template <code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code>
|
||||
member function taking the target type.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Within each group, all the family members differ only in the number of parameters
|
||||
allowed.
|
||||
</p>
|
||||
<p>
|
||||
This library provides an overloaded set of helper template functions to construct
|
||||
these factories without requiring unnecessary template parameters:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span>
|
||||
<span class="identifier">InPlaceFactoryN</span> <span class="special"><</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span>
|
||||
<span class="identifier">TypedInPlaceFactoryN</span> <span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
In-place factories can be used generically by the wrapper and user as follows:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">InPlaceFactory</span> <span class="special">></span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="keyword">template</span> <span class="special"><</span><span class="identifier">X</span><span class="special">></span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">in_place</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are implemented in the headers: <a href="../../../../../../boost/utility/in_place_factory.hpp" target="_top">in_place_factory.hpp</a>
|
||||
and <a href="../../../../../../boost/utility/typed_in_place_factory.hpp" target="_top">typed_in_place_factory.hpp</a>
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="rebinding_semantics_for_assignment_of_optional_references.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="a_note_about_optional_bool_.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
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|
||||
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|
||||
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|
||||
@ -1,87 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>IO operators</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
|
||||
<link rel="home" href="../../index.html" title="Boost.Optional">
|
||||
<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="prev" href="relational_operators.html" title="Relational operators">
|
||||
<link rel="next" href="optional_references.html" title="Optional references">
|
||||
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|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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||||
<table cellpadding="2" width="100%"><tr>
|
||||
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
|
||||
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|
||||
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|
||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
|
||||
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<hr>
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||||
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||||
<a accesskey="p" href="relational_operators.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.io_operators"></a><a class="link" href="io_operators.html" title="IO operators">IO operators</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
It is possible to use <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
with IO streams, provided that <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
can be used with streams. IOStream operators are defined in a separate header.
|
||||
</p>
|
||||
<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">o1</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</span> <span class="special">>></span> <span class="identifier">oN</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
<p>
|
||||
The current implementation does not guarantee any particular output. What
|
||||
it guarantees is that if streaming out and then back in <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
gives the same value, then streaming out and then back in <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
will also give back the same result:
|
||||
</p>
|
||||
<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">sstream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">x1</span><span class="special">,</span> <span class="identifier">x2</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">;</span>
|
||||
<span class="identifier">s</span> <span class="special"><<</span> <span class="identifier">o1</span> <span class="special"><<</span> <span class="identifier">oN</span><span class="special">;</span>
|
||||
<span class="identifier">s</span> <span class="special">>></span> <span class="identifier">x1</span> <span class="special">>></span> <span class="identifier">x2</span><span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">x1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">x2</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="relational_operators.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
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|
||||
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||||
</html>
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||||
@ -1,129 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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||||
<title>Motivation</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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|
||||
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|
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<table cellpadding="2" width="100%"><tr>
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|
||||
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||||
<hr>
|
||||
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|
||||
<a accesskey="p" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="design_overview.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.motivation"></a><a class="link" href="motivation.html" title="Motivation">Motivation</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not
|
||||
have a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or
|
||||
uses a runtime check and throws an exception if the postcondition is violated.
|
||||
This is a reasonable choice for example, for function (A), because the lack
|
||||
of a proper return value is directly related to an invalid parameter (out
|
||||
of domain argument), so it is appropriate to require the callee to supply
|
||||
only parameters in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail
|
||||
just because it can't find a value to return; so it is incorrect to consider
|
||||
such a situation an error and assert or throw an exception. This function
|
||||
must return, and somehow, must tell the callee that it is not returning a
|
||||
meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error
|
||||
to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside
|
||||
itself, but in many applications, it is just impractical for performance
|
||||
reasons to treat this as an error (because detecting that the polygon has
|
||||
no area might be too expensive to be required to be tested previously), and
|
||||
either an arbitrary point (typically at infinity) is returned, or some efficient
|
||||
way to tell the callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved
|
||||
to communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>,
|
||||
points at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when
|
||||
such values do not exist. In these cases, the usual alternative is either
|
||||
to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound
|
||||
type, such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the
|
||||
result which indicates if the result is meaningful, has the advantage that
|
||||
can be turned into a consistent idiom since the first element of the pair
|
||||
can be whatever the function would conceptually return. For example, the
|
||||
last two functions could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error
|
||||
prone since the user can easily use the function result (first element of
|
||||
the pair) without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="design_overview.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -1,115 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Optional references</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
|
||||
<link rel="home" href="../../index.html" title="Boost.Optional">
|
||||
<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="prev" href="io_operators.html" title="IO operators">
|
||||
<link rel="next" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">
|
||||
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|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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||||
<table cellpadding="2" width="100%"><tr>
|
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
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|
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
|
||||
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||||
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|
||||
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|
||||
<a accesskey="p" href="io_operators.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="rebinding_semantics_for_assignment_of_optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional
|
||||
references</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
This library allows the template parameter <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
to be of reference type: <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>, and to some extent, <code class="computeroutput"><span class="identifier">T</span>
|
||||
<span class="keyword">const</span><span class="special">&</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
However, since references are not real objects some restrictions apply and
|
||||
some operations are not available in this case:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Converting constructors
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Converting assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace construction
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access via pointer
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Also, even though <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
treats it wrapped pseudo-object much as a real value, a true real reference
|
||||
is stored so aliasing will ocurr:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Copies of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
will copy the references but all these references will nonetheless refer
|
||||
to the same object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access will actually provide access to the referenced object rather
|
||||
than the reference itself.
|
||||
</li>
|
||||
</ul></div>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
On compilers that do not conform to Standard C++ rules of reference binding,
|
||||
operations on optional references might give adverse results: rather than
|
||||
binding a reference to a designated object they may create an unexpected
|
||||
temporary and bind to it. For more details see <a class="link" href="../dependencies_and_portability/optional_reference_binding.html" title="Optional Reference Binding">Dependencies
|
||||
and Portability section</a>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.optional_references.rvalue_references"></a></span><a class="link" href="optional_references.html#boost_optional.tutorial.optional_references.rvalue_references">Rvalue
|
||||
references</a>
|
||||
</h5>
|
||||
<p>
|
||||
Rvalue references and lvalue references to const have the ability in C++
|
||||
to extend the life time of a temporary they bind to. Optional references
|
||||
do not have this capability, therefore to avoid surprising effects it is
|
||||
not possible to initialize an optional references from a temporary. Optional
|
||||
rvalue references are disabled altogether. Also, the initialization and assignment
|
||||
of an optional reference to const from rvalue reference is disabled.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// legal</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// illegal</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="io_operators.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="rebinding_semantics_for_assignment_of_optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -1,223 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Performance considerations</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<link rel="home" href="../../index.html" title="Boost.Optional">
|
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<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="prev" href="type_requirements.html" title="Type requirements">
|
||||
<link rel="next" href="../../optional/reference.html" title="Reference">
|
||||
</head>
|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
|
||||
<table cellpadding="2" width="100%"><tr>
|
||||
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
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||||
<td align="center"><a href="../../../../../../index.html">Home</a></td>
|
||||
<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="type_requirements.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../../optional/reference.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.performance_considerations"></a><a class="link" href="performance_considerations.html" title="Performance considerations">Performance
|
||||
considerations</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Technical details aside, the memory layout of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is more-less this:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">aligned_storage_t</span><span class="special"><</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">t</span><span class="special">),</span> <span class="keyword">alignof</span><span class="special">(</span><span class="identifier">T</span><span class="special">)></span> <span class="identifier">_storage</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
But for the purpose of this analysis, considering memory layouts, we can
|
||||
think of it as:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
|
||||
<span class="identifier">T</span> <span class="identifier">_storage</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Given type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>, and
|
||||
assuming that <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span> <span class="special">==</span>
|
||||
<span class="number">4</span></code>, we will get <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>)</span>
|
||||
<span class="special">==</span> <span class="number">8</span></code>.
|
||||
This is so because of the alignment rules, for our two members we get the
|
||||
following alignment:
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align1.png" alt="opt_align1"></span>
|
||||
</p>
|
||||
<p>
|
||||
This means you can fit twice as many <code class="computeroutput"><span class="keyword">int</span></code>s
|
||||
as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>s into
|
||||
the same space of memory. Therefore, if the size of the objects is critical
|
||||
for your application (e.g., because you want to utilize your CPU cache in
|
||||
order to gain performance) and you have determined you are willing to trade
|
||||
the code clarity, it is recommended that you simply go with type <code class="computeroutput"><span class="keyword">int</span></code> and use some 'magic value' to represent
|
||||
<span class="emphasis"><em>not-an-int</em></span>.
|
||||
</p>
|
||||
<p>
|
||||
Even if you cannot spare any value of <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
to represent <span class="emphasis"><em>not-an-int</em></span> (e.g., because every value is
|
||||
useful, or you do want to signal <span class="emphasis"><em>not-an-int</em></span> explicitly),
|
||||
at least for <code class="computeroutput"><span class="identifier">Trivial</span></code> types
|
||||
you should consider storing the value and the <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
flag representing the <span class="emphasis"><em>null-state</em></span> separately. Consider
|
||||
the following class:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Its memory layout can be depicted as follows:
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align2.png" alt="opt_align2"></span>
|
||||
</p>
|
||||
<p>
|
||||
This is exactly the same as if we had the following members:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_min</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_max</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
But when they are stored separately, we at least have an option to reorder
|
||||
them like this:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_min</span><span class="special">;</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_max</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Which gives us the following layout (and smaller total size):
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align3.png" alt="opt_align3"></span>
|
||||
</p>
|
||||
<p>
|
||||
Sometimes it requires detailed consideration what data we make optional.
|
||||
In our case above, if we determine that both minimum and maximum value can
|
||||
be provided or not provided together, but one is never provided without the
|
||||
other, we can make only one optional memebr:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Limits</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Limits</span><span class="special">></span> <span class="identifier">_limits</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
This would give us the following layout:
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align4.png" alt="opt_align4"></span>
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.performance_considerations.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.performance_considerations.optional_function_parameters"></a></span><a class="link" href="performance_considerations.html#boost_optional.tutorial.performance_considerations.optional_function_parameters">Optional
|
||||
function parameters</a>
|
||||
</h5>
|
||||
<p>
|
||||
Having function parameters of type <code class="computeroutput"><span class="keyword">const</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span></code>
|
||||
may incur certain unexpected run-time cost connected to copy construction
|
||||
of <code class="computeroutput"><span class="identifier">T</span></code>. Consider the following
|
||||
code.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">>&</span> <span class="identifier">v</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">v</span><span class="special">)</span> <span class="identifier">doSomethingWith</span><span class="special">(*</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="keyword">else</span> <span class="identifier">doSomethingElse</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
|
||||
<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// no copy</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// copy constructor of Big</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
No copy elision or move semantics can save us from copying type <code class="computeroutput"><span class="identifier">Big</span></code> here. Not that we need any copy, but
|
||||
this is how <code class="computeroutput"><span class="identifier">optional</span></code> works.
|
||||
In order to avoid copying in this case, one could provide second overload
|
||||
of <code class="computeroutput"><span class="identifier">fun</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Big</span><span class="special">&</span> <span class="identifier">v</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">doSomethingWith</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
|
||||
<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// no copy</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// no copy: second overload selected</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Alternatively, you could consider using an optional reference instead:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">Big</span><span class="special">&></span> <span class="identifier">v</span><span class="special">)</span> <span class="comment">// note where the reference is</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">v</span><span class="special">)</span> <span class="identifier">doSomethingWith</span><span class="special">(*</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="keyword">else</span> <span class="identifier">doSomethingElse</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
|
||||
<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// doesn't compile</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// no copy</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="type_requirements.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../../optional/reference.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
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||||
</html>
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@ -1,149 +0,0 @@
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||||
<html>
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||||
<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references"></a><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">Rebinding
|
||||
semantics for assignment of optional references</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
If you assign to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
the effect is to bind (for the first time) to the object. Clearly, there
|
||||
is no other choice.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If you assign to a bare C++ reference, the assignment is forwarded to the
|
||||
referenced object; its value changes but the reference is never rebound.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span>
|
||||
<span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'</span>
|
||||
</pre>
|
||||
<p>
|
||||
Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>,
|
||||
the effect is to <span class="bold"><strong>rebind</strong></span> to the new object
|
||||
instead of assigning the referee. This is unlike bare C++ references.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span>
|
||||
</pre>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a>
|
||||
</h5>
|
||||
<p>
|
||||
Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> references has been
|
||||
chosen to provide <span class="bold"><strong>consistency among initialization
|
||||
states</strong></span> even at the expense of lack of consistency with the semantics
|
||||
of bare C++ references. It is true that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code>
|
||||
strives to behave as much as possible as <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
does whenever it is initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>,
|
||||
doing so would result in inconsistent behavior w.r.t to the lvalue initialization
|
||||
state.
|
||||
</p>
|
||||
<p>
|
||||
Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
forwarding assignment to the referenced object (thus changing the referenced
|
||||
object value but not rebinding), and consider the following code:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">get</span><span class="special">();</span>
|
||||
<span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">rx</span><span class="special">);</span>
|
||||
<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
What does the assignment do?
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>,
|
||||
the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
(we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>).
|
||||
But what if <code class="computeroutput"><span class="identifier">a</span></code> is already
|
||||
<span class="emphasis"><em>initialized</em></span>? it would change the value of the referenced
|
||||
object (whatever that is); which is inconsistent with the other possible
|
||||
case.
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
would assign just like <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code> does, you would never be able to use
|
||||
Optional's assignment without explicitly handling the previous initialization
|
||||
state unless your code is capable of functioning whether after the assignment,
|
||||
<code class="computeroutput"><span class="identifier">a</span></code> aliases the same object
|
||||
as <code class="computeroutput"><span class="identifier">b</span></code> or not.
|
||||
</p>
|
||||
<p>
|
||||
That is, you would have to discriminate in order to be consistent.
|
||||
</p>
|
||||
<p>
|
||||
If in your code rebinding to another object is not an option, then it is
|
||||
very likely that binding for the first time isn't either. In such case, assignment
|
||||
to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
shall be prohibited. It is quite possible that in such a scenario it is a
|
||||
precondition that the lvalue must be already initialized. If it isn't, then
|
||||
binding for the first time is OK while rebinding is not which is IMO very
|
||||
unlikely. In such a scenario, you can assign the value itself directly, as
|
||||
in:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span>
|
||||
<span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_optional.tutorial.relational_operators"></a><a class="link" href="relational_operators.html" title="Relational operators">Relational
|
||||
operators</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
<a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>. Two optional
|
||||
objects containing a value compare in the same way as their contained values.
|
||||
The uninitialized state of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is treated as a distinct value, equal to itself, and unequal to any value
|
||||
of type <code class="computeroutput"><span class="identifier">T</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o0</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span>
|
||||
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">!=</span> <span class="identifier">o0</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">oN</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">!=</span> <span class="identifier">o1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">==</span> <span class="identifier">o0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
The converting constructor from <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
as well as from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> implies the existence and semantics
|
||||
of the mixed comparison between <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> as
|
||||
well as between <code class="computeroutput"><span class="identifier">none_t</span></code> and
|
||||
<code class="computeroutput"><span class="identifier">optionl</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">!=</span> <span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This mixed comparison has a practical interpretation, which is occasionally
|
||||
useful:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">ask_user</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">choice</span> <span class="special">==</span> <span class="number">2</span><span class="special">)</span>
|
||||
<span class="identifier">start_procedure_2</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
In the above example, the meaning of the comparison is 'user chose number
|
||||
2'. If user chose nothing, he didn't choose number 2.
|
||||
</p>
|
||||
<p>
|
||||
In case where <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
compared to <code class="computeroutput"><span class="identifier">none</span></code>, it is not
|
||||
required that <code class="computeroutput"><span class="identifier">T</span></code> be <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</p>
|
||||
<p>
|
||||
In a similar manner, type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. The optional
|
||||
object containing no value is compared less than any value of <code class="computeroutput"><span class="identifier">T</span></code>. To illustrate this, if the default ordering
|
||||
of <code class="computeroutput"><span class="identifier">size_t</span></code> is {<code class="computeroutput"><span class="number">0</span></code>, <code class="computeroutput"><span class="number">1</span></code>,
|
||||
<code class="computeroutput"><span class="number">2</span></code>, ...}, the default ordering
|
||||
of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">size_t</span><span class="special">></span></code>
|
||||
is {<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>, <code class="computeroutput"><span class="number">0</span></code>,
|
||||
<code class="computeroutput"><span class="number">1</span></code>, <code class="computeroutput"><span class="number">2</span></code>,
|
||||
...}. This order does not have a practical interpretation. The goal is to
|
||||
have any semantically correct default ordering in order for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> to
|
||||
be usable in ordered associative containers (wherever <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is usable).
|
||||
</p>
|
||||
<p>
|
||||
Mixed relational operators are the only case where the contained value of
|
||||
an optional object can be inspected without the usage of value accessing
|
||||
function (<code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code>,
|
||||
<code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>).
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="when_to_use_optional.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="io_operators.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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@ -1,109 +0,0 @@
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||||
<html>
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||||
<head>
|
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>Type requirements</title>
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<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<a accesskey="p" href="exception_safety_guarantees.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="performance_considerations.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.type_requirements"></a><a class="link" href="type_requirements.html" title="Type requirements">Type requirements</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
The very minimum requirement of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is that <code class="computeroutput"><span class="identifier">T</span></code> is a complete type
|
||||
and that it has a publicly accessible destructor. <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
doesn't even need to be constructible. You can use a very minimum interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span><span class="special">;</span> <span class="comment">// uninitialized</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// check if initialized</span>
|
||||
<span class="identifier">assert</span><span class="special">(!</span><span class="identifier">o</span><span class="special">);</span> <span class="comment">//</span>
|
||||
<span class="identifier">o</span><span class="special">.</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// always throws</span>
|
||||
</pre>
|
||||
<p>
|
||||
But this is practically useless. In order for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
to be able to do anything useful and offer all the spectrum of ways of accessing
|
||||
the contained value, <code class="computeroutput"><span class="identifier">T</span></code> needs
|
||||
to have at least one accessible constructor. In that case you need to initialize
|
||||
the optional object with function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>, or if your compiler does not support it,
|
||||
resort to <a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span><span class="special">;</span>
|
||||
<span class="identifier">o</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"T"</span><span class="special">,</span> <span class="string">"ctor"</span><span class="special">,</span> <span class="string">"params"</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
|
||||
can be easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
and be passed by value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>();</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>
|
||||
and can be easily initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">o</span><span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is not <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>, it is still possible to
|
||||
reset the value of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
using function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
|
||||
<span class="identifier">o</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="identifier">make_another_T</span><span class="special">());</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>
|
||||
(both <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
|
||||
<code class="computeroutput"><span class="identifier">MoveAssignable</span></code>) then <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <code class="computeroutput"><span class="identifier">Moveable</span></code> and additionally
|
||||
can be constructed and assigned from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Similarly, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Copyable</span></code> (both <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> and <code class="computeroutput"><span class="identifier">CopyAssignable</span></code>) then <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is also <code class="computeroutput"><span class="identifier">Copyable</span></code> and additionally
|
||||
can be constructed and assigned from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> <span class="emphasis"><em>is not</em></span>
|
||||
required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a>.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
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|
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<div class="spirit-nav">
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<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.when_to_use_optional"></a><a class="link" href="when_to_use_optional.html" title="When to use Optional">When to
|
||||
use Optional</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
It is recommended to use <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
in situations where there is exactly one, clear (to all parties) reason for
|
||||
having no value of type <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
and where the lack of value is as natural as having any regular value of
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>. One example of such situation
|
||||
is asking the user in some GUI form to optionally specify some limit on an
|
||||
<code class="computeroutput"><span class="keyword">int</span></code> value, but the user is allowed
|
||||
to say 'I want the number not to be constrained by the maximum'. For another
|
||||
example, consider a config parameter specifying how many threads the application
|
||||
should launch. Leaving this parameter unspecified means that the application
|
||||
should decide itself. For yet another example, consider a function returning
|
||||
the index of the smallest element in a <code class="computeroutput"><span class="identifier">vector</span></code>.
|
||||
We need to be prepared for the situation, where the <code class="computeroutput"><span class="identifier">vector</span></code>
|
||||
is empty. Therefore a natural signature for such function would be:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">find_smallest_elem</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">vec</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
Here, having received an empty <code class="computeroutput"><span class="identifier">vec</span></code>
|
||||
and having no <code class="computeroutput"><span class="identifier">size_t</span></code> to return
|
||||
is not a <span class="emphasis"><em>failure</em></span> but a <span class="emphasis"><em>normal</em></span>,
|
||||
albeit irregular, situation.
|
||||
</p>
|
||||
<p>
|
||||
Another typical situation is to indicate that we do not have a value yet,
|
||||
but we expect to have it later. This notion can be used in implementing solutions
|
||||
like lazy initialization or a two-phase initialization.
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> can be used to take
|
||||
a non-<a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a> type <code class="computeroutput"><span class="identifier">T</span></code> and create a sibling type with a default
|
||||
constructor. This is a way to add a <span class="emphasis"><em>null-state</em></span> to any
|
||||
type that doesn't have it already.
|
||||
</p>
|
||||
<p>
|
||||
Sometimes type <code class="computeroutput"><span class="identifier">T</span></code> already
|
||||
provides a built-in null-state, but it may still be useful to wrap it into
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code>. Consider <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
|
||||
When you read a piece of text from a GUI form or a DB table, it is hardly
|
||||
ever that the empty string indicates anything else but a missing text. And
|
||||
some data bases do not even distinguish between a null string entry and a
|
||||
non-null string of length 0. Still, it may be practical to use <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">string</span><span class="special">></span></code>
|
||||
to indicate in the returned type that we want to treat the empty string in
|
||||
a special dedicated program path:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">if</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">name</span> <span class="special">=</span> <span class="identifier">ask_user_name</span><span class="special">())</span> <span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(*</span><span class="identifier">name</span> <span class="special">!=</span> <span class="string">""</span><span class="special">);</span>
|
||||
<span class="identifier">logon_as</span><span class="special">(*</span><span class="identifier">name</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">else</span> <span class="special">{</span>
|
||||
<span class="identifier">skip_logon</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
In the example above, the assertion indicates that if we choose to use this
|
||||
technique, we must translate the empty string state to an optional object
|
||||
with no contained value (inside function <code class="computeroutput"><span class="identifier">ask_user_name</span></code>).
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.when_to_use_optional.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.when_to_use_optional.not_recommended_usages"></a></span><a class="link" href="when_to_use_optional.html#boost_optional.tutorial.when_to_use_optional.not_recommended_usages">Not
|
||||
recommended usages</a>
|
||||
</h5>
|
||||
<p>
|
||||
It is not recommended to use <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
to indicate that we were not able to compute a value because of a <span class="emphasis"><em>failure</em></span>.
|
||||
It is difficult to define what a failure is, but it usually has one common
|
||||
characteristic: an associated information on the cause of the failure. This
|
||||
can be the type and member data of an exception object, or an error code.
|
||||
It is a bad design to signal a failure and not inform about the cause. If
|
||||
you do not want to use exceptions, and do not like the fact that by returning
|
||||
error codes you cannot return the computed value, you can use <a href="https://github.com/ptal/Boost.Expected" target="_top">Expected</a>
|
||||
library. It is sort of <a href="../../../../../variant/index.html" target="_top">Boost.Variant</a>
|
||||
that contains either a computed value or a reason why the computation failed.
|
||||
</p>
|
||||
<p>
|
||||
Sometimes the distinction into what is a failure and what is a valid but
|
||||
irregular result is blurry and depends on a particular usage and personal
|
||||
preference. Consider a function that converts a <code class="computeroutput"><span class="identifier">string</span></code>
|
||||
to an <code class="computeroutput"><span class="keyword">int</span></code>. Is it a failure that
|
||||
you cannot convert? It might in some cases, but in other you may call it
|
||||
exactly for the purpose of figuring out if a given <code class="computeroutput"><span class="identifier">string</span></code>
|
||||
is convertible, and you are not even interested in the resulting value. Sometimes
|
||||
when a conversion fails you may not consider it a failure, but you need to
|
||||
know why it cannot be converted; for instance at which character it is determined
|
||||
that the conversion is impossible. In this case returning <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
will not suffice. Finally, there is a use case where an input string that
|
||||
does not represent an <code class="computeroutput"><span class="keyword">int</span></code> is
|
||||
not a failure condition, but during the conversion we use resources whose
|
||||
acquisition may fail. In that case the natural representation is to both
|
||||
return <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> and
|
||||
signal failure:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert1</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">string</span><span class="special">&</span> <span class="identifier">str</span><span class="special">);</span> <span class="comment">// throws</span>
|
||||
<span class="identifier">expected</span><span class="special"><</span><span class="identifier">ErrorT</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">convert2</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">string</span><span class="special">&</span> <span class="identifier">str</span><span class="special">);</span> <span class="comment">// return either optional or error</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="design_overview/the_interface.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="relational_operators.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
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|
||||
@ -1,43 +1,35 @@
|
||||
<html>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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|
||||
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|
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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|
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<a accesskey="p" href="exception_safety_guarantees.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="dependencies_and_portability.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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|
||||
<div class="section">
|
||||
<div class="section" title="Type requirements">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.type_requirements"></a><a class="link" href="type_requirements.html" title="Type requirements">Type requirements</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
At the very minimum for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
to work with a minimum interface it is required that <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
has a publicly accessible no-throw destructor. In that case you need to initialize
|
||||
the optional object with function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code> or use <span class="bold"><strong>InPlaceFactories</strong></span>.
|
||||
Additionally, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is also <code class="computeroutput"><span class="identifier">Moveable</span></code> and can be
|
||||
easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
and be passed by value. Additionally, if <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">Copyable</span></code>, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <code class="computeroutput"><span class="identifier">Copyable</span></code> and can be easily
|
||||
initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
In general, <code class="computeroutput"><span class="identifier">T</span></code> must be <a href="../../../../utility/CopyConstructible.html" target="_top">Copy Constructible</a> and
|
||||
have a no-throw destructor. The copy-constructible requirement is not needed
|
||||
if <span class="bold"><strong>InPlaceFactories</strong></span> are used.
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> <span class="underline">is
|
||||
@ -47,7 +39,7 @@
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
<td align="right"><div class="copyright-footer">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -55,7 +47,7 @@
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="exception_safety_guarantees.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="dependencies_and_portability.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<a accesskey="p" href="exception_safety_guarantees.html"><img src="../../../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="implementation_notes.html"><img src="../../../../../doc/html/images/next.png" alt="Next"></a>
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</div>
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</body>
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</html>
|
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|
||||
582
doc/html/boostbook.css
Normal file
582
doc/html/boostbook.css
Normal file
@ -0,0 +1,582 @@
|
||||
/*=============================================================================
|
||||
Copyright (c) 2004 Joel de Guzman
|
||||
http://spirit.sourceforge.net/
|
||||
|
||||
Use, modification and distribution is subject to the Boost Software
|
||||
License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
=============================================================================*/
|
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/*=============================================================================
|
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Body defaults
|
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=============================================================================*/
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body
|
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{
|
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margin: 1em;
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font-family: sans-serif;
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}
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/*=============================================================================
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Paragraphs
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=============================================================================*/
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p
|
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{
|
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text-align: left;
|
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font-size: 10pt;
|
||||
line-height: 1.15;
|
||||
}
|
||||
|
||||
/*=============================================================================
|
||||
Program listings
|
||||
=============================================================================*/
|
||||
|
||||
/* Code on paragraphs */
|
||||
p tt.computeroutput
|
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|
||||
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|
||||
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|
||||
|
||||
pre.synopsis
|
||||
{
|
||||
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|
||||
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|
||||
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|
||||
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|
||||
|
||||
.programlisting,
|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
|
||||
/* Program listings in tables don't get borders */
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||||
td .programlisting,
|
||||
td .screen
|
||||
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|
||||
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|
||||
padding: 0pc 0pc 0pc 0pc;
|
||||
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|
||||
|
||||
/*=============================================================================
|
||||
Headings
|
||||
=============================================================================*/
|
||||
|
||||
h1, h2, h3, h4, h5, h6
|
||||
{
|
||||
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|
||||
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||||
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|
||||
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||||
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||||
h1 { font: 140% }
|
||||
h2 { font: bold 140% }
|
||||
h3 { font: bold 130% }
|
||||
h4 { font: bold 120% }
|
||||
h5 { font: italic 110% }
|
||||
h6 { font: italic 100% }
|
||||
|
||||
/* Top page titles */
|
||||
title,
|
||||
h1.title,
|
||||
h2.title
|
||||
h3.title,
|
||||
h4.title,
|
||||
h5.title,
|
||||
h6.title,
|
||||
.refentrytitle
|
||||
{
|
||||
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|
||||
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|
||||
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|
||||
|
||||
h1.title { font-size: 140% }
|
||||
h2.title { font-size: 140% }
|
||||
h3.title { font-size: 130% }
|
||||
h4.title { font-size: 120% }
|
||||
h5.title { font-size: 110% }
|
||||
h6.title { font-size: 100% }
|
||||
|
||||
.section h1
|
||||
{
|
||||
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|
||||
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|
||||
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|
||||
|
||||
.section h2 { font-size: 140% }
|
||||
.section h3 { font-size: 130% }
|
||||
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|
||||
.section h5 { font-size: 110% }
|
||||
.section h6 { font-size: 100% }
|
||||
|
||||
/* Code on titles */
|
||||
h1 tt.computeroutput { font-size: 140% }
|
||||
h2 tt.computeroutput { font-size: 140% }
|
||||
h3 tt.computeroutput { font-size: 130% }
|
||||
h4 tt.computeroutput { font-size: 120% }
|
||||
h5 tt.computeroutput { font-size: 110% }
|
||||
h6 tt.computeroutput { font-size: 100% }
|
||||
|
||||
/*=============================================================================
|
||||
Author
|
||||
=============================================================================*/
|
||||
|
||||
h3.author
|
||||
{
|
||||
font-size: 100%
|
||||
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|
||||
|
||||
/*=============================================================================
|
||||
Lists
|
||||
=============================================================================*/
|
||||
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||||
li
|
||||
{
|
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|
||||
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|
||||
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|
||||
|
||||
/* Unordered lists */
|
||||
ul
|
||||
{
|
||||
text-align: left;
|
||||
}
|
||||
|
||||
/* Ordered lists */
|
||||
ol
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||||
{
|
||||
text-align: left;
|
||||
}
|
||||
|
||||
/*=============================================================================
|
||||
Links
|
||||
=============================================================================*/
|
||||
|
||||
a
|
||||
{
|
||||
text-decoration: none; /* no underline */
|
||||
}
|
||||
|
||||
a:hover
|
||||
{
|
||||
text-decoration: underline;
|
||||
}
|
||||
|
||||
/*=============================================================================
|
||||
Spirit style navigation
|
||||
=============================================================================*/
|
||||
|
||||
.spirit-nav
|
||||
{
|
||||
text-align: right;
|
||||
}
|
||||
|
||||
.spirit-nav a
|
||||
{
|
||||
color: white;
|
||||
padding-left: 0.5em;
|
||||
}
|
||||
|
||||
.spirit-nav img
|
||||
{
|
||||
border-width: 0px;
|
||||
}
|
||||
|
||||
/*=============================================================================
|
||||
Table of contents
|
||||
=============================================================================*/
|
||||
|
||||
.toc
|
||||
{
|
||||
margin: 1pc 4% 0pc 4%;
|
||||
padding: 0.1pc 1pc 0.1pc 1pc;
|
||||
font-size: 10pt;
|
||||
line-height: 1.15;
|
||||
}
|
||||
|
||||
.toc-main
|
||||
{
|
||||
text-align: center;
|
||||
margin: 3pc 16% 3pc 16%;
|
||||
padding: 3pc 1pc 3pc 1pc;
|
||||
line-height: 0.1;
|
||||
}
|
||||
|
||||
.boost-toc
|
||||
{
|
||||
float: right;
|
||||
padding: 0.5pc;
|
||||
}
|
||||
|
||||
/*=============================================================================
|
||||
Tables
|
||||
=============================================================================*/
|
||||
|
||||
.table-title,
|
||||
div.table p.title
|
||||
{
|
||||
margin-left: 4%;
|
||||
padding-right: 0.5em;
|
||||
padding-left: 0.5em;
|
||||
}
|
||||
|
||||
.informaltable table,
|
||||
.table table
|
||||
{
|
||||
width: 92%;
|
||||
margin-left: 4%;
|
||||
margin-right: 4%;
|
||||
}
|
||||
|
||||
div.informaltable table,
|
||||
div.table table
|
||||
{
|
||||
padding: 4px;
|
||||
}
|
||||
|
||||
/* Table Cells */
|
||||
div.informaltable table tr td,
|
||||
div.table table tr td
|
||||
{
|
||||
padding: 0.5em;
|
||||
text-align: left;
|
||||
}
|
||||
|
||||
div.informaltable table tr th,
|
||||
div.table table tr th
|
||||
{
|
||||
padding: 0.5em 0.5em 0.5em 0.5em;
|
||||
border: 1pt solid white;
|
||||
font-size: 120%;
|
||||
}
|
||||
|
||||
/*=============================================================================
|
||||
Blurbs
|
||||
=============================================================================*/
|
||||
|
||||
div.note,
|
||||
div.tip,
|
||||
div.important,
|
||||
div.caution,
|
||||
div.warning,
|
||||
div.sidebar
|
||||
{
|
||||
font-size: 10pt;
|
||||
line-height: 1.2;
|
||||
display: block;
|
||||
margin: 1pc 4% 0pc 4%;
|
||||
padding: 0.5pc 0.5pc 0.5pc 0.5pc;
|
||||
}
|
||||
|
||||
div.sidebar img
|
||||
{
|
||||
padding: 1pt;
|
||||
}
|
||||
|
||||
|
||||
|
||||
/*=============================================================================
|
||||
Callouts
|
||||
=============================================================================*/
|
||||
.line_callout_bug img
|
||||
{
|
||||
float: left;
|
||||
position:relative;
|
||||
left: 4px;
|
||||
top: -12px;
|
||||
clear: left;
|
||||
margin-left:-22px;
|
||||
}
|
||||
|
||||
.callout_bug img
|
||||
{
|
||||
}
|
||||
|
||||
|
||||
|
||||
/*=============================================================================
|
||||
Variable Lists
|
||||
=============================================================================*/
|
||||
|
||||
/* Make the terms in definition lists bold */
|
||||
div.variablelist dl dt,
|
||||
span.term
|
||||
{
|
||||
font-weight: bold;
|
||||
font-size: 10pt;
|
||||
}
|
||||
|
||||
div.variablelist table tbody tr td
|
||||
{
|
||||
text-align: left;
|
||||
vertical-align: top;
|
||||
padding: 0em 2em 0em 0em;
|
||||
font-size: 10pt;
|
||||
margin: 0em 0em 0.5em 0em;
|
||||
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|
||||
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|
||||
|
||||
/* Make the terms in definition lists bold */
|
||||
div.variablelist dl dt
|
||||
{
|
||||
margin-bottom: 0.2em;
|
||||
}
|
||||
|
||||
div.variablelist dl dd
|
||||
{
|
||||
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|
||||
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|
||||
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|
||||
|
||||
div.variablelist table tbody tr td p
|
||||
div.variablelist dl dd p
|
||||
{
|
||||
margin: 0em 0em 0.5em 0em;
|
||||
line-height: 1;
|
||||
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|
||||
|
||||
/*=============================================================================
|
||||
Misc
|
||||
=============================================================================*/
|
||||
|
||||
/* Title of books and articles in bibliographies */
|
||||
span.title
|
||||
{
|
||||
font-style: italic;
|
||||
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|
||||
|
||||
span.underline
|
||||
{
|
||||
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|
||||
}
|
||||
|
||||
span.strikethrough
|
||||
{
|
||||
text-decoration: line-through;
|
||||
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|
||||
|
||||
/* Copyright, Legal Notice */
|
||||
div div.legalnotice p
|
||||
{
|
||||
text-align: left
|
||||
}
|
||||
|
||||
/*=============================================================================
|
||||
Colors
|
||||
=============================================================================*/
|
||||
|
||||
@media screen
|
||||
{
|
||||
/* Links */
|
||||
a
|
||||
{
|
||||
color: #0C7445;
|
||||
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|
||||
|
||||
a:visited
|
||||
{
|
||||
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|
||||
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|
||||
|
||||
h1 a, h2 a, h3 a, h4 a, h5 a, h6 a,
|
||||
h1 a:hover, h2 a:hover, h3 a:hover, h4 a:hover, h5 a:hover, h6 a:hover,
|
||||
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|
||||
{
|
||||
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|
||||
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|
||||
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|
||||
|
||||
/* Syntax Highlighting */
|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
|
||||
/* Copyright, Legal Notice */
|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
|
||||
div div.legalnotice p
|
||||
{
|
||||
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|
||||
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|
||||
|
||||
/* Program listing */
|
||||
pre.synopsis
|
||||
{
|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
|
||||
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|
||||
.screen
|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
|
||||
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|
||||
td .screen
|
||||
{
|
||||
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|
||||
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|
||||
|
||||
/* Blurbs */
|
||||
div.note,
|
||||
div.tip,
|
||||
div.important,
|
||||
div.caution,
|
||||
div.warning,
|
||||
div.sidebar
|
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{
|
||||
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|
||||
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|
||||
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|
||||
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|
||||
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|
||||
|
||||
/* Table of contents */
|
||||
.toc
|
||||
{
|
||||
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|
||||
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|
||||
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|
||||
background-color: #FAFFFB;
|
||||
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|
||||
|
||||
/* Table of contents */
|
||||
.toc-main
|
||||
{
|
||||
border: 1px solid #DCDCDC;
|
||||
border-bottom: 3px solid #9D9D9D;
|
||||
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|
||||
background-color: #FAFFFB;
|
||||
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|
||||
|
||||
|
||||
/* Tables */
|
||||
div.informaltable table tr td,
|
||||
div.table table tr td
|
||||
{
|
||||
border: 1px solid #DCDCDC;
|
||||
background-color: #FAFFFB;
|
||||
}
|
||||
|
||||
div.informaltable table tr th,
|
||||
div.table table tr th
|
||||
{
|
||||
background-color: #E3F9E4;
|
||||
border: 1px solid #DCDCDC;
|
||||
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|
||||
|
||||
/* Misc */
|
||||
span.highlight
|
||||
{
|
||||
color: #00A000;
|
||||
}
|
||||
}
|
||||
|
||||
@media print
|
||||
{
|
||||
/* Links */
|
||||
a
|
||||
{
|
||||
color: black;
|
||||
}
|
||||
|
||||
a:visited
|
||||
{
|
||||
color: black;
|
||||
}
|
||||
|
||||
.spirit-nav
|
||||
{
|
||||
display: none;
|
||||
}
|
||||
|
||||
/* Program listing */
|
||||
pre.synopsis
|
||||
{
|
||||
border: 1px solid gray;
|
||||
background-color: #FAFFFB;
|
||||
}
|
||||
|
||||
.programlisting,
|
||||
.screen
|
||||
{
|
||||
border: 1px solid gray;
|
||||
background-color: #FAFFFB;
|
||||
}
|
||||
|
||||
td .programlisting,
|
||||
td .screen
|
||||
{
|
||||
border: 0px solid #DCDCDC;
|
||||
}
|
||||
|
||||
/* Table of contents */
|
||||
.toc
|
||||
{
|
||||
border: 1px solid #DCDCDC;
|
||||
border-bottom: 3px solid #9D9D9D;
|
||||
border-right: 3px solid #9D9D9D;
|
||||
background-color: #FAFFFB;
|
||||
}
|
||||
|
||||
/* Table of contents */
|
||||
.toc-main
|
||||
{
|
||||
border: 1px solid #DCDCDC;
|
||||
border-bottom: 3px solid #9D9D9D;
|
||||
border-right: 3px solid #9D9D9D;
|
||||
background-color: #FAFFFB;
|
||||
}
|
||||
|
||||
.informaltable table,
|
||||
.table table
|
||||
{
|
||||
border: 1px solid #DCDCDC;
|
||||
border-bottom: 3px solid #9D9D9D;
|
||||
border-right: 3px solid #9D9D9D;
|
||||
border-collapse: collapse;
|
||||
background-color: #FAFFFB;
|
||||
}
|
||||
|
||||
/* Tables */
|
||||
div.informaltable table tr td,
|
||||
div.table table tr td
|
||||
{
|
||||
border: 1px solid #DCDCDC;
|
||||
background-color: #FAFFFB;
|
||||
}
|
||||
|
||||
div.informaltable table tr th,
|
||||
div.table table tr th
|
||||
{
|
||||
border: 1px solid #DCDCDC;
|
||||
background-color: #FAFFFB;
|
||||
}
|
||||
|
||||
/* Misc */
|
||||
span.highlight
|
||||
{
|
||||
font-weight: bold;
|
||||
}
|
||||
}
|
||||
Binary file not shown.
|
Before Width: | Height: | Size: 417 B |
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|
Before Width: | Height: | Size: 472 B |
Binary file not shown.
|
Before Width: | Height: | Size: 468 B |
Binary file not shown.
|
Before Width: | Height: | Size: 428 B |
@ -1,34 +1,33 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Boost.Optional</title>
|
||||
<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
|
||||
<link rel="home" href="index.html" title="Boost.Optional">
|
||||
<link rel="next" href="boost_optional/quick_start.html" title="Quick Start">
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
|
||||
<title>Chapter<EFBFBD>1.<2E>Boost.Optional</title>
|
||||
<link rel="stylesheet" href="boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
|
||||
<link rel="home" href="index.html" title="Chapter<EFBFBD>1.<2E>Boost.Optional">
|
||||
<link rel="next" href="boost_optional/development.html" title="Development">
|
||||
</head>
|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
|
||||
<table cellpadding="2" width="100%"><tr>
|
||||
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td>
|
||||
<td align="center"><a href="../../../../index.html">Home</a></td>
|
||||
<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="../../../libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../more/index.htm">More</a></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav"><a accesskey="n" href="boost_optional/quick_start.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a></div>
|
||||
<div class="chapter">
|
||||
<div class="spirit-nav"><a accesskey="n" href="boost_optional/development.html"><img src="../../../../doc/html/images/next.png" alt="Next"></a></div>
|
||||
<div class="chapter" title="Chapter<65>1.<2E>Boost.Optional">
|
||||
<div class="titlepage"><div>
|
||||
<div><h2 class="title">
|
||||
<a name="optional"></a>Boost.Optional</h2></div>
|
||||
<a name="optional"></a>Chapter<EFBFBD>1.<2E>Boost.Optional</h2></div>
|
||||
<div><div class="author"><h3 class="author">
|
||||
<span class="firstname">Fernando Luis</span> <span class="surname">Cacciola Carballal</span>
|
||||
</h3></div></div>
|
||||
<div><p class="copyright">Copyright © 2003-2007 Fernando Luis Cacciola Carballal</p></div>
|
||||
<div><p class="copyright">Copyright © 2014, 2015 Andrzej Krzemieński</p></div>
|
||||
<div><div class="legalnotice">
|
||||
<a name="optional.legal"></a><p>
|
||||
<div><p class="copyright">Copyright <EFBFBD> 2003 -2007 Fernando Luis Cacciola Carballal</p></div>
|
||||
<div><div class="legalnotice" title="Legal Notice">
|
||||
<a name="id553943"></a><p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
@ -36,120 +35,141 @@
|
||||
</div></div>
|
||||
<div class="toc">
|
||||
<p><b>Table of Contents</b></p>
|
||||
<dl class="toc">
|
||||
<dt><span class="section"><a href="index.html#optional.introduction">Introduction</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start.html">Quick Start</a></span></dt>
|
||||
<dl>
|
||||
<dt><span class="section"><a href="index.html#optional.motivation">Motivation</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/development.html">Development</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start.html#boost_optional.quick_start.optional_return_values">Optional
|
||||
<dt><span class="section"><a href="boost_optional/development.html#boost_optional.development.the_models">The models</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/development.html#boost_optional.development.the_semantics">The semantics</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/development.html#boost_optional.development.the_interface">The Interface</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="boost_optional/synopsis.html">Synopsis</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/detailed_semantics.html">Detailed Semantics</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/examples.html">Examples</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.optional_return_values">Optional
|
||||
return values</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/optional_automatic_variables.html">Optional
|
||||
automatic variables</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/optional_data_members.html">Optional
|
||||
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.optional_local_variables">Optional
|
||||
local variables</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.optional_data_members">Optional
|
||||
data members</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/bypassing_unnecessary_default_construction.html">Bypassing
|
||||
unnecessary default construction</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/storage_in_containers.html">Storage
|
||||
in containers</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="optional/tutorial.html">Tutorial</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="optional/tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/design_overview.html">Design Overview</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/when_to_use_optional.html">When to
|
||||
use Optional</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/relational_operators.html">Relational
|
||||
operators</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/io_operators.html">IO operators</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/optional_references.html">Optional
|
||||
references</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/rebinding_semantics_for_assignment_of_optional_references.html">Rebinding
|
||||
semantics for assignment of optional references</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/in_place_factories.html">In-Place
|
||||
Factories</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/a_note_about_optional_bool_.html">A
|
||||
note about optional<bool></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/exception_safety_guarantees.html">Exception
|
||||
Safety Guarantees</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/performance_considerations.html">Performance
|
||||
considerations</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="optional/reference.html">Reference</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="optional/reference.html#boost_optional.reference.header__boost_none_hpp_">Header
|
||||
<boost/none.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_bad_optional_access_hpp_.html">Header
|
||||
<boost/optional/bad_optional_access.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/io_header.html">Header <boost/optional/optional_io.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html">Header
|
||||
<boost/optional/optional_fwd.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="optional/reference/header__boost_optional_optional_hpp_.html">Header
|
||||
<boost/optional/optional.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_hpp_.html">Header
|
||||
<boost/optional.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction">Bypassing
|
||||
expensive unnecessary default construction</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="boost_optional/optional_references.html">Optional references</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/rebinding_semantics_for_assignment_of_optional_references.html">Rebinding
|
||||
semantics for assignment of optional references</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/in_place_factories.html">In-Place Factories</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/a_note_about_optional_bool_.html">A note about
|
||||
optional<bool></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/exception_safety_guarantees.html">Exception Safety
|
||||
Guarantees</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/type_requirements.html">Type requirements</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/implementation_notes.html">Implementation Notes</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html">Dependencies
|
||||
and Portability</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies">Dependencies</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability/optional_reference_binding.html">Optional
|
||||
Reference Binding</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="boost_optional/relnotes.html">Release Notes</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/acknowledgements.html">Acknowledgements</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/acknowledgments.html">Acknowledgments</a></span></dt>
|
||||
</dl>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="section" title="Motivation">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="optional.introduction"></a><a class="link" href="index.html#optional.introduction" title="Introduction">Introduction</a>
|
||||
<a name="optional.motivation"></a><a class="link" href="index.html#optional.motivation" title="Motivation">Motivation</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
Class template <code class="computeroutput"><span class="identifier">optional</span></code> is
|
||||
a wrapper for representing 'optional' (or 'nullable') objects who may not (yet)
|
||||
contain a valid value. Optional objects offer full value semantics; they are
|
||||
good for passing by value and usage inside STL containers. This is a header-only
|
||||
library.
|
||||
Consider these functions which should return a value but which might not have
|
||||
a value to return:
|
||||
</p>
|
||||
<h4>
|
||||
<a name="optional.introduction.h0"></a>
|
||||
<span class="phrase"><a name="optional.introduction.problem"></a></span><a class="link" href="index.html#optional.introduction.problem">Problem</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span>
|
||||
<span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Suppose we want to read a parameter form a config file which represents some
|
||||
integral value, let's call it <code class="computeroutput"><span class="string">"MaxValue"</span></code>.
|
||||
It is possible that this parameter is not specified; such situation is no error.
|
||||
It is valid to not specify the parameter and in that case the program is supposed
|
||||
to behave slightly differently. Also, suppose that any possible value of type
|
||||
<code class="computeroutput"><span class="keyword">int</span></code> is a valid value for <code class="computeroutput"><span class="string">"MaxValue"</span></code>, so we cannot just use
|
||||
<code class="computeroutput"><span class="special">-</span><span class="number">1</span></code>
|
||||
to represent the absence of the parameter in the config file.
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<h4>
|
||||
<a name="optional.introduction.h1"></a>
|
||||
<span class="phrase"><a name="optional.introduction.solution"></a></span><a class="link" href="index.html#optional.introduction.solution">Solution</a>
|
||||
</h4>
|
||||
<p>
|
||||
This is how you solve it with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>:
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or uses
|
||||
a runtime check and throws an exception if the postcondition is violated. This
|
||||
is a reasonable choice for example, for function (A), because the lack of a
|
||||
proper return value is directly related to an invalid parameter (out of domain
|
||||
argument), so it is appropriate to require the callee to supply only parameters
|
||||
in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">getConfigParam</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">);</span> <span class="comment">// return either an int or a `not-an-int`</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">getConfigParam</span><span class="special">(</span><span class="string">"MaxValue"</span><span class="special">))</span> <span class="comment">// did I get a real int?</span>
|
||||
<span class="identifier">runWithMax</span><span class="special">(*</span><span class="identifier">oi</span><span class="special">);</span> <span class="comment">// use my int</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="identifier">runWithNoMax</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail just
|
||||
because it can't find a value to return; so it is incorrect to consider such
|
||||
a situation an error and assert or throw an exception. This function must return,
|
||||
and somehow, must tell the callee that it is not returning a meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error to
|
||||
ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside itself,
|
||||
but in many applications, it is just impractical for performance reasons to
|
||||
treat this as an error (because detecting that the polygon has no area might
|
||||
be too expensive to be required to be tested previously), and either an arbitrary
|
||||
point (typically at infinity) is returned, or some efficient way to tell the
|
||||
callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved to
|
||||
communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>, points
|
||||
at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when such
|
||||
values do not exist. In these cases, the usual alternative is either to use
|
||||
a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> in place
|
||||
of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound type, such
|
||||
as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the result
|
||||
which indicates if the result is meaningful, has the advantage that can be
|
||||
turned into a consistent idiom since the first element of the pair can be whatever
|
||||
the function would conceptually return. For example, the last two functions
|
||||
could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly inexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error prone
|
||||
since the user can easily use the function result (first element of the pair)
|
||||
without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
<p>
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"><p><small>Last revised: October 20, 2015 at 21:17:45 GMT</small></p></td>
|
||||
<td align="left"><p><small>Last revised: November 20, 2009 at 10:24:28 GMT</small></p></td>
|
||||
<td align="right"><div class="copyright-footer"></div></td>
|
||||
</tr></table>
|
||||
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|
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<dt><span class="section"><a href="../boost_optional/reference/io_header.html">Header <boost/optional/optional_io.hpp></a></span></dt>
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<boost/optional/optional_fwd.hpp></a></span></dt>
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|
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</h4></div></div></div>
|
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<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">none_t</span> <span class="special">{/*</span> <span class="identifier">see</span> <span class="identifier">below</span> <span class="special">*/};</span>
|
||||
|
||||
<span class="keyword">const</span> <span class="identifier">none_t</span> <span class="identifier">none</span> <span class="special">(/*</span> <span class="identifier">see</span> <span class="identifier">below</span> <span class="special">*/);</span>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
<p>
|
||||
Class <code class="computeroutput"><span class="identifier">none_t</span></code> is meant to
|
||||
serve as a tag for selecting appropriate overloads of from <code class="computeroutput"><span class="identifier">optional</span></code>'s interface. It is an empty,
|
||||
trivially copyable class with disabled default constructor.
|
||||
</p>
|
||||
<p>
|
||||
Constant <code class="computeroutput"><span class="identifier">none</span></code> is used to
|
||||
indicate an optional object that does not contain a value in initialization,
|
||||
assignment and relational operations of <code class="computeroutput"><span class="identifier">optional</span></code>.
|
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
|
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<pre class="programlisting"><code class="computeroutput"><span class="comment">// In Header: <</span></code><a href="../../../../../../boost/optional/optional.hpp" target="_top">boost/optional/optional.hpp</a><span class="comment">></span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span> <span class="special">:</span>
|
||||
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">value_type</span><span class="special">;</span>
|
||||
|
||||
<span class="comment">// (If T is of reference type, the parameters and results by reference are by value)</span>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_constructor"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_constructor_none_t"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_constructor_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_constructor_move_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// [new in 1.34]</span>
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_constructor_bool_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_constructor_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_move_constructor_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_constructor_other_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_move_constructor_other_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_equal_none_t"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_equal_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_move_equal_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_equal_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_move_equal_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_equal_other_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_move_equal_other_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span> <span class="special">(</span> <span class="identifier">Args</span><span class="special">...&&</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_emplace"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_asterisk_move"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_value_move"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_value_or"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_value_or_move"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or_eval</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_value_or_call"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or_eval</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_value_or_call_move"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">explicit</span> <span class="keyword">operator</span> <span class="keyword">bool</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_bool"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_operator_not"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// deprecated methods</span>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_reset"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_reset_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_is_initialized"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_operator_compare_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_operator_compare_not_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_operator_compare_less_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">></span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_operator_compare_greater_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_operator_compare_less_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">>=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_operator_compare_greater_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_operator_compare_equal_optional_none"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_operator_compare_not_equal_optional_none"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_make_optional_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_make_optional_bool_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_optional_value_or</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>*</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_pointer</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_pointer</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics.html#reference_swap_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<div class="section">
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||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="optional.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/design_overview.html">Design Overview</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/when_to_use_optional.html">When to
|
||||
use Optional</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/relational_operators.html">Relational
|
||||
operators</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/io_operators.html">IO operators</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/optional_references.html">Optional
|
||||
references</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/rebinding_semantics_for_assignment_of_optional_references.html">Rebinding
|
||||
semantics for assignment of optional references</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/in_place_factories.html">In-Place
|
||||
Factories</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/a_note_about_optional_bool_.html">A
|
||||
note about optional<bool></a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/exception_safety_guarantees.html">Exception
|
||||
Safety Guarantees</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/performance_considerations.html">Performance
|
||||
considerations</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.motivation"></a><a class="link" href="tutorial.html#boost_optional.tutorial.motivation" title="Motivation">Motivation</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not
|
||||
have a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or
|
||||
uses a runtime check and throws an exception if the postcondition is violated.
|
||||
This is a reasonable choice for example, for function (A), because the lack
|
||||
of a proper return value is directly related to an invalid parameter (out
|
||||
of domain argument), so it is appropriate to require the callee to supply
|
||||
only parameters in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail
|
||||
just because it can't find a value to return; so it is incorrect to consider
|
||||
such a situation an error and assert or throw an exception. This function
|
||||
must return, and somehow, must tell the callee that it is not returning a
|
||||
meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error
|
||||
to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside
|
||||
itself, but in many applications, it is just impractical for performance
|
||||
reasons to treat this as an error (because detecting that the polygon has
|
||||
no area might be too expensive to be required to be tested previously), and
|
||||
either an arbitrary point (typically at infinity) is returned, or some efficient
|
||||
way to tell the callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved
|
||||
to communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>,
|
||||
points at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when
|
||||
such values do not exist. In these cases, the usual alternative is either
|
||||
to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound
|
||||
type, such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the
|
||||
result which indicates if the result is meaningful, has the advantage that
|
||||
can be turned into a consistent idiom since the first element of the pair
|
||||
can be whatever the function would conceptually return. For example, the
|
||||
last two functions could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error
|
||||
prone since the user can easily use the function result (first element of
|
||||
the pair) without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../boost_optional/quick_start/storage_in_containers.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../boost_optional/tutorial/design_overview.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
|
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</div>
|
||||
</body>
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</html>
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26
doc/implementation_notes.qbk
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doc/implementation_notes.qbk
Normal file
@ -0,0 +1,26 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
|
||||
|
||||
[section Implementation Notes]
|
||||
|
||||
`optional<T>` is currently implemented using a custom aligned storage facility
|
||||
built from `alignment_of` and `type_with_alignment` (both from Type Traits). It
|
||||
uses a separate boolean flag to indicate the initialization state.
|
||||
Placement new with `T`'s copy constructor and `T`'s destructor are explicitly used
|
||||
to initialize,copy and destroy optional values.
|
||||
As a result, `T`'s default constructor is effectively by-passed, but the exception
|
||||
guarantees are basic.
|
||||
It is planned to replace the current implementation with another with stronger
|
||||
exception safety, such as a future `boost::variant`.
|
||||
|
||||
[endsect]
|
||||
|
||||
@ -1,14 +1,49 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
Copyright (c) 2014 Andrzej Krzemienski
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
[library Boost.Optional
|
||||
[quickbook 1.4]
|
||||
[authors [Cacciola Carballal, Fernando Luis]]
|
||||
[copyright 2003-2007 Fernando Luis Cacciola Carballal]
|
||||
[category miscellaneous]
|
||||
[id optional]
|
||||
[dirname optional]
|
||||
[purpose
|
||||
Discriminated-union wrapper for optional values
|
||||
]
|
||||
[source-mode c++]
|
||||
[license
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
[@http://www.boost.org/LICENSE_1_0.txt])
|
||||
]
|
||||
]
|
||||
|
||||
[/ Macros will be used for links so we have a central place to change them ]
|
||||
|
||||
|
||||
[/ Cited Boost resources ]
|
||||
|
||||
[def __BOOST_VARIANT__ [@../../../variant/index.html Boost.Variant]]
|
||||
[def __BOOST_TRIBOOL__ [@../../../../doc/html/tribool.html boost::tribool]]
|
||||
|
||||
[def __OPTIONAL_POINTEE__ [@../../../utility/OptionalPointee.html OptionalPointee]]
|
||||
[def __COPY_CONSTRUCTIBLE__ [@../../../utility/CopyConstructible.html Copy Constructible]]
|
||||
[def __FUNCTION_EQUAL_POINTEES__ [@../../../utility/OptionalPointee.html#equal `equal_pointees()`]]
|
||||
[def __FUNCTION_LESS_POINTEES__ [@../../../utility/OptionalPointee.html#less `less_pointees()`]]
|
||||
|
||||
[def __IN_PLACE_FACTORY_HPP__ [@../../../../boost/utility/in_place_factory.hpp in_place_factory.hpp]]
|
||||
[def __TYPED_IN_PLACE_FACTORY_HPP__ [@../../../../boost/utility/typed_in_place_factory.hpp typed_in_place_factory.hpp]]
|
||||
|
||||
[/ Other web resources ]
|
||||
|
||||
[def __HASKELL__ [@http://www.haskell.org/ Haskell]]
|
||||
[def __SGI_DEFAULT_CONSTRUCTIBLE__ [@http://www.sgi.com/tech/stl/DefaultConstructible.html Default Constructible]]
|
||||
|
||||
|
||||
[/ Icons ]
|
||||
|
||||
[def __SPACE__ [$images/space.png]]
|
||||
[def __GO_TO__ [$images/callouts/R.png]]
|
||||
|
||||
|
||||
[section Motivation]
|
||||
|
||||
Consider these functions which should return a value but which might not have
|
||||
@ -64,7 +99,7 @@ functions could have the following interface:
|
||||
std::pair<char,bool> get_async_input();
|
||||
std::pair<point,bool> polygon::get_any_point_effectively_inside();
|
||||
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
These functions use a consistent interface for dealing with possibly inexistent
|
||||
results:
|
||||
|
||||
std::pair<point,bool> p = poly.get_any_point_effectively_inside();
|
||||
@ -78,3 +113,14 @@ without ever checking if it has a valid value.
|
||||
Clearly, we need a better idiom.
|
||||
|
||||
[endsect]
|
||||
|
||||
|
||||
[include development.qbk]
|
||||
[include reference.qbk]
|
||||
[include examples.qbk]
|
||||
[include special_cases.qbk]
|
||||
[include implementation_notes.qbk]
|
||||
[include dependencies.qbk]
|
||||
[include acknowledgments.qbk]
|
||||
|
||||
|
||||
881
doc/reference.qbk
Normal file
881
doc/reference.qbk
Normal file
@ -0,0 +1,881 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
http://www.boost.org/LICENSE_1_0.txt)
|
||||
]
|
||||
|
||||
|
||||
[section Synopsis]
|
||||
|
||||
namespace boost {
|
||||
|
||||
template<class T>
|
||||
class optional
|
||||
{
|
||||
public :
|
||||
|
||||
// (If T is of reference type, the parameters and results by reference are by value)
|
||||
|
||||
optional () ; ``[link reference_optional_constructor __GO_TO__]``
|
||||
|
||||
optional ( none_t ) ; ``[link reference_optional_constructor_none_t __GO_TO__]``
|
||||
|
||||
optional ( T const& v ) ; ``[link reference_optional_constructor_value __GO_TO__]``
|
||||
|
||||
// [new in 1.34]
|
||||
optional ( bool condition, T const& v ) ; ``[link reference_optional_constructor_bool_value __GO_TO__]``
|
||||
|
||||
optional ( optional const& rhs ) ; ``[link reference_optional_constructor_optional __GO_TO__]``
|
||||
|
||||
template<class U> explicit optional ( optional<U> const& rhs ) ; ``[link reference_optional_constructor_other_optional __GO_TO__]``
|
||||
|
||||
template<class InPlaceFactory> explicit optional ( InPlaceFactory const& f ) ; ``[link reference_optional_constructor_factory __GO_TO__]``
|
||||
|
||||
template<class TypedInPlaceFactory> explicit optional ( TypedInPlaceFactory const& f ) ; ``[link reference_optional_constructor_factory __GO_TO__]``
|
||||
|
||||
optional& operator = ( none_t ) ; ``[/[link reference_optional_operator_equal_none_t __GO_TO__]]``
|
||||
|
||||
optional& operator = ( T const& v ) ; ``[link reference_optional_operator_equal_value __GO_TO__]``
|
||||
|
||||
optional& operator = ( optional const& rhs ) ; ``[link reference_optional_operator_equal_optional __GO_TO__]``
|
||||
|
||||
template<class U> optional& operator = ( optional<U> const& rhs ) ; ``[link reference_optional_operator_equal_other_optional __GO_TO__]``
|
||||
|
||||
template<class InPlaceFactory> optional& operator = ( InPlaceFactory const& f ) ; ``[/[link reference_optional_operator_equal_factory __GO_TO__]]``
|
||||
|
||||
template<class TypedInPlaceFactory> optional& operator = ( TypedInPlaceFactory const& f ) ; ``[/[link reference_optional_operator_equal_factory __GO_TO__]]``
|
||||
|
||||
T const& get() const ; ``[link reference_optional_get __GO_TO__]``
|
||||
T& get() ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
// [new in 1.34]
|
||||
T const& get_value_or( T const& default ) const ; ``[link reference_optional_get_value_or_value __GO_TO__]``
|
||||
|
||||
T const* operator ->() const ; ``[link reference_optional_operator_arrow __GO_TO__]``
|
||||
T* operator ->() ; ``[link reference_optional_operator_arrow __GO_TO__]``
|
||||
|
||||
T const& operator *() const ; ``[link reference_optional_get __GO_TO__]``
|
||||
T& operator *() ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
T const* get_ptr() const ; ``[link reference_optional_get_ptr __GO_TO__]``
|
||||
T* get_ptr() ; ``[link reference_optional_get_ptr __GO_TO__]``
|
||||
|
||||
operator unspecified-bool-type() const ; ``[link reference_optional_operator_bool __GO_TO__]``
|
||||
|
||||
bool operator!() const ; ``[link reference_optional_operator_not __GO_TO__]``
|
||||
|
||||
// deprecated methods
|
||||
|
||||
// (deprecated)
|
||||
void reset() ; ``[link reference_optional_reset __GO_TO__]``
|
||||
|
||||
// (deprecated)
|
||||
void reset ( T const& ) ; ``[link reference_optional_reset_value __GO_TO__]``
|
||||
|
||||
// (deprecated)
|
||||
bool is_initialized() const ; ``[link reference_optional_is_initialized __GO_TO__]``
|
||||
|
||||
};
|
||||
|
||||
template<class T> inline bool operator == ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_equal_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator != ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_not_equal_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator < ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_less_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator > ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_greater_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator <= ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_less_or_equal_optional_optional __GO_TO__]``
|
||||
|
||||
template<class T> inline bool operator >= ( optional<T> const& x, optional<T> const& y ) ; ``[link reference_operator_compare_greater_or_equal_optional_optional __GO_TO__]``
|
||||
|
||||
// [new in 1.34]
|
||||
template<class T> inline optional<T> make_optional ( T const& v ) ; ``[link reference_make_optional_value __GO_TO__]``
|
||||
|
||||
// [new in 1.34]
|
||||
template<class T> inline optional<T> make_optional ( bool condition, T const& v ) ; ``[link reference_make_optional_bool_value __GO_TO__]``
|
||||
|
||||
// [new in 1.34]
|
||||
template<class T> inline T const& get_optional_value_or ( optional<T> const& opt, T const& default ) ; ``[link reference_optional_get_value_or_value __GO_TO__]``
|
||||
|
||||
template<class T> inline T const& get ( optional<T> const& opt ) ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
template<class T> inline T& get ( optional<T> & opt ) ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
template<class T> inline T const* get ( optional<T> const* opt ) ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
template<class T> inline T* get ( optional<T>* opt ) ; ``[link reference_optional_get __GO_TO__]``
|
||||
|
||||
template<class T> inline T const* get_pointer ( optional<T> const& opt ) ; ``[link reference_optional_get_ptr __GO_TO__]``
|
||||
|
||||
template<class T> inline T* get_pointer ( optional<T> & opt ) ; ``[link reference_optional_get_ptr __GO_TO__]``
|
||||
|
||||
template<class T> inline void swap( optional<T>& x, optional<T>& y ) ; ``[link reference_swap_optional_optional __GO_TO__]``
|
||||
|
||||
} // namespace boost
|
||||
|
||||
|
||||
[endsect]
|
||||
|
||||
[section Detailed Semantics]
|
||||
|
||||
Because `T` might be of reference type, in the sequel, those entries whose
|
||||
semantic depends on `T` being of reference type or not will be distinguished
|
||||
using the following convention:
|
||||
|
||||
* If the entry reads: `optional<T`['(not a ref)]`>`, the description
|
||||
corresponds only to the case where `T` is not of reference type.
|
||||
* If the entry reads: `optional<T&>`, the description corresponds only to
|
||||
the case where `T` is of reference type.
|
||||
* If the entry reads: `optional<T>`, the description is the same for both
|
||||
cases.
|
||||
|
||||
[note
|
||||
The following section contains various `assert()` which are used only to show
|
||||
the postconditions as sample code. It is not implied that the type `T` must
|
||||
support each particular expression but that if the expression is supported,
|
||||
the implied condition holds.
|
||||
]
|
||||
|
||||
__SPACE__
|
||||
|
||||
[heading optional class member functions]
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_constructor]
|
||||
|
||||
[: `optional<T>::optional();`]
|
||||
|
||||
* [*Effect:] Default-Constructs an `optional`.
|
||||
* [*Postconditions:] `*this` is [_uninitialized].
|
||||
* [*Throws:] Nothing.
|
||||
* Notes: T's default constructor [_is not] called.
|
||||
* [*Example:]
|
||||
``
|
||||
optional<T> def ;
|
||||
assert ( !def ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_constructor_none_t]
|
||||
|
||||
[: `optional<T>::optional( none_t );`]
|
||||
|
||||
* [*Effect:] Constructs an `optional` uninitialized.
|
||||
* [*Postconditions:] `*this` is [_uninitialized].
|
||||
* [*Throws:] Nothing.
|
||||
* [*Notes:] `T`'s default constructor [_is not] called. The expression
|
||||
`boost::none` denotes an instance of `boost::none_t` that can be used as
|
||||
the parameter.
|
||||
* [*Example:]
|
||||
``
|
||||
#include <boost/none.hpp>
|
||||
optional<T> n(none) ;
|
||||
assert ( !n ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_constructor_value]
|
||||
|
||||
[: `optional<T `['(not a ref)]`>::optional( T const& v )`]
|
||||
|
||||
* [*Effect:] Directly-Constructs an `optional`.
|
||||
* [*Postconditions:] `*this` is [_initialized] and its value is a['copy]
|
||||
of `v`.
|
||||
* [*Throws:] Whatever `T::T( T const& )` throws.
|
||||
* [*Notes: ] `T::T( T const& )` is called.
|
||||
* [*Exception Safety:] Exceptions can only be thrown during
|
||||
`T::T( T const& );` in that case, this constructor has no effect.
|
||||
* [*Example:]
|
||||
``
|
||||
T v;
|
||||
optional<T> opt(v);
|
||||
assert ( *opt == v ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[: `optional<T&>::optional( T& ref )`]
|
||||
|
||||
* [*Effect:] Directly-Constructs an `optional`.
|
||||
* [*Postconditions:] `*this` is [_initialized] and its value is an instance
|
||||
of an internal type wrapping the reference `ref`.
|
||||
* [*Throws:] Nothing.
|
||||
* [*Example:]
|
||||
``
|
||||
T v;
|
||||
T& vref = v ;
|
||||
optional<T&> opt(vref);
|
||||
assert ( *opt == v ) ;
|
||||
++ v ; // mutate referee
|
||||
assert (*opt == v);
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_constructor_bool_value]
|
||||
|
||||
[: `optional<T` ['(not a ref)]`>::optional( bool condition, T const& v ) ;` ]
|
||||
[: `optional<T&> ::optional( bool condition, T& v ) ;` ]
|
||||
|
||||
* If condition is true, same as:
|
||||
|
||||
[: `optional<T` ['(not a ref)]`>::optional( T const& v )`]
|
||||
[: `optional<T&> ::optional( T& v )`]
|
||||
|
||||
* otherwise, same as:
|
||||
|
||||
[: `optional<T ['(not a ref)]>::optional()`]
|
||||
[: `optional<T&> ::optional()`]
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_constructor_optional]
|
||||
|
||||
[: `optional<T `['(not a ref)]`>::optional( optional const& rhs );`]
|
||||
|
||||
* [*Effect:] Copy-Constructs an `optional`.
|
||||
* [*Postconditions:] If rhs is initialized, `*this` is initialized and
|
||||
its value is a ['copy] of the value of `rhs`; else `*this` is uninitialized.
|
||||
* [*Throws:] Whatever `T::T( T const& )` throws.
|
||||
* [*Notes:] If rhs is initialized, `T::T(T const& )` is called.
|
||||
* [*Exception Safety:] Exceptions can only be thrown during
|
||||
`T::T( T const& );` in that case, this constructor has no effect.
|
||||
* [*Example:]
|
||||
``
|
||||
optional<T> uninit ;
|
||||
assert (!uninit);
|
||||
|
||||
optional<T> uinit2 ( uninit ) ;
|
||||
assert ( uninit2 == uninit );
|
||||
|
||||
optional<T> init( T(2) );
|
||||
assert ( *init == T(2) ) ;
|
||||
|
||||
optional<T> init2 ( init ) ;
|
||||
assert ( init2 == init ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[: `optional<T&>::optional( optional const& rhs );`]
|
||||
|
||||
* [*Effect:] Copy-Constructs an `optional`.
|
||||
* [*Postconditions:] If `rhs` is initialized, `*this` is initialized and its
|
||||
value is another reference to the same object referenced by `*rhs`; else
|
||||
`*this` is uninitialized.
|
||||
* [*Throws:] Nothing.
|
||||
* [*Notes:] If `rhs` is initialized, both `*this` and `*rhs` will reefer to the
|
||||
same object (they alias).
|
||||
* [*Example:]
|
||||
``
|
||||
optional<T&> uninit ;
|
||||
assert (!uninit);
|
||||
|
||||
optional<T&> uinit2 ( uninit ) ;
|
||||
assert ( uninit2 == uninit );
|
||||
|
||||
T v = 2 ; T& ref = v ;
|
||||
optional<T> init(ref);
|
||||
assert ( *init == v ) ;
|
||||
|
||||
optional<T> init2 ( init ) ;
|
||||
assert ( *init2 == v ) ;
|
||||
|
||||
v = 3 ;
|
||||
|
||||
assert ( *init == 3 ) ;
|
||||
assert ( *init2 == 3 ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_constructor_other_optional]
|
||||
|
||||
[: `template<U> explicit optional<T` ['(not a ref)]`>::optional( optional<U> const& rhs );`]
|
||||
|
||||
* [*Effect:] Copy-Constructs an `optional`.
|
||||
* [*Postconditions:] If `rhs` is initialized, `*this` is initialized and its
|
||||
value is a ['copy] of the value of rhs converted to type `T`; else `*this` is
|
||||
uninitialized.
|
||||
* [*Throws:] Whatever `T::T( U const& )` throws.
|
||||
* [*Notes: ] `T::T( U const& )` is called if `rhs` is initialized, which requires a
|
||||
valid conversion from `U` to `T`.
|
||||
* [*Exception Safety:] Exceptions can only be thrown during `T::T( U const& );`
|
||||
in that case, this constructor has no effect.
|
||||
* [*Example:]
|
||||
``
|
||||
optional<double> x(123.4);
|
||||
assert ( *x == 123.4 ) ;
|
||||
|
||||
optional<int> y(x) ;
|
||||
assert( *y == 123 ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_constructor_factory]
|
||||
|
||||
[: `template<InPlaceFactory> explicit optional<T` ['(not a ref)]`>::optional( InPlaceFactory const& f );`]
|
||||
[: `template<TypedInPlaceFactory> explicit optional<T` ['(not a ref)]`>::optional( TypedInPlaceFactory const& f );`]
|
||||
|
||||
* [*Effect:] Constructs an `optional` with a value of `T` obtained from the
|
||||
factory.
|
||||
* [*Postconditions: ] `*this` is [_initialized] and its value is ['directly given]
|
||||
from the factory `f` (i.e., the value [_is not copied]).
|
||||
* [*Throws:] Whatever the `T` constructor called by the factory throws.
|
||||
* [*Notes:] See [link boost_optional.in_place_factories In-Place Factories]
|
||||
* [*Exception Safety:] Exceptions can only be thrown during the call to
|
||||
the `T` constructor used by the factory; in that case, this constructor has
|
||||
no effect.
|
||||
* [*Example:]
|
||||
``
|
||||
class C { C ( char, double, std::string ) ; } ;
|
||||
|
||||
C v('A',123.4,"hello");
|
||||
|
||||
optional<C> x( in_place ('A', 123.4, "hello") ); // InPlaceFactory used
|
||||
optional<C> y( in_place<C>('A', 123.4, "hello") ); // TypedInPlaceFactory used
|
||||
|
||||
assert ( *x == v ) ;
|
||||
assert ( *y == v ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_operator_equal_value]
|
||||
|
||||
[: `optional& optional<T` ['(not a ref)]`>::operator= ( T const& rhs ) ;`]
|
||||
|
||||
* [*Effect:] Assigns the value `rhs` to an `optional`.
|
||||
* [*Postconditions: ] `*this` is initialized and its value is a ['copy] of `rhs`.
|
||||
* [*Throws:] Whatever `T::operator=( T const& )` or `T::T(T const&)` throws.
|
||||
* [*Notes:] If `*this` was initialized, `T`'s assignment operator is used,
|
||||
otherwise, its copy-constructor is used.
|
||||
* [*Exception Safety:] In the event of an exception, the initialization
|
||||
state of `*this` is unchanged and its value unspecified as far as `optional`
|
||||
is concerned (it is up to `T`'s `operator=()`). If `*this` is initially
|
||||
uninitialized and `T`'s ['copy constructor] fails, `*this` is left properly
|
||||
uninitialized.
|
||||
* [*Example:]
|
||||
``
|
||||
T x;
|
||||
optional<T> def ;
|
||||
optional<T> opt(x) ;
|
||||
|
||||
T y;
|
||||
def = y ;
|
||||
assert ( *def == y ) ;
|
||||
opt = y ;
|
||||
assert ( *opt == y ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[: `optional<T&>& optional<T&>::operator= ( T& const& rhs ) ;`]
|
||||
|
||||
* [*Effect:] (Re)binds thee wrapped reference.
|
||||
* [*Postconditions: ] `*this` is initialized and it references the same
|
||||
object referenced by `rhs`.
|
||||
* [*Notes:] If `*this` was initialized, is is ['rebound] to the new object.
|
||||
See [link boost_optional.rebinding_semantics_for_assignment_of_optional_references here] for details on this behavior.
|
||||
* [*Example:]
|
||||
``
|
||||
int a = 1 ;
|
||||
int b = 2 ;
|
||||
T& ra = a ;
|
||||
T& rb = b ;
|
||||
optional<int&> def ;
|
||||
optional<int&> opt(ra) ;
|
||||
|
||||
def = rb ; // binds 'def' to 'b' through 'rb'
|
||||
assert ( *def == b ) ;
|
||||
*def = a ; // changes the value of 'b' to a copy of the value of 'a'
|
||||
assert ( b == a ) ;
|
||||
int c = 3;
|
||||
int& rc = c ;
|
||||
opt = rc ; // REBINDS to 'c' through 'rc'
|
||||
c = 4 ;
|
||||
assert ( *opt == 4 ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_operator_equal_optional]
|
||||
|
||||
[: `optional& optional<T` ['(not a ref)]`>::operator= ( optional const& rhs ) ;`]
|
||||
|
||||
* [*Effect:] Assigns another `optional` to an `optional`.
|
||||
* [*Postconditions:] If `rhs` is initialized, `*this` is initialized and
|
||||
its value is a ['copy] of the value of `rhs`; else `*this` is uninitialized.
|
||||
* [*Throws:] Whatever `T::operator( T const&)` or `T::T( T const& )` throws.
|
||||
* [*Notes:] If both `*this` and `rhs` are initially initialized, `T`'s
|
||||
['assignment operator] is used. If `*this` is initially initialized but `rhs` is
|
||||
uninitialized, `T`'s [destructor] is called. If `*this` is initially uninitialized
|
||||
but `rhs` is initialized, `T`'s ['copy constructor] is called.
|
||||
* [*Exception Safety:] In the event of an exception, the initialization state of
|
||||
`*this` is unchanged and its value unspecified as far as optional is concerned
|
||||
(it is up to `T`'s `operator=()`). If `*this` is initially uninitialized and
|
||||
`T`'s ['copy constructor] fails, `*this` is left properly uninitialized.
|
||||
* [*Example:]
|
||||
``
|
||||
T v;
|
||||
optional<T> opt(v);
|
||||
optional<T> def ;
|
||||
|
||||
opt = def ;
|
||||
assert ( !def ) ;
|
||||
// previous value (copy of 'v') destroyed from within 'opt'.
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[: `optional<T&> & optional<T&>::operator= ( optional<T&> const& rhs ) ;`]
|
||||
|
||||
* [*Effect:] (Re)binds thee wrapped reference.
|
||||
* [*Postconditions:] If `*rhs` is initialized, `*this` is initialized and it
|
||||
references the same object referenced by `*rhs`; otherwise, `*this` is
|
||||
uninitialized (and references no object).
|
||||
* [*Notes:] If `*this` was initialized and so is *rhs, this is is ['rebound] to
|
||||
the new object. See [link boost_optional.rebinding_semantics_for_assignment_of_optional_references here] for details on this behavior.
|
||||
* [*Example:]
|
||||
``
|
||||
int a = 1 ;
|
||||
int b = 2 ;
|
||||
T& ra = a ;
|
||||
T& rb = b ;
|
||||
optional<int&> def ;
|
||||
optional<int&> ora(ra) ;
|
||||
optional<int&> orb(rb) ;
|
||||
|
||||
def = orb ; // binds 'def' to 'b' through 'rb' wrapped within 'orb'
|
||||
assert ( *def == b ) ;
|
||||
*def = ora ; // changes the value of 'b' to a copy of the value of 'a'
|
||||
assert ( b == a ) ;
|
||||
int c = 3;
|
||||
int& rc = c ;
|
||||
optional<int&> orc(rc) ;
|
||||
ora = orc ; // REBINDS ora to 'c' through 'rc'
|
||||
c = 4 ;
|
||||
assert ( *ora == 4 ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_operator_equal_other_optional]
|
||||
|
||||
[: `template<U> optional& optional<T` ['(not a ref)]`>::operator= ( optional<U> const& rhs ) ;`]
|
||||
|
||||
* [*Effect:] Assigns another convertible optional to an optional.
|
||||
* [*Postconditions:] If `rhs` is initialized, `*this` is initialized and
|
||||
its value is a ['copy] of the value of `rhs` ['converted] to type `T`; else
|
||||
`*this` is uninitialized.
|
||||
* [*Throws:] Whatever `T::operator=( U const& )` or `T::T( U const& )` throws.
|
||||
* [*Notes:] If both `*this` and rhs are initially initialized, `T`'s
|
||||
['assignment operator] (from `U`) is used. If `*this` is initially initialized
|
||||
but `rhs` is uninitialized, `T`'s ['destructor] is called. If `*this` is
|
||||
initially uninitialized but rhs is initialized, `T`'s ['converting constructor]
|
||||
(from `U`) is called.
|
||||
* [*Exception Safety:] In the event of an exception, the initialization state
|
||||
of `*this` is unchanged and its value unspecified as far as optional is
|
||||
concerned (it is up to `T`'s `operator=()`). If `*this` is initially
|
||||
uninitialized and `T`'s converting constructor fails, `*this` is left properly
|
||||
uninitialized.
|
||||
* [*Example:]
|
||||
``
|
||||
T v;
|
||||
optional<T> opt0(v);
|
||||
optional<U> opt1;
|
||||
|
||||
opt1 = opt0 ;
|
||||
assert ( *opt1 == static_cast<U>(v) ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_reset_value]
|
||||
|
||||
[: `void optional<T` ['(not a ref)]`>::reset( T const& v ) ;`]
|
||||
* [*Deprecated:] same as `operator= ( T const& v) ;`
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_reset]
|
||||
|
||||
[: `void optional<T>::reset() ;`]
|
||||
* [*Deprecated:] Same as `operator=( detail::none_t );`
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_get]
|
||||
|
||||
[: `T const& optional<T` ['(not a ref)]`>::operator*() const ;`]
|
||||
[: `T& optional<T` ['(not a ref)]`>::operator*();`]
|
||||
[: `T const& optional<T` ['(not a ref)]`>::get() const ;`]
|
||||
[: `T& optional<T` ['(not a ref)]`>::get() ;`]
|
||||
|
||||
[: `inline T const& get ( optional<T` ['(not a ref)]`> const& ) ;`]
|
||||
[: `inline T& get ( optional<T` ['(not a ref)]`> &) ;`]
|
||||
|
||||
* [*Requirements:] `*this` is initialized
|
||||
* [*Returns:] A reference to the contained value
|
||||
* [*Throws:] Nothing.
|
||||
* [*Notes:] The requirement is asserted via `BOOST_ASSERT()`.
|
||||
* [*Example:]
|
||||
``
|
||||
T v ;
|
||||
optional<T> opt ( v );
|
||||
T const& u = *opt;
|
||||
assert ( u == v ) ;
|
||||
T w ;
|
||||
*opt = w ;
|
||||
assert ( *opt == w ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_get_value_or_value]
|
||||
|
||||
[: `T const& optional<T` ['(not a ref)]`>::get_value_or( T const& default) const ;`]
|
||||
[: `T& optional<T` ['(not a ref)]`>::get_value_or( T& default ) ;`]
|
||||
|
||||
[: `inline T const& get_optional_value_or ( optional<T` ['(not a ref)]`> const& o, T const& default ) ;`]
|
||||
[: `inline T& get_optional_value_or ( optional<T` ['(not a ref)]`>& o, T& default ) ;`]
|
||||
|
||||
* [*Returns:] A reference to the contained value, if any, or `default`.
|
||||
* [*Throws:] Nothing.
|
||||
* [*Example:]
|
||||
``
|
||||
T v, z ;
|
||||
optional<T> def;
|
||||
T const& y = def.get_value_or(z);
|
||||
assert ( y == z ) ;
|
||||
|
||||
optional<T> opt ( v );
|
||||
T const& u = get_optional_value_or(opt,z);
|
||||
assert ( u == v ) ;
|
||||
assert ( u != z ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[: `T const& optional<T&>::operator*() const ;`]
|
||||
[: `T & optional<T&>::operator*();`]
|
||||
[: `T const& optional<T&>::get() const ;`]
|
||||
[: `T& optional<T&>::get() ;`]
|
||||
|
||||
[: `inline T const& get ( optional<T&> const& ) ;`]
|
||||
[: `inline T& get ( optional<T&> &) ;`]
|
||||
|
||||
* [*Requirements: ] `*this` is initialized
|
||||
* [*Returns:] [_The] reference contained.
|
||||
* [*Throws:] Nothing.
|
||||
* [*Notes:] The requirement is asserted via `BOOST_ASSERT()`.
|
||||
* [*Example:]
|
||||
``
|
||||
T v ;
|
||||
T& vref = v ;
|
||||
optional<T&> opt ( vref );
|
||||
T const& vref2 = *opt;
|
||||
assert ( vref2 == v ) ;
|
||||
++ v ;
|
||||
assert ( *opt == v ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_get_ptr]
|
||||
|
||||
[: `T const* optional<T` ['(not a ref)]`>::get_ptr() const ;`]
|
||||
[: `T* optional<T` ['(not a ref)]`>::get_ptr() ;`]
|
||||
|
||||
[: `inline T const* get_pointer ( optional<T` ['(not a ref)]`> const& ) ;`]
|
||||
[: `inline T* get_pointer ( optional<T` ['(not a ref)]`> &) ;`]
|
||||
|
||||
* [*Returns:] If `*this` is initialized, a pointer to the contained value;
|
||||
else `0` (['null]).
|
||||
* [*Throws:] Nothing.
|
||||
* [*Notes:] The contained value is permanently stored within `*this`, so you
|
||||
should not hold nor delete this pointer
|
||||
* [*Example:]
|
||||
``
|
||||
T v;
|
||||
optional<T> opt(v);
|
||||
optional<T> const copt(v);
|
||||
T* p = opt.get_ptr() ;
|
||||
T const* cp = copt.get_ptr();
|
||||
assert ( p == get_pointer(opt) );
|
||||
assert ( cp == get_pointer(copt) ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_operator_arrow]
|
||||
|
||||
[: `T const* optional<T` ['(not a ref)]`>::operator ->() const ;`]
|
||||
[: `T* optional<T` ['(not a ref)]`>::operator ->() ;`]
|
||||
|
||||
* [*Requirements: ] `*this` is initialized.
|
||||
* [*Returns:] A pointer to the contained value.
|
||||
* [*Throws:] Nothing.
|
||||
* [*Notes:] The requirement is asserted via `BOOST_ASSERT()`.
|
||||
* [*Example:]
|
||||
``
|
||||
struct X { int mdata ; } ;
|
||||
X x ;
|
||||
optional<X> opt (x);
|
||||
opt->mdata = 2 ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_operator_bool]
|
||||
|
||||
[: `optional<T>::operator `['unspecified-bool-type]`() const ;`]
|
||||
|
||||
* [*Returns:] An unspecified value which if used on a boolean context
|
||||
is equivalent to (`get() != 0`)
|
||||
* [*Throws:] Nothing.
|
||||
* [*Example:]
|
||||
``
|
||||
optional<T> def ;
|
||||
assert ( def == 0 );
|
||||
optional<T> opt ( v ) ;
|
||||
assert ( opt );
|
||||
assert ( opt != 0 );
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_operator_not]
|
||||
|
||||
[: `bool optional<T>::operator!() ;`]
|
||||
|
||||
* [*Returns:] If `*this` is uninitialized, `true`; else `false`.
|
||||
* [*Throws:] Nothing.
|
||||
* [*Notes:] This operator is provided for those compilers which can't
|
||||
use the ['unspecified-bool-type operator] in certain boolean contexts.
|
||||
* [*Example:]
|
||||
``
|
||||
optional<T> opt ;
|
||||
assert ( !opt );
|
||||
*opt = some_T ;
|
||||
|
||||
// Notice the "double-bang" idiom here.
|
||||
assert ( !!opt ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_optional_is_initialized]
|
||||
|
||||
[: `bool optional<T>::is_initialized() const ;`]
|
||||
|
||||
* [*Returns: ] `true` if the `optional` is initialized, `false` otherwise.
|
||||
* [*Throws:] Nothing.
|
||||
* [*Example:]
|
||||
``
|
||||
optional<T> def ;
|
||||
assert ( !def.is_initialized() );
|
||||
optional<T> opt ( v ) ;
|
||||
assert ( opt.is_initialized() );
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[heading Free functions]
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_make_optional_value]
|
||||
|
||||
[: `optional<T` ['(not a ref)]`> make_optional( T const& v )`]
|
||||
|
||||
* [*Returns: ] `optional<T>(v)` for the ['deduced] type `T` of `v`.
|
||||
* [*Example:]
|
||||
``
|
||||
template<class T> void foo ( optional<T> const& opt ) ;
|
||||
|
||||
foo ( make_optional(1+1) ) ; // Creates an optional<int>
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_make_optional_bool_value]
|
||||
|
||||
[: `optional<T` ['(not a ref)]`> make_optional( bool condition, T const& v )`]
|
||||
|
||||
* [*Returns: ] `optional<T>(condition,v)` for the ['deduced] type `T` of `v`.
|
||||
* [*Example:]
|
||||
``
|
||||
optional<double> calculate_foo()
|
||||
{
|
||||
double val = compute_foo();
|
||||
return make_optional(is_not_nan_and_finite(val),val);
|
||||
}
|
||||
|
||||
optional<double> v = calculate_foo();
|
||||
if ( !v )
|
||||
error("foo wasn't computed");
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_operator_compare_equal_optional_optional]
|
||||
|
||||
[: `bool operator == ( optional<T> const& x, optional<T> const& y );`]
|
||||
|
||||
* [*Returns:] If both `x` and `y` are initialized, `(*x == *y)`. If only
|
||||
`x` or `y` is initialized, `false`. If both are uninitialized, `true`.
|
||||
* [*Throws:] Nothing.
|
||||
* [*Notes:] Pointers have shallow relational operators while `optional` has
|
||||
deep relational operators. Do not use `operator ==` directly in generic
|
||||
code which expect to be given either an `optional<T>` or a pointer; use
|
||||
__FUNCTION_EQUAL_POINTEES__ instead
|
||||
* [*Example:]
|
||||
``
|
||||
T x(12);
|
||||
T y(12);
|
||||
T z(21);
|
||||
optional<T> def0 ;
|
||||
optional<T> def1 ;
|
||||
optional<T> optX(x);
|
||||
optional<T> optY(y);
|
||||
optional<T> optZ(z);
|
||||
|
||||
// Identity always hold
|
||||
assert ( def0 == def0 );
|
||||
assert ( optX == optX );
|
||||
|
||||
// Both uninitialized compare equal
|
||||
assert ( def0 == def1 );
|
||||
|
||||
// Only one initialized compare unequal.
|
||||
assert ( def0 != optX );
|
||||
|
||||
// Both initialized compare as (*lhs == *rhs)
|
||||
assert ( optX == optY ) ;
|
||||
assert ( optX != optZ ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_operator_compare_less_optional_optional]
|
||||
|
||||
[: `bool operator < ( optional<T> const& x, optional<T> const& y );`]
|
||||
|
||||
* [*Returns:] If `y` is not initialized, `false`. If `y` is initialized
|
||||
and `x` is not initialized, `true`. If both `x` and `y` are initialized,
|
||||
`(*x < *y)`.
|
||||
* [*Throws:] Nothing.
|
||||
* [*Notes:] Pointers have shallow relational operators while `optional` has
|
||||
deep relational operators. Do not use `operator <` directly in generic code
|
||||
which expect to be given either an `optional<T>` or a pointer; use __FUNCTION_LESS_POINTEES__ instead.
|
||||
* [*Example:]
|
||||
``
|
||||
T x(12);
|
||||
T y(34);
|
||||
optional<T> def ;
|
||||
optional<T> optX(x);
|
||||
optional<T> optY(y);
|
||||
|
||||
// Identity always hold
|
||||
assert ( !(def < def) );
|
||||
assert ( optX == optX );
|
||||
|
||||
// Both uninitialized compare equal
|
||||
assert ( def0 == def1 );
|
||||
|
||||
// Only one initialized compare unequal.
|
||||
assert ( def0 != optX );
|
||||
|
||||
// Both initialized compare as (*lhs == *rhs)
|
||||
assert ( optX == optY ) ;
|
||||
assert ( optX != optZ ) ;
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_operator_compare_not_equal_optional_optional]
|
||||
|
||||
[: `bool operator != ( optional<T> const& x, optional<T> const& y );`]
|
||||
|
||||
* [*Returns: ] `!( x == y );`
|
||||
* [*Throws:] Nothing.
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_operator_compare_greater_optional_optional]
|
||||
|
||||
[: `bool operator > ( optional<T> const& x, optional<T> const& y );`]
|
||||
|
||||
* [*Returns: ] `( y < x );`
|
||||
* [*Throws:] Nothing.
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_operator_compare_less_or_equal_optional_optional]
|
||||
|
||||
[: `bool operator <= ( optional<T> const& x, optional<T> const& y );`]
|
||||
|
||||
* [*Returns: ] `!( y<x );`
|
||||
* [*Throws:] Nothing.
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_operator_compare_greater_or_equal_optional_optional]
|
||||
|
||||
[: `bool operator >= ( optional<T> const& x, optional<T> const& y );`]
|
||||
|
||||
* [*Returns: ] `!( x<y );`
|
||||
* [*Throws:] Nothing.
|
||||
|
||||
__SPACE__
|
||||
|
||||
[#reference_swap_optional_optional]
|
||||
|
||||
[: `void swap ( optional<T>& x, optional<T>& y );`]
|
||||
|
||||
* [*Effect:] If both `x` and `y` are initialized, calls `swap(*x,*y)`
|
||||
using `std::swap`. If only one is initialized, say `x`, calls:
|
||||
`y.reset(*x); x.reset();` If none is initialized, does nothing.
|
||||
* [*Postconditions:] The states of `x` and `y` interchanged.
|
||||
* [*Throws:] If both are initialized, whatever `swap(T&,T&)` throws. If only
|
||||
one is initialized, whatever `T::T ( T const& )` throws.
|
||||
* [*Notes:] If both are initialized, `swap(T&,T&)` is used unqualified but
|
||||
with `std::swap` introduced in scope.
|
||||
If only one is initialized, `T::~T()` and `T::T( T const& )` is called.
|
||||
* [*Exception Safety:] If both are initialized, this operation has the
|
||||
exception safety guarantees of `swap(T&,T&)`.
|
||||
If only one is initialized, it has the same basic guarantee as
|
||||
`optional<T>::reset( T const& )`.
|
||||
* [*Example:]
|
||||
``
|
||||
T x(12);
|
||||
T y(21);
|
||||
optional<T> def0 ;
|
||||
optional<T> def1 ;
|
||||
optional<T> optX(x);
|
||||
optional<T> optY(y);
|
||||
|
||||
boost::swap(def0,def1); // no-op
|
||||
|
||||
boost::swap(def0,optX);
|
||||
assert ( *def0 == x );
|
||||
assert ( !optX );
|
||||
|
||||
boost::swap(def0,optX); // Get back to original values
|
||||
|
||||
boost::swap(optX,optY);
|
||||
assert ( *optX == y );
|
||||
assert ( *optY == x );
|
||||
``
|
||||
|
||||
[endsect]
|
||||
372
doc/special_cases.qbk
Normal file
372
doc/special_cases.qbk
Normal file
@ -0,0 +1,372 @@
|
||||
|
||||
[section Optional references]
|
||||
|
||||
This library allows the template parameter `T` to be of reference type:
|
||||
`T&`, and to some extent, `T const&`.
|
||||
|
||||
However, since references are not real objects some restrictions apply and
|
||||
some operations are not available in this case:
|
||||
|
||||
* Converting constructors
|
||||
* Converting assignment
|
||||
* InPlace construction
|
||||
* InPlace assignment
|
||||
* Value-access via pointer
|
||||
|
||||
Also, even though `optional<T&>` treats it wrapped pseudo-object much as
|
||||
a real value, a true real reference is stored so aliasing will ocurr:
|
||||
|
||||
* Copies of `optional<T&>` will copy the references but all these references
|
||||
will nonetheless reefer to the same object.
|
||||
* Value-access will actually provide access to the referenced object
|
||||
rather than the reference itself.
|
||||
|
||||
[endsect]
|
||||
|
||||
[section Rebinding semantics for assignment of optional references]
|
||||
|
||||
If you assign to an ['uninitialized ] `optional<T&>` the effect is to bind (for
|
||||
the first time) to the object. Clearly, there is no other choice.
|
||||
|
||||
int x = 1 ;
|
||||
int& rx = x ;
|
||||
optional<int&> ora ;
|
||||
optional<int&> orb(x) ;
|
||||
ora = orb ; // now 'ora' is bound to 'x' through 'rx'
|
||||
*ora = 2 ; // Changes value of 'x' through 'ora'
|
||||
assert(x==2);
|
||||
|
||||
If you assign to a bare C++ reference, the assignment is forwarded to the
|
||||
referenced object; it's value changes but the reference is never rebound.
|
||||
|
||||
int a = 1 ;
|
||||
int& ra = a ;
|
||||
int b = 2 ;
|
||||
int& rb = b ;
|
||||
ra = rb ; // Changes the value of 'a' to 'b'
|
||||
assert(a==b);
|
||||
b = 3 ;
|
||||
assert(ra!=b); // 'ra' is not rebound to 'b'
|
||||
|
||||
Now, if you assign to an ['initialized ] `optional<T&>`, the effect is to
|
||||
[*rebind] to the new object instead of assigning the referee. This is unlike
|
||||
bare C++ references.
|
||||
|
||||
int a = 1 ;
|
||||
int b = 2 ;
|
||||
int& ra = a ;
|
||||
int& rb = b ;
|
||||
optional<int&> ora(ra) ;
|
||||
optional<int&> orb(rb) ;
|
||||
ora = orb ; // 'ora' is rebound to 'b'
|
||||
*ora = 3 ; // Changes value of 'b' (not 'a')
|
||||
assert(a==1);
|
||||
assert(b==3);
|
||||
|
||||
[heading Rationale]
|
||||
|
||||
Rebinding semantics for the assignment of ['initialized ] `optional` references has
|
||||
been chosen to provide [*consistency among initialization states] even at the
|
||||
expense of lack of consistency with the semantics of bare C++ references.
|
||||
It is true that `optional<U>` strives to behave as much as possible as `U`
|
||||
does whenever it is initialized; but in the case when `U` is `T&`, doing so would
|
||||
result in inconsistent behavior w.r.t to the lvalue initialization state.
|
||||
|
||||
Imagine `optional<T&>` forwarding assignment to the referenced object (thus
|
||||
changing the referenced object value but not rebinding), and consider the
|
||||
following code:
|
||||
|
||||
optional<int&> a = get();
|
||||
int x = 1 ;
|
||||
int& rx = x ;
|
||||
optional<int&> b(rx);
|
||||
a = b ;
|
||||
|
||||
What does the assignment do?
|
||||
|
||||
If `a` is ['uninitialized], the answer is clear: it binds to `x` (we now have
|
||||
another reference to `x`).
|
||||
But what if `a` is already ['initialized]? it would change the value of the
|
||||
referenced object (whatever that is); which is inconsistent with the other
|
||||
possible case.
|
||||
|
||||
If `optional<T&>` would assign just like `T&` does, you would never be able to
|
||||
use Optional's assignment without explicitly handling the previous
|
||||
initialization state unless your code is capable of functioning whether
|
||||
after the assignment, `a` aliases the same object as `b` or not.
|
||||
|
||||
That is, you would have to discriminate in order to be consistency.
|
||||
|
||||
If in your code rebinding to another object is not an option, then is very
|
||||
likely that binding for the fist time isn't either. In such case, assignment
|
||||
to an ['uninitialized ] `optional<T&>` shall be prohibited. It is quite possible
|
||||
that in such scenario the precondition that the lvalue must be already
|
||||
initialized exist. If it doesn't, then binding for the first time is OK
|
||||
while rebinding is not which is IMO very unlikely.
|
||||
In such scenario, you can assign the value itself directly, as in:
|
||||
|
||||
assert(!!opt);
|
||||
*opt=value;
|
||||
|
||||
[endsect]
|
||||
|
||||
[section In-Place Factories]
|
||||
|
||||
One of the typical problems with wrappers and containers is that their
|
||||
interfaces usually provide an operation to initialize or assign the
|
||||
contained object as a copy of some other object. This not only requires the
|
||||
underlying type to be __COPY_CONSTRUCTIBLE__, but also requires the existence of
|
||||
a fully constructed object, often temporary, just to follow the copy from:
|
||||
|
||||
struct X
|
||||
{
|
||||
X ( int, std:::string ) ;
|
||||
} ;
|
||||
|
||||
class W
|
||||
{
|
||||
X wrapped_ ;
|
||||
|
||||
public:
|
||||
|
||||
W ( X const& x ) : wrapped_(x) {}
|
||||
} ;
|
||||
|
||||
void foo()
|
||||
{
|
||||
// Temporary object created.
|
||||
W ( X(123,"hello") ) ;
|
||||
}
|
||||
|
||||
A solution to this problem is to support direct construction of the
|
||||
contained object right in the container's storage.
|
||||
In this scheme, the user only needs to supply the arguments to the
|
||||
constructor to use in the wrapped object construction.
|
||||
|
||||
class W
|
||||
{
|
||||
X wrapped_ ;
|
||||
|
||||
public:
|
||||
|
||||
W ( X const& x ) : wrapped_(x) {}
|
||||
W ( int a0, std::string a1) : wrapped_(a0,a1) {}
|
||||
} ;
|
||||
|
||||
void foo()
|
||||
{
|
||||
// Wrapped object constructed in-place
|
||||
// No temporary created.
|
||||
W (123,"hello") ;
|
||||
}
|
||||
|
||||
A limitation of this method is that it doesn't scale well to wrapped
|
||||
objects with multiple constructors nor to generic code were the constructor
|
||||
overloads are unknown.
|
||||
|
||||
The solution presented in this library is the family of [*InPlaceFactories]
|
||||
and [*TypedInPlaceFactories].
|
||||
These factories are a family of classes which encapsulate an increasing
|
||||
number of arbitrary constructor parameters and supply a method to construct
|
||||
an object of a given type using those parameters at an address specified by
|
||||
the user via placement new.
|
||||
|
||||
For example, one member of this family looks like:
|
||||
|
||||
template<class T,class A0, class A1>
|
||||
class TypedInPlaceFactory2
|
||||
{
|
||||
A0 m_a0 ; A1 m_a1 ;
|
||||
|
||||
public:
|
||||
|
||||
TypedInPlaceFactory2( A0 const& a0, A1 const& a1 ) : m_a0(a0), m_a1(a1) {}
|
||||
|
||||
void construct ( void* p ) { new (p) T(m_a0,m_a1) ; }
|
||||
} ;
|
||||
|
||||
A wrapper class aware of this can use it as:
|
||||
|
||||
class W
|
||||
{
|
||||
X wrapped_ ;
|
||||
|
||||
public:
|
||||
|
||||
W ( X const& x ) : wrapped_(x) {}
|
||||
W ( TypedInPlaceFactory2 const& fac ) { fac.construct(&wrapped_) ; }
|
||||
} ;
|
||||
|
||||
void foo()
|
||||
{
|
||||
// Wrapped object constructed in-place via a TypedInPlaceFactory.
|
||||
// No temporary created.
|
||||
W ( TypedInPlaceFactory2<X,int,std::string>(123,"hello")) ;
|
||||
}
|
||||
|
||||
The factories are divided in two groups:
|
||||
|
||||
* [_TypedInPlaceFactories]: those which take the target type as a primary
|
||||
template parameter.
|
||||
* [_InPlaceFactories]: those with a template `construct(void*)` member
|
||||
function taking the target type.
|
||||
|
||||
Within each group, all the family members differ only in the number of
|
||||
parameters allowed.
|
||||
|
||||
This library provides an overloaded set of helper template functions to
|
||||
construct these factories without requiring unnecessary template parameters:
|
||||
|
||||
template<class A0,...,class AN>
|
||||
InPlaceFactoryN <A0,...,AN> in_place ( A0 const& a0, ..., AN const& aN) ;
|
||||
|
||||
template<class T,class A0,...,class AN>
|
||||
TypedInPlaceFactoryN <T,A0,...,AN> in_place ( T const& a0, A0 const& a0, ..., AN const& aN) ;
|
||||
|
||||
In-place factories can be used generically by the wrapper and user as follows:
|
||||
|
||||
class W
|
||||
{
|
||||
X wrapped_ ;
|
||||
|
||||
public:
|
||||
|
||||
W ( X const& x ) : wrapped_(x) {}
|
||||
|
||||
template< class InPlaceFactory >
|
||||
W ( InPlaceFactory const& fac ) { fac.template <X>construct(&wrapped_) ; }
|
||||
|
||||
} ;
|
||||
|
||||
void foo()
|
||||
{
|
||||
// Wrapped object constructed in-place via a InPlaceFactory.
|
||||
// No temporary created.
|
||||
W ( in_place(123,"hello") ) ;
|
||||
}
|
||||
|
||||
The factories are implemented in the headers: __IN_PLACE_FACTORY_HPP__ and __TYPED_IN_PLACE_FACTORY_HPP__
|
||||
|
||||
[endsect]
|
||||
|
||||
[section A note about optional<bool>]
|
||||
|
||||
`optional<bool>` should be used with special caution and consideration.
|
||||
|
||||
First, it is functionally similar to a tristate boolean (false,maybe,true)
|
||||
—such as __BOOST_TRIBOOL__— except that in a tristate boolean, the maybe state
|
||||
[_represents a valid value], unlike the corresponding state of an uninitialized
|
||||
`optional<bool>`.
|
||||
It should be carefully considered if an `optional<bool>` instead of a `tribool`
|
||||
is really needed.
|
||||
|
||||
Second, `optional<>` provides an implicit conversion to `bool`. This
|
||||
conversion refers to the initialization state and not to the contained value.
|
||||
Using `optional<bool>` can lead to subtle errors due to the implicit `bool`
|
||||
conversion:
|
||||
|
||||
void foo ( bool v ) ;
|
||||
void bar()
|
||||
{
|
||||
optional<bool> v = try();
|
||||
|
||||
// The following intended to pass the value of 'v' to foo():
|
||||
foo(v);
|
||||
// But instead, the initialization state is passed
|
||||
// due to a typo: it should have been foo(*v).
|
||||
}
|
||||
|
||||
The only implicit conversion is to `bool`, and it is safe in the sense that
|
||||
typical integral promotions don't apply (i.e. if `foo()` takes an `int`
|
||||
instead, it won't compile).
|
||||
|
||||
[endsect]
|
||||
|
||||
[section Exception Safety Guarantees]
|
||||
|
||||
Because of the current implementation (see [link boost_optional.implementation_notes Implementation Notes]), all of the assignment methods:
|
||||
|
||||
* `optional<T>::operator= ( optional<T> const& )`
|
||||
* `optional<T>::operator= ( T const& )`
|
||||
* `template<class U> optional<T>::operator= ( optional<U> const& )`
|
||||
* `template<class InPlaceFactory> optional<T>::operator= ( InPlaceFactory const& )`
|
||||
* `template<class TypedInPlaceFactory> optional<T>::operator= ( TypedInPlaceFactory const& ) `
|
||||
* `optional<T>:::reset ( T const&)`
|
||||
|
||||
Can only ['guarantee] the [_basic exception safety]: The lvalue optional is
|
||||
left [_uninitialized] if an exception is thrown (any previous value is ['first]
|
||||
destroyed using `T::~T()`)
|
||||
|
||||
On the other hand, the ['uninitializing] methods:
|
||||
|
||||
* `optional<T>::operator= ( detail::none_t )`
|
||||
* `optional<T>::reset()`
|
||||
|
||||
Provide the no-throw guarantee (assuming a no-throw `T::~T()`)
|
||||
|
||||
However, since `optional<>` itself doesn't throw any exceptions, the only
|
||||
source for exceptions here are `T`'s constructor, so if you know the exception
|
||||
guarantees for `T::T ( T const& )`, you know that `optional`'s assignment and
|
||||
reset has the same guarantees.
|
||||
|
||||
//
|
||||
// Case 1: Exception thrown during assignment.
|
||||
//
|
||||
T v0(123);
|
||||
optional<T> opt0(v0);
|
||||
try
|
||||
{
|
||||
T v1(456);
|
||||
optional<T> opt1(v1);
|
||||
opt0 = opt1 ;
|
||||
|
||||
// If no exception was thrown, assignment succeeded.
|
||||
assert( *opt0 == v1 ) ;
|
||||
}
|
||||
catch(...)
|
||||
{
|
||||
// If any exception was thrown, 'opt0' is reset to uninitialized.
|
||||
assert( !opt0 ) ;
|
||||
}
|
||||
|
||||
//
|
||||
// Case 2: Exception thrown during reset(v)
|
||||
//
|
||||
T v0(123);
|
||||
optional<T> opt(v0);
|
||||
try
|
||||
{
|
||||
T v1(456);
|
||||
opt.reset ( v1 ) ;
|
||||
|
||||
// If no exception was thrown, reset succeeded.
|
||||
assert( *opt == v1 ) ;
|
||||
}
|
||||
catch(...)
|
||||
{
|
||||
// If any exception was thrown, 'opt' is reset to uninitialized.
|
||||
assert( !opt ) ;
|
||||
}
|
||||
|
||||
[heading Swap]
|
||||
|
||||
`void swap( optional<T>&, optional<T>& )` has the same exception guarantee
|
||||
as `swap(T&,T&)` when both optionals are initialized.
|
||||
If only one of the optionals is initialized, it gives the same ['basic]
|
||||
exception guarantee as `optional<T>::reset( T const& )` (since
|
||||
`optional<T>::reset()` doesn't throw).
|
||||
If none of the optionals is initialized, it has no-throw guarantee
|
||||
since it is a no-op.
|
||||
|
||||
[endsect]
|
||||
|
||||
[section Type requirements]
|
||||
|
||||
In general, `T` must be __COPY_CONSTRUCTIBLE__ and have a no-throw destructor.
|
||||
The copy-constructible requirement is not needed if [*InPlaceFactories] are used.
|
||||
|
||||
`T` [_is not] required to be __SGI_DEFAULT_CONSTRUCTIBLE__.
|
||||
|
||||
[endsect]
|
||||
|
||||
|
||||
28
include/boost/detail/none_t.hpp
Normal file
28
include/boost/detail/none_t.hpp
Normal file
@ -0,0 +1,28 @@
|
||||
// Copyright (C) 2003, Fernando Luis Cacciola Carballal.
|
||||
//
|
||||
// Use, modification, and distribution is subject to the Boost Software
|
||||
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
// http://www.boost.org/LICENSE_1_0.txt)
|
||||
//
|
||||
// See http://www.boost.org/lib/optional for documentation.
|
||||
//
|
||||
// You are welcome to contact the author at:
|
||||
// fernando_cacciola@hotmail.com
|
||||
//
|
||||
#ifndef BOOST_DETAIL_NONE_T_17SEP2003_HPP
|
||||
#define BOOST_DETAIL_NONE_T_17SEP2003_HPP
|
||||
|
||||
namespace boost {
|
||||
|
||||
namespace detail {
|
||||
|
||||
struct none_helper{};
|
||||
|
||||
typedef int none_helper::*none_t ;
|
||||
|
||||
} // namespace detail
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif
|
||||
|
||||
@ -1,5 +1,4 @@
|
||||
// Copyright (C) 2003, Fernando Luis Cacciola Carballal.
|
||||
// Copyright (C) 2014 Andrzej Krzemienski.
|
||||
//
|
||||
// Distributed under the Boost Software License, Version 1.0.
|
||||
// (See accompanying file LICENSE_1_0.txt or copy at
|
||||
@ -21,39 +20,9 @@
|
||||
|
||||
namespace boost {
|
||||
|
||||
#ifdef BOOST_OPTIONAL_USE_OLD_DEFINITION_OF_NONE
|
||||
|
||||
none_t const none = (static_cast<none_t>(0)) ;
|
||||
|
||||
#elif defined BOOST_OPTIONAL_USE_SINGLETON_DEFINITION_OF_NONE
|
||||
|
||||
namespace detail { namespace optional_detail {
|
||||
|
||||
// the trick here is to make boost::none defined once as a global but in a header file
|
||||
template <typename T>
|
||||
struct none_instance
|
||||
{
|
||||
static const T instance;
|
||||
};
|
||||
|
||||
template <typename T>
|
||||
const T none_instance<T>::instance = T(); // global, but because 'tis a template, no cpp file required
|
||||
|
||||
} } // namespace detail::optional_detail
|
||||
|
||||
|
||||
namespace {
|
||||
// TU-local
|
||||
const none_t& none = detail::optional_detail::none_instance<none_t>::instance;
|
||||
}
|
||||
|
||||
#else
|
||||
|
||||
const none_t none ((none_t::init_tag()));
|
||||
|
||||
#endif // older definitions
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif // header guard
|
||||
#endif
|
||||
|
||||
|
||||
@ -1,5 +1,4 @@
|
||||
// Copyright (C) 2003, Fernando Luis Cacciola Carballal.
|
||||
// Copyright (C) 2014 Andrzej Krzemienski.
|
||||
//
|
||||
// Use, modification, and distribution is subject to the Boost Software
|
||||
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
@ -15,26 +14,11 @@
|
||||
|
||||
namespace boost {
|
||||
|
||||
#ifdef BOOST_OPTIONAL_USE_OLD_DEFINITION_OF_NONE
|
||||
|
||||
namespace detail { struct none_helper{}; }
|
||||
|
||||
typedef int detail::none_helper::*none_t ;
|
||||
|
||||
#elif defined BOOST_OPTIONAL_USE_SINGLETON_DEFINITION_OF_NONE
|
||||
|
||||
class none_t {};
|
||||
|
||||
#else
|
||||
|
||||
struct none_t
|
||||
{
|
||||
struct init_tag{};
|
||||
explicit none_t(init_tag){} // to prevent default constructor
|
||||
};
|
||||
|
||||
#endif // old implementation workarounds
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif // header guard
|
||||
#endif
|
||||
|
||||
|
||||
@ -1,32 +0,0 @@
|
||||
// Copyright (C) 2014, Andrzej Krzemienski.
|
||||
//
|
||||
// Use, modification, and distribution is subject to the Boost Software
|
||||
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
// http://www.boost.org/LICENSE_1_0.txt)
|
||||
//
|
||||
// See http://www.boost.org/libs/optional for documentation.
|
||||
//
|
||||
// You are welcome to contact the author at:
|
||||
// akrzemi1@gmail.com
|
||||
//
|
||||
#ifndef BOOST_BAD_OPTIONAL_ACCESS_22MAY2014_HPP
|
||||
#define BOOST_BAD_OPTIONAL_ACCESS_22MAY2014_HPP
|
||||
|
||||
#include <stdexcept>
|
||||
#if __cplusplus < 201103L
|
||||
#include <string> // to make converting-ctor std::string(char const*) visible
|
||||
#endif
|
||||
|
||||
namespace boost {
|
||||
|
||||
class bad_optional_access : public std::logic_error
|
||||
{
|
||||
public:
|
||||
bad_optional_access()
|
||||
: std::logic_error("Attempted to access the value of an uninitialized optional object.")
|
||||
{}
|
||||
};
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif
|
||||
File diff suppressed because it is too large
Load Diff
@ -4,23 +4,22 @@
|
||||
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
// http://www.boost.org/LICENSE_1_0.txt)
|
||||
//
|
||||
// See http://www.boost.org/libs/optional for documentation.
|
||||
// See http://www.boost.org/lib/optional for documentation.
|
||||
//
|
||||
// You are welcome to contact the author at:
|
||||
// fernando_cacciola@hotmail.com
|
||||
//
|
||||
// Revisions:
|
||||
// 10 May 2008 (added swap related forward declaration) Niels Dekker
|
||||
//
|
||||
//
|
||||
#ifndef BOOST_OPTIONAL_OPTIONAL_FWD_FLC_19NOV2002_HPP
|
||||
#define BOOST_OPTIONAL_OPTIONAL_FWD_FLC_19NOV2002_HPP
|
||||
|
||||
|
||||
namespace boost {
|
||||
|
||||
template<class T> class optional ;
|
||||
|
||||
template<class T> void swap ( optional<T>& , optional<T>& );
|
||||
template<class T> void swap ( optional<T>& , optional<T>& ) ;
|
||||
|
||||
template<class T> struct optional_swap_should_use_default_constructor ;
|
||||
|
||||
|
||||
@ -4,7 +4,7 @@
|
||||
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
// http://www.boost.org/LICENSE_1_0.txt)
|
||||
//
|
||||
// See http://www.boost.org/libs/optional for documentation.
|
||||
// See http://www.boost.org/lib/optional for documentation.
|
||||
//
|
||||
// You are welcome to contact the author at:
|
||||
// fernando_cacciola@hotmail.com
|
||||
@ -12,37 +12,39 @@
|
||||
#ifndef BOOST_OPTIONAL_OPTIONAL_IO_FLC_19NOV2002_HPP
|
||||
#define BOOST_OPTIONAL_OPTIONAL_IO_FLC_19NOV2002_HPP
|
||||
|
||||
#include <istream>
|
||||
#include <ostream>
|
||||
#if defined __GNUC__
|
||||
# if (__GNUC__ == 2 && __GNUC_MINOR__ <= 97)
|
||||
# define BOOST_OPTIONAL_NO_TEMPLATED_STREAMS
|
||||
# endif
|
||||
#endif // __GNUC__
|
||||
|
||||
#if defined BOOST_OPTIONAL_NO_TEMPLATED_STREAMS
|
||||
# include <iostream>
|
||||
#else
|
||||
# include <istream>
|
||||
# include <ostream>
|
||||
#endif
|
||||
|
||||
|
||||
#include "boost/none.hpp"
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#include "boost/utility/value_init.hpp"
|
||||
|
||||
namespace boost
|
||||
{
|
||||
|
||||
template<class CharType, class CharTrait>
|
||||
inline
|
||||
std::basic_ostream<CharType, CharTrait>&
|
||||
operator<<(std::basic_ostream<CharType, CharTrait>& out, none_t)
|
||||
{
|
||||
if (out.good())
|
||||
{
|
||||
out << "--";
|
||||
}
|
||||
|
||||
return out;
|
||||
}
|
||||
|
||||
#if defined (BOOST_NO_TEMPLATED_STREAMS)
|
||||
template<class T>
|
||||
inline std::ostream& operator<<(std::ostream& out, optional<T> const& v)
|
||||
#else
|
||||
template<class CharType, class CharTrait, class T>
|
||||
inline
|
||||
std::basic_ostream<CharType, CharTrait>&
|
||||
operator<<(std::basic_ostream<CharType, CharTrait>& out, optional<T> const& v)
|
||||
#endif
|
||||
{
|
||||
if (out.good())
|
||||
if ( out.good() )
|
||||
{
|
||||
if (!v)
|
||||
if ( !v )
|
||||
out << "--" ;
|
||||
else out << ' ' << *v ;
|
||||
}
|
||||
@ -50,39 +52,27 @@ operator<<(std::basic_ostream<CharType, CharTrait>& out, optional<T> const& v)
|
||||
return out;
|
||||
}
|
||||
|
||||
#if defined (BOOST_NO_TEMPLATED_STREAMS)
|
||||
template<class T>
|
||||
inline std::istream& operator>>(std::istream& in, optional<T>& v)
|
||||
#else
|
||||
template<class CharType, class CharTrait, class T>
|
||||
inline
|
||||
std::basic_istream<CharType, CharTrait>&
|
||||
operator>>(std::basic_istream<CharType, CharTrait>& in, optional<T>& v)
|
||||
#endif
|
||||
{
|
||||
if (in.good())
|
||||
if ( in.good() )
|
||||
{
|
||||
int d = in.get();
|
||||
if (d == ' ')
|
||||
if ( d == ' ' )
|
||||
{
|
||||
T x;
|
||||
T x ;
|
||||
in >> x;
|
||||
#ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
|
||||
v = boost::move(x);
|
||||
#else
|
||||
v = x;
|
||||
#endif
|
||||
v = x ;
|
||||
}
|
||||
else
|
||||
{
|
||||
if (d == '-')
|
||||
{
|
||||
d = in.get();
|
||||
|
||||
if (d == '-')
|
||||
{
|
||||
v = none;
|
||||
return in;
|
||||
}
|
||||
}
|
||||
|
||||
in.setstate( std::ios::failbit );
|
||||
}
|
||||
v = optional<T>() ;
|
||||
}
|
||||
|
||||
return in;
|
||||
|
||||
@ -1,16 +0,0 @@
|
||||
{
|
||||
"key": "optional",
|
||||
"boost-version": "1.30.0",
|
||||
"name": "Optional",
|
||||
"authors": [
|
||||
"Fernando Cacciola"
|
||||
],
|
||||
"description": "A value-semantic, type-safe wrapper for representing 'optional' (or 'nullable') objects of a given type. An optional object may or may not contain a value of the underlying type.",
|
||||
"category": [
|
||||
"Data structures"
|
||||
],
|
||||
"maintainers": [
|
||||
"Fernando Cacciola <fernando_cacciola -at- ciudad.com.ar>",
|
||||
"Andrzej Krzemienski <akrzemi1 -at- gmail.com>"
|
||||
]
|
||||
}
|
||||
@ -1,16 +1,15 @@
|
||||
# Boost.Optional Library test Jamfile
|
||||
# Boost.Optional Library test Jamfile
|
||||
#
|
||||
# Copyright (C) 2003, Fernando Luis Cacciola Carballal.
|
||||
# Copyright (C) 2014, 2015 Andrzej Krzemienski
|
||||
# Copyright (C) 2003, Fernando Luis Cacciola Carballal.
|
||||
#
|
||||
# Use, modification, and distribution is subject to the Boost Software
|
||||
# License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
# http://www.boost.org/LICENSE_1_0.txt)
|
||||
# This material is provided "as is", with absolutely no warranty expressed
|
||||
# or implied. Any use is at your own risk.
|
||||
#
|
||||
# See http://www.boost.org/libs/optional for documentation.
|
||||
#
|
||||
# You are welcome to contact the author at:
|
||||
# akrzemi1@gmail.com
|
||||
# Permission to use or copy this software for any purpose is hereby granted
|
||||
# without fee, provided the above notices are retained on all copies.
|
||||
# Permission to modify the code and to distribute modified code is granted,
|
||||
# provided the above notices are retained, and a notice that the code was
|
||||
# modified is included with the above copyright notice.
|
||||
#
|
||||
|
||||
import testing ;
|
||||
@ -18,27 +17,10 @@ import testing ;
|
||||
{
|
||||
test-suite optional :
|
||||
[ run optional_test.cpp ]
|
||||
[ run optional_test_swap.cpp ]
|
||||
[ run optional_test_conversions_from_U.cpp ]
|
||||
[ run optional_test_tie.cpp ]
|
||||
[ run optional_test_ref_assign_portable_minimum.cpp ]
|
||||
[ run optional_test_ref_assign_mutable_int.cpp ]
|
||||
[ run optional_test_ref_assign_const_int.cpp ]
|
||||
[ run optional_test_ref_converting_ctor.cpp ]
|
||||
[ run optional_test_ref_convert_assign_non_int.cpp ]
|
||||
[ run optional_test_ref_convert_assign_mutable_int.cpp ]
|
||||
[ run optional_test_ref_convert_assign_const_int.cpp ]
|
||||
[ run optional_test_ref_portable_minimum.cpp ]
|
||||
[ run optional_test_ref_move.cpp ]
|
||||
[ run optional_test_inplace_factory.cpp ]
|
||||
[ run optional_test_ref.cpp ]
|
||||
[ run optional_test_inplace.cpp ]
|
||||
[ run optional_test_io.cpp ]
|
||||
[ run optional_test_move.cpp ]
|
||||
[ run optional_test_noexcept_move.cpp ]
|
||||
[ run optional_test_equals_none.cpp ]
|
||||
[ run optional_test_value_access.cpp ]
|
||||
[ run optional_test_emplace.cpp ]
|
||||
[ run optional_test_minimum_requirements.cpp ]
|
||||
[ run optional_test_msvc_bug_workaround.cpp ]
|
||||
[ compile-fail optional_test_fail1.cpp ]
|
||||
[ compile-fail optional_test_fail3a.cpp ]
|
||||
[ compile-fail optional_test_fail3b.cpp ]
|
||||
@ -47,18 +29,5 @@ import testing ;
|
||||
[ compile-fail optional_test_ref_fail4.cpp ]
|
||||
[ compile-fail optional_test_inplace_fail.cpp ]
|
||||
[ compile-fail optional_test_inplace_fail2.cpp ]
|
||||
[ compile-fail optional_test_fail_implicit_bool_convert.cpp ]
|
||||
[ compile-fail optional_test_fail_copying_a_moveable_type.cpp ]
|
||||
[ compile-fail optional_test_fail_optional_rvalue_ref.cpp ]
|
||||
[ compile-fail optional_test_ref_fail_init_from_Trefref.cpp ]
|
||||
[ compile-fail optional_test_ref_fail_init_from_Urefref.cpp ]
|
||||
[ compile-fail optional_test_ref_fail_assign_from_Trefref.cpp ]
|
||||
[ compile-fail optional_test_ref_fail_assign_from_Urefref.cpp ]
|
||||
[ compile-fail optional_test_fail_convert_from_null.cpp ]
|
||||
[ compile-fail optional_test_fail_explicit_convert_in_value_or.cpp ]
|
||||
[ compile-fail optional_test_fail_explicit_convert_in_value_or_call.cpp ]
|
||||
[ compile-fail optional_test_fail_io_without_io.cpp ]
|
||||
[ compile-fail optional_test_fail_none_io_without_io.cpp ]
|
||||
[ compile-fail optional_test_fail_convert_assign_of_enums.cpp ]
|
||||
;
|
||||
}
|
||||
|
||||
@ -1,197 +0,0 @@
|
||||
// Copyright (C) 2014 Andrzej Krzemienski.
|
||||
//
|
||||
// Use, modification, and distribution is subject to the Boost Software
|
||||
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
// http://www.boost.org/LICENSE_1_0.txt)
|
||||
//
|
||||
// See http://www.boost.org/lib/optional for documentation.
|
||||
//
|
||||
// You are welcome to contact the author at: akrzemi1@gmail.com
|
||||
|
||||
#ifndef BOOST_OPTIONAL_TEST_OPTIONAL_REF_ASSIGN_TEST_DEFS_AK_07JAN2015_HPP
|
||||
#define BOOST_OPTIONAL_TEST_OPTIONAL_REF_ASSIGN_TEST_DEFS_AK_07JAN2015_HPP
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/core/addressof.hpp"
|
||||
#include "testable_classes.hpp"
|
||||
|
||||
using boost::optional;
|
||||
using boost::none;
|
||||
using boost::addressof;
|
||||
|
||||
template <typename T>
|
||||
void test_copy_assignment_for_const()
|
||||
{
|
||||
const typename concrete_type_of<T>::type v(2);
|
||||
optional<const T&> o;
|
||||
o = optional<const T&>(v);
|
||||
|
||||
BOOST_TEST(o);
|
||||
BOOST_TEST(o != none);
|
||||
BOOST_TEST(addressof(*o) == addressof(v));
|
||||
BOOST_TEST(val(*o) == val(v));
|
||||
BOOST_TEST(val(*o) == 2);
|
||||
}
|
||||
|
||||
template <typename T>
|
||||
void test_copy_assignment_for_noconst_const()
|
||||
{
|
||||
typename concrete_type_of<T>::type v(2);
|
||||
optional<const T&> o;
|
||||
o = optional<const T&>(v);
|
||||
|
||||
BOOST_TEST(o);
|
||||
BOOST_TEST(o != none);
|
||||
BOOST_TEST(addressof(*o) == addressof(v));
|
||||
BOOST_TEST(val(*o) == val(v));
|
||||
BOOST_TEST(val(*o) == 2);
|
||||
|
||||
val(v) = 9;
|
||||
BOOST_TEST(addressof(*o) == addressof(v));
|
||||
BOOST_TEST_EQ(val(*o), val(v));
|
||||
BOOST_TEST_EQ(val(*o), 9);
|
||||
BOOST_TEST_EQ(val(v), 9);
|
||||
}
|
||||
|
||||
template <typename T>
|
||||
void test_copy_assignment_for()
|
||||
{
|
||||
typename concrete_type_of<T>::type v(2);
|
||||
optional<T&> o;
|
||||
o = optional<T&>(v);
|
||||
|
||||
BOOST_TEST(o);
|
||||
BOOST_TEST(o != none);
|
||||
BOOST_TEST(addressof(*o) == addressof(v));
|
||||
BOOST_TEST(val(*o) == val(v));
|
||||
BOOST_TEST(val(*o) == 2);
|
||||
|
||||
val(v) = 9;
|
||||
BOOST_TEST(addressof(*o) == addressof(v));
|
||||
BOOST_TEST_EQ(val(*o), val(v));
|
||||
BOOST_TEST_EQ(val(*o), 9);
|
||||
BOOST_TEST_EQ(val(v), 9);
|
||||
|
||||
val(*o) = 7;
|
||||
BOOST_TEST(addressof(*o) == addressof(v));
|
||||
BOOST_TEST_EQ(val(*o), val(v));
|
||||
BOOST_TEST_EQ(val(*o), 7);
|
||||
BOOST_TEST_EQ(val(v), 7);
|
||||
}
|
||||
|
||||
template <typename T>
|
||||
void test_rebinding_assignment_semantics_const()
|
||||
{
|
||||
const typename concrete_type_of<T>::type v(2), w(7);
|
||||
optional<const T&> o(v);
|
||||
|
||||
BOOST_TEST(o);
|
||||
BOOST_TEST(addressof(*o) == addressof(v));
|
||||
BOOST_TEST_EQ(val(*o), val(v));
|
||||
BOOST_TEST_EQ(val(*o), 2);
|
||||
|
||||
o = optional<const T&>(w);
|
||||
BOOST_TEST_EQ(val(v), 2);
|
||||
|
||||
BOOST_TEST(o);
|
||||
BOOST_TEST(addressof(*o) != addressof(v));
|
||||
BOOST_TEST_NE(val(*o), val(v));
|
||||
BOOST_TEST_NE(val(*o), 2);
|
||||
|
||||
BOOST_TEST(addressof(*o) == addressof(w));
|
||||
BOOST_TEST_EQ(val(*o), val(w));
|
||||
BOOST_TEST_EQ(val(*o), 7);
|
||||
}
|
||||
|
||||
template <typename T>
|
||||
void test_rebinding_assignment_semantics_noconst_const()
|
||||
{
|
||||
typename concrete_type_of<T>::type v(2), w(7);
|
||||
optional<const T&> o(v);
|
||||
|
||||
BOOST_TEST(o);
|
||||
BOOST_TEST(addressof(*o) == addressof(v));
|
||||
BOOST_TEST_EQ(val(*o), val(v));
|
||||
BOOST_TEST_EQ(val(*o), 2);
|
||||
|
||||
o = optional<const T&>(w);
|
||||
BOOST_TEST_EQ(val(v), 2);
|
||||
|
||||
BOOST_TEST(o);
|
||||
BOOST_TEST(addressof(*o) != addressof(v));
|
||||
BOOST_TEST_NE(val(*o), val(v));
|
||||
BOOST_TEST_NE(val(*o), 2);
|
||||
|
||||
BOOST_TEST(addressof(*o) == addressof(w));
|
||||
BOOST_TEST_EQ(val(*o), val(w));
|
||||
BOOST_TEST_EQ(val(*o), 7);
|
||||
}
|
||||
|
||||
template <typename T>
|
||||
void test_rebinding_assignment_semantics()
|
||||
{
|
||||
typename concrete_type_of<T>::type v(2), w(7);
|
||||
optional<T&> o(v);
|
||||
|
||||
BOOST_TEST(o);
|
||||
BOOST_TEST(addressof(*o) == addressof(v));
|
||||
BOOST_TEST_EQ(val(*o), val(v));
|
||||
BOOST_TEST_EQ(val(*o), 2);
|
||||
|
||||
o = optional<T&>(w);
|
||||
BOOST_TEST_EQ(val(v), 2);
|
||||
|
||||
BOOST_TEST(o);
|
||||
BOOST_TEST(addressof(*o) != addressof(v));
|
||||
BOOST_TEST_NE(val(*o), val(v));
|
||||
BOOST_TEST_NE(val(*o), 2);
|
||||
|
||||
BOOST_TEST(addressof(*o) == addressof(w));
|
||||
BOOST_TEST_EQ(val(*o), val(w));
|
||||
BOOST_TEST_EQ(val(*o), 7);
|
||||
|
||||
val(*o) = 8;
|
||||
BOOST_TEST(addressof(*o) == addressof(w));
|
||||
BOOST_TEST_EQ(val(*o), val(w));
|
||||
BOOST_TEST_EQ(val(*o), 8);
|
||||
BOOST_TEST_EQ(val(w), 8);
|
||||
BOOST_TEST_EQ(val(v), 2);
|
||||
}
|
||||
|
||||
template <typename T>
|
||||
void test_converting_assignment()
|
||||
{
|
||||
typename concrete_type_of<T>::type v1(1), v2(2), v3(3);
|
||||
optional<T&> oA(v1), oB(none);
|
||||
|
||||
oA = v2;
|
||||
BOOST_TEST(oA);
|
||||
BOOST_TEST(addressof(*oA) == addressof(v2));
|
||||
|
||||
oB = v3;
|
||||
BOOST_TEST(oB);
|
||||
BOOST_TEST(addressof(*oB) == addressof(v3));
|
||||
}
|
||||
|
||||
template <typename T>
|
||||
void test_converting_assignment_for_noconst_const()
|
||||
{
|
||||
typename concrete_type_of<T>::type v1(1), v2(2), v3(3);
|
||||
optional<const T&> oA(v1), oB(none);
|
||||
|
||||
oA = v2;
|
||||
BOOST_TEST(oA);
|
||||
BOOST_TEST(addressof(*oA) == addressof(v2));
|
||||
|
||||
oB = v3;
|
||||
BOOST_TEST(oB);
|
||||
BOOST_TEST(addressof(*oB) == addressof(v3));
|
||||
}
|
||||
|
||||
#endif //BOOST_OPTIONAL_TEST_OPTIONAL_REF_ASSIGN_TEST_DEFS_AK_07JAN2015_HPP
|
||||
|
||||
@ -30,7 +30,7 @@
|
||||
|
||||
#include "boost/none.hpp"
|
||||
|
||||
#include "boost/core/lightweight_test.hpp"
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
@ -46,12 +46,12 @@ void test_implicit_construction ( optional<X> opt, X const& v, X const& z )
|
||||
|
||||
void test_default_implicit_construction ( double, optional<double> opt )
|
||||
{
|
||||
BOOST_TEST(!opt);
|
||||
BOOST_CHECK(!opt);
|
||||
}
|
||||
|
||||
void test_default_implicit_construction ( X const&, optional<X> opt )
|
||||
{
|
||||
BOOST_TEST(!opt);
|
||||
BOOST_CHECK(!opt);
|
||||
}
|
||||
|
||||
//
|
||||
@ -158,27 +158,27 @@ void test_basics( T const* )
|
||||
ob.reset();
|
||||
check_is_pending_dtor( ARG(T) ) ;
|
||||
check_uninitialized(ob);
|
||||
|
||||
|
||||
}
|
||||
|
||||
template<class T>
|
||||
void test_conditional_ctor_and_get_valur_or ( T const* )
|
||||
{
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
|
||||
T a(321);
|
||||
|
||||
|
||||
T z(123);
|
||||
|
||||
|
||||
optional<T> const cdef0(false,a);
|
||||
|
||||
|
||||
optional<T> def0(false,a);
|
||||
optional<T> def1 = boost::make_optional(false,a); // T is not within boost so ADL won't find make_optional unqualified
|
||||
check_uninitialized(def0);
|
||||
check_uninitialized(def1);
|
||||
|
||||
optional<T> const co0(true,a);
|
||||
|
||||
|
||||
optional<T> o0(true,a);
|
||||
optional<T> o1 = boost::make_optional(true,a); // T is not within boost so ADL won't find make_optional unqualified
|
||||
|
||||
@ -186,67 +186,67 @@ void test_conditional_ctor_and_get_valur_or ( T const* )
|
||||
check_initialized(o1);
|
||||
check_value(o0,a,z);
|
||||
check_value(o1,a,z);
|
||||
|
||||
|
||||
T b = def0.get_value_or(z);
|
||||
BOOST_TEST( b == z ) ;
|
||||
|
||||
BOOST_CHECK( b == z ) ;
|
||||
|
||||
b = get_optional_value_or(def0,z);
|
||||
BOOST_TEST( b == z ) ;
|
||||
|
||||
BOOST_CHECK( b == z ) ;
|
||||
|
||||
b = o0.get_value_or(z);
|
||||
BOOST_TEST( b == a ) ;
|
||||
BOOST_CHECK( b == a ) ;
|
||||
|
||||
b = get_optional_value_or(o0,z);
|
||||
BOOST_TEST( b == a ) ;
|
||||
|
||||
|
||||
BOOST_CHECK( b == a ) ;
|
||||
|
||||
|
||||
T const& crz = z ;
|
||||
T& rz = z ;
|
||||
|
||||
|
||||
T const& crzz = def0.get_value_or(crz);
|
||||
BOOST_TEST( crzz == crz ) ;
|
||||
|
||||
BOOST_CHECK( crzz == crz ) ;
|
||||
|
||||
T& rzz = def0.get_value_or(rz);
|
||||
BOOST_TEST( rzz == rz ) ;
|
||||
|
||||
BOOST_CHECK( rzz == rz ) ;
|
||||
|
||||
T const& crzzz = get_optional_value_or(cdef0,crz);
|
||||
BOOST_TEST( crzzz == crz ) ;
|
||||
|
||||
BOOST_CHECK( crzzz == crz ) ;
|
||||
|
||||
T& rzzz = get_optional_value_or(def0,rz);
|
||||
BOOST_TEST( rzzz == rz ) ;
|
||||
|
||||
BOOST_CHECK( rzzz == rz ) ;
|
||||
|
||||
T const& crb = o0.get_value_or(crz);
|
||||
BOOST_TEST( crb == a ) ;
|
||||
|
||||
BOOST_CHECK( crb == a ) ;
|
||||
|
||||
T& rb = o0.get_value_or(rz);
|
||||
BOOST_TEST( rb == b ) ;
|
||||
|
||||
BOOST_CHECK( rb == b ) ;
|
||||
|
||||
T const& crbb = get_optional_value_or(co0,crz);
|
||||
BOOST_TEST( crbb == b ) ;
|
||||
|
||||
BOOST_CHECK( crbb == b ) ;
|
||||
|
||||
T const& crbbb = get_optional_value_or(o0,crz);
|
||||
BOOST_TEST( crbbb == b ) ;
|
||||
|
||||
BOOST_CHECK( crbbb == b ) ;
|
||||
|
||||
T& rbb = get_optional_value_or(o0,rz);
|
||||
BOOST_TEST( rbb == b ) ;
|
||||
|
||||
BOOST_CHECK( rbb == b ) ;
|
||||
|
||||
T& ra = a ;
|
||||
|
||||
|
||||
optional<T&> defref(false,ra);
|
||||
BOOST_TEST(!defref);
|
||||
|
||||
BOOST_CHECK(!defref);
|
||||
|
||||
optional<T&> ref(true,ra);
|
||||
BOOST_TEST(!!ref);
|
||||
|
||||
BOOST_CHECK(!!ref);
|
||||
|
||||
a = T(432);
|
||||
|
||||
BOOST_TEST( *ref == a ) ;
|
||||
|
||||
|
||||
BOOST_CHECK( *ref == a ) ;
|
||||
|
||||
T& r1 = defref.get_value_or(z);
|
||||
BOOST_TEST( r1 == z ) ;
|
||||
|
||||
BOOST_CHECK( r1 == z ) ;
|
||||
|
||||
T& r2 = ref.get_value_or(z);
|
||||
BOOST_TEST( r2 == a ) ;
|
||||
BOOST_CHECK( r2 == a ) ;
|
||||
}
|
||||
|
||||
//
|
||||
@ -262,18 +262,18 @@ void test_direct_value_manip( T const* )
|
||||
optional<T> const c_opt0(x) ;
|
||||
optional<T> opt0(x);
|
||||
|
||||
BOOST_TEST( c_opt0.get().V() == x.V() ) ;
|
||||
BOOST_TEST( opt0.get().V() == x.V() ) ;
|
||||
BOOST_CHECK( c_opt0.get().V() == x.V() ) ;
|
||||
BOOST_CHECK( opt0.get().V() == x.V() ) ;
|
||||
|
||||
BOOST_TEST( c_opt0->V() == x.V() ) ;
|
||||
BOOST_TEST( opt0->V() == x.V() ) ;
|
||||
BOOST_CHECK( c_opt0->V() == x.V() ) ;
|
||||
BOOST_CHECK( opt0->V() == x.V() ) ;
|
||||
|
||||
BOOST_TEST( (*c_opt0).V() == x.V() ) ;
|
||||
BOOST_TEST( (* opt0).V() == x.V() ) ;
|
||||
BOOST_CHECK( (*c_opt0).V() == x.V() ) ;
|
||||
BOOST_CHECK( (* opt0).V() == x.V() ) ;
|
||||
|
||||
T y(4);
|
||||
opt0 = y ;
|
||||
BOOST_TEST( get(opt0).V() == y.V() ) ;
|
||||
BOOST_CHECK( get(opt0).V() == y.V() ) ;
|
||||
}
|
||||
|
||||
//
|
||||
@ -291,45 +291,45 @@ void test_uninitialized_access( T const* )
|
||||
{
|
||||
// This should throw because 'def' is uninitialized
|
||||
T const& n = def.get() ;
|
||||
boost::ignore_unused(n);
|
||||
unused_variable(n);
|
||||
passed = true ;
|
||||
}
|
||||
catch (...) {}
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
|
||||
passed = false ;
|
||||
try
|
||||
{
|
||||
// This should throw because 'def' is uninitialized
|
||||
T const& n = *def ;
|
||||
boost::ignore_unused(n);
|
||||
unused_variable(n);
|
||||
passed = true ;
|
||||
}
|
||||
catch (...) {}
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
|
||||
passed = false ;
|
||||
try
|
||||
{
|
||||
T v(5) ;
|
||||
boost::ignore_unused(v);
|
||||
unused_variable(v);
|
||||
// This should throw because 'def' is uninitialized
|
||||
*def = v ;
|
||||
passed = true ;
|
||||
}
|
||||
catch (...) {}
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
|
||||
passed = false ;
|
||||
try
|
||||
{
|
||||
// This should throw because 'def' is uninitialized
|
||||
T v = *(def.operator->()) ;
|
||||
boost::ignore_unused(v);
|
||||
unused_variable(v);
|
||||
passed = true ;
|
||||
}
|
||||
catch (...) {}
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
}
|
||||
|
||||
#if BOOST_WORKAROUND( BOOST_INTEL_CXX_VERSION, <= 700) // Intel C++ 7.0
|
||||
@ -374,7 +374,7 @@ void test_throwing_direct_init( T const* )
|
||||
}
|
||||
catch ( ... ){}
|
||||
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
check_is_not_pending_copy( ARG(T) );
|
||||
check_instance_count(count, ARG(T) );
|
||||
|
||||
@ -410,7 +410,7 @@ void test_throwing_val_assign_on_uninitialized( T const* )
|
||||
}
|
||||
catch ( ... ) {}
|
||||
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
|
||||
check_is_not_pending_copy( ARG(T) );
|
||||
check_instance_count(count, ARG(T) );
|
||||
@ -456,7 +456,7 @@ void test_throwing_val_assign_on_initialized( T const* )
|
||||
}
|
||||
catch ( ... ) {}
|
||||
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
|
||||
check_is_not_pending_assign( ARG(T) );
|
||||
check_instance_count(count, ARG(T) );
|
||||
@ -495,7 +495,7 @@ void test_throwing_copy_initialization( T const* )
|
||||
}
|
||||
catch ( ... ) {}
|
||||
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
|
||||
check_is_not_pending_copy( ARG(T) );
|
||||
check_instance_count(count, ARG(T) );
|
||||
@ -538,7 +538,7 @@ void test_throwing_assign_to_uninitialized( T const* )
|
||||
}
|
||||
catch ( ... ) {}
|
||||
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
|
||||
check_is_not_pending_copy( ARG(T) );
|
||||
check_instance_count(count, ARG(T) );
|
||||
@ -581,7 +581,7 @@ void test_throwing_assign_to_initialized( T const* )
|
||||
}
|
||||
catch ( ... ) {}
|
||||
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
|
||||
// opt0 was left uninitialized
|
||||
check_is_not_pending_assign( ARG(T) );
|
||||
@ -661,7 +661,7 @@ void test_throwing_swap( T const* )
|
||||
}
|
||||
catch ( ... ) {}
|
||||
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
|
||||
// optional's swap doesn't affect the initialized states of the arguments. Therefore,
|
||||
// the following must hold:
|
||||
@ -692,7 +692,7 @@ void test_throwing_swap( T const* )
|
||||
}
|
||||
catch ( ... ) {}
|
||||
|
||||
BOOST_TEST(!passed);
|
||||
BOOST_CHECK(!passed);
|
||||
|
||||
check_uninitialized(opt0);
|
||||
check_initialized(opt1);
|
||||
@ -718,71 +718,71 @@ void test_relops( T const* )
|
||||
optional<T> opt0(v0);
|
||||
optional<T> opt1(v1);
|
||||
optional<T> opt2(v2);
|
||||
|
||||
|
||||
// Check identity
|
||||
BOOST_TEST ( def0 == def0 ) ;
|
||||
BOOST_TEST ( opt0 == opt0 ) ;
|
||||
BOOST_TEST ( !(def0 != def0) ) ;
|
||||
BOOST_TEST ( !(opt0 != opt0) ) ;
|
||||
BOOST_CHECK ( def0 == def0 ) ;
|
||||
BOOST_CHECK ( opt0 == opt0 ) ;
|
||||
BOOST_CHECK ( !(def0 != def0) ) ;
|
||||
BOOST_CHECK ( !(opt0 != opt0) ) ;
|
||||
|
||||
// Check when both are uininitalized.
|
||||
BOOST_TEST ( def0 == def1 ) ; // both uninitialized compare equal
|
||||
BOOST_TEST ( !(def0 < def1) ) ; // uninitialized is never less than uninitialized
|
||||
BOOST_TEST ( !(def0 > def1) ) ; // uninitialized is never greater than uninitialized
|
||||
BOOST_TEST ( !(def0 != def1) ) ;
|
||||
BOOST_TEST ( def0 <= def1 ) ;
|
||||
BOOST_TEST ( def0 >= def1 ) ;
|
||||
BOOST_CHECK ( def0 == def1 ) ; // both uninitialized compare equal
|
||||
BOOST_CHECK ( !(def0 < def1) ) ; // uninitialized is never less than uninitialized
|
||||
BOOST_CHECK ( !(def0 > def1) ) ; // uninitialized is never greater than uninitialized
|
||||
BOOST_CHECK ( !(def0 != def1) ) ;
|
||||
BOOST_CHECK ( def0 <= def1 ) ;
|
||||
BOOST_CHECK ( def0 >= def1 ) ;
|
||||
|
||||
// Check when only lhs is uninitialized.
|
||||
BOOST_TEST ( def0 != opt0 ) ; // uninitialized is never equal to initialized
|
||||
BOOST_TEST ( !(def0 == opt0) ) ;
|
||||
BOOST_TEST ( def0 < opt0 ) ; // uninitialized is always less than initialized
|
||||
BOOST_TEST ( !(def0 > opt0) ) ;
|
||||
BOOST_TEST ( def0 <= opt0 ) ;
|
||||
BOOST_TEST ( !(def0 >= opt0) ) ;
|
||||
BOOST_CHECK ( def0 != opt0 ) ; // uninitialized is never equal to initialized
|
||||
BOOST_CHECK ( !(def0 == opt0) ) ;
|
||||
BOOST_CHECK ( def0 < opt0 ) ; // uninitialized is always less than initialized
|
||||
BOOST_CHECK ( !(def0 > opt0) ) ;
|
||||
BOOST_CHECK ( def0 <= opt0 ) ;
|
||||
BOOST_CHECK ( !(def0 >= opt0) ) ;
|
||||
|
||||
// Check when only rhs is uninitialized.
|
||||
BOOST_TEST ( opt0 != def0 ) ; // initialized is never equal to uninitialized
|
||||
BOOST_TEST ( !(opt0 == def0) ) ;
|
||||
BOOST_TEST ( !(opt0 < def0) ) ; // initialized is never less than uninitialized
|
||||
BOOST_TEST ( opt0 > def0 ) ;
|
||||
BOOST_TEST ( !(opt0 <= def0) ) ;
|
||||
BOOST_TEST ( opt0 >= opt0 ) ;
|
||||
BOOST_CHECK ( opt0 != def0 ) ; // initialized is never equal to uninitialized
|
||||
BOOST_CHECK ( !(opt0 == def0) ) ;
|
||||
BOOST_CHECK ( !(opt0 < def0) ) ; // initialized is never less than uninitialized
|
||||
BOOST_CHECK ( opt0 > def0 ) ;
|
||||
BOOST_CHECK ( !(opt0 <= def0) ) ;
|
||||
BOOST_CHECK ( opt0 >= opt0 ) ;
|
||||
|
||||
// If both are initialized, values are compared
|
||||
BOOST_TEST ( opt0 != opt1 ) ;
|
||||
BOOST_TEST ( opt1 == opt2 ) ;
|
||||
BOOST_TEST ( opt0 < opt1 ) ;
|
||||
BOOST_TEST ( opt1 > opt0 ) ;
|
||||
BOOST_TEST ( opt1 <= opt2 ) ;
|
||||
BOOST_TEST ( opt1 >= opt0 ) ;
|
||||
|
||||
BOOST_CHECK ( opt0 != opt1 ) ;
|
||||
BOOST_CHECK ( opt1 == opt2 ) ;
|
||||
BOOST_CHECK ( opt0 < opt1 ) ;
|
||||
BOOST_CHECK ( opt1 > opt0 ) ;
|
||||
BOOST_CHECK ( opt1 <= opt2 ) ;
|
||||
BOOST_CHECK ( opt1 >= opt0 ) ;
|
||||
|
||||
// Compare against a value directly
|
||||
BOOST_TEST ( opt0 == v0 ) ;
|
||||
BOOST_TEST ( opt0 != v1 ) ;
|
||||
BOOST_TEST ( opt1 == v2 ) ;
|
||||
BOOST_TEST ( opt0 < v1 ) ;
|
||||
BOOST_TEST ( opt1 > v0 ) ;
|
||||
BOOST_TEST ( opt1 <= v2 ) ;
|
||||
BOOST_TEST ( opt1 >= v0 ) ;
|
||||
BOOST_TEST ( v0 != opt1 ) ;
|
||||
BOOST_TEST ( v1 == opt2 ) ;
|
||||
BOOST_TEST ( v0 < opt1 ) ;
|
||||
BOOST_TEST ( v1 > opt0 ) ;
|
||||
BOOST_TEST ( v1 <= opt2 ) ;
|
||||
BOOST_TEST ( v1 >= opt0 ) ;
|
||||
BOOST_TEST ( def0 != v0 ) ;
|
||||
BOOST_TEST ( !(def0 == v0) ) ;
|
||||
BOOST_TEST ( def0 < v0 ) ;
|
||||
BOOST_TEST ( !(def0 > v0) ) ;
|
||||
BOOST_TEST ( def0 <= v0 ) ;
|
||||
BOOST_TEST ( !(def0 >= v0) ) ;
|
||||
BOOST_TEST ( v0 != def0 ) ;
|
||||
BOOST_TEST ( !(v0 == def0) ) ;
|
||||
BOOST_TEST ( !(v0 < def0) ) ;
|
||||
BOOST_TEST ( v0 > def0 ) ;
|
||||
BOOST_TEST ( !(v0 <= def0) ) ;
|
||||
BOOST_TEST ( v0 >= opt0 ) ;
|
||||
BOOST_CHECK ( opt0 == v0 ) ;
|
||||
BOOST_CHECK ( opt0 != v1 ) ;
|
||||
BOOST_CHECK ( opt1 == v2 ) ;
|
||||
BOOST_CHECK ( opt0 < v1 ) ;
|
||||
BOOST_CHECK ( opt1 > v0 ) ;
|
||||
BOOST_CHECK ( opt1 <= v2 ) ;
|
||||
BOOST_CHECK ( opt1 >= v0 ) ;
|
||||
BOOST_CHECK ( v0 != opt1 ) ;
|
||||
BOOST_CHECK ( v1 == opt2 ) ;
|
||||
BOOST_CHECK ( v0 < opt1 ) ;
|
||||
BOOST_CHECK ( v1 > opt0 ) ;
|
||||
BOOST_CHECK ( v1 <= opt2 ) ;
|
||||
BOOST_CHECK ( v1 >= opt0 ) ;
|
||||
BOOST_CHECK ( def0 != v0 ) ;
|
||||
BOOST_CHECK ( !(def0 == v0) ) ;
|
||||
BOOST_CHECK ( def0 < v0 ) ;
|
||||
BOOST_CHECK ( !(def0 > v0) ) ;
|
||||
BOOST_CHECK ( def0 <= v0 ) ;
|
||||
BOOST_CHECK ( !(def0 >= v0) ) ;
|
||||
BOOST_CHECK ( v0 != def0 ) ;
|
||||
BOOST_CHECK ( !(v0 == def0) ) ;
|
||||
BOOST_CHECK ( !(v0 < def0) ) ;
|
||||
BOOST_CHECK ( v0 > def0 ) ;
|
||||
BOOST_CHECK ( !(v0 <= def0) ) ;
|
||||
BOOST_CHECK ( v0 >= opt0 ) ;
|
||||
}
|
||||
|
||||
template<class T>
|
||||
@ -796,16 +796,16 @@ void test_none( T const* )
|
||||
optional<T> def1(none) ;
|
||||
optional<T> non_def( T(1234) ) ;
|
||||
|
||||
BOOST_TEST ( def0 == none ) ;
|
||||
BOOST_TEST ( non_def != none ) ;
|
||||
BOOST_TEST ( !def1 ) ;
|
||||
BOOST_TEST ( !(non_def < none) ) ;
|
||||
BOOST_TEST ( non_def > none ) ;
|
||||
BOOST_TEST ( !(non_def <= none) ) ;
|
||||
BOOST_TEST ( non_def >= none ) ;
|
||||
BOOST_CHECK ( def0 == none ) ;
|
||||
BOOST_CHECK ( non_def != none ) ;
|
||||
BOOST_CHECK ( !def1 ) ;
|
||||
BOOST_CHECK ( !(non_def < none) ) ;
|
||||
BOOST_CHECK ( non_def > none ) ;
|
||||
BOOST_CHECK ( !(non_def <= none) ) ;
|
||||
BOOST_CHECK ( non_def >= none ) ;
|
||||
|
||||
non_def = none ;
|
||||
BOOST_TEST ( !non_def ) ;
|
||||
BOOST_CHECK ( !non_def ) ;
|
||||
|
||||
test_default_implicit_construction(T(1),none);
|
||||
}
|
||||
@ -819,13 +819,13 @@ void test_arrow( T const* )
|
||||
|
||||
optional<T> oa(a) ;
|
||||
optional<T> const coa(a) ;
|
||||
|
||||
BOOST_TEST ( coa->V() == 1234 ) ;
|
||||
|
||||
|
||||
BOOST_CHECK ( coa->V() == 1234 ) ;
|
||||
|
||||
oa->V() = 4321 ;
|
||||
|
||||
BOOST_TEST ( a.V() = 1234 ) ;
|
||||
BOOST_TEST ( (*oa).V() = 4321 ) ;
|
||||
|
||||
BOOST_CHECK ( a.V() = 1234 ) ;
|
||||
BOOST_CHECK ( (*oa).V() = 4321 ) ;
|
||||
}
|
||||
|
||||
void test_with_builtin_types()
|
||||
@ -840,21 +840,11 @@ void test_with_builtin_types()
|
||||
test_none( ARG(double) ) ;
|
||||
}
|
||||
|
||||
// MSVC < 11.0 doesn't destroy X when we call ptr->VBase::VBase.
|
||||
// Make sure that we work around this bug.
|
||||
struct VBase : virtual X
|
||||
{
|
||||
VBase(int v) : X(v) {}
|
||||
// MSVC 8.0 doesn't generate this correctly...
|
||||
VBase(const VBase& other) : X(static_cast<const X&>(other)) {}
|
||||
};
|
||||
|
||||
void test_with_class_type()
|
||||
{
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
test_basics( ARG(X) );
|
||||
test_basics( ARG(VBase) );
|
||||
test_conditional_ctor_and_get_valur_or( ARG(X) );
|
||||
test_direct_value_manip( ARG(X) );
|
||||
test_uninitialized_access( ARG(X) );
|
||||
@ -869,7 +859,7 @@ void test_with_class_type()
|
||||
test_relops( ARG(X) ) ;
|
||||
test_none( ARG(X) ) ;
|
||||
test_arrow( ARG(X) ) ;
|
||||
BOOST_TEST ( X::count == 0 ) ;
|
||||
BOOST_CHECK ( X::count == 0 ) ;
|
||||
}
|
||||
|
||||
int eat ( bool ) { return 1 ; }
|
||||
@ -888,7 +878,7 @@ void test_no_implicit_conversions_impl( T const& )
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
optional<T> def ;
|
||||
BOOST_TEST ( eat(def) == 0 ) ;
|
||||
BOOST_CHECK ( eat(def) == 0 ) ;
|
||||
}
|
||||
|
||||
void test_no_implicit_conversions()
|
||||
@ -906,49 +896,390 @@ void test_no_implicit_conversions()
|
||||
test_no_implicit_conversions_impl(p);
|
||||
}
|
||||
|
||||
|
||||
// Test for support for classes with overridden operator&
|
||||
class CustomAddressOfClass
|
||||
struct A {} ;
|
||||
void test_conversions1()
|
||||
{
|
||||
int n;
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
public:
|
||||
CustomAddressOfClass() : n(0) {}
|
||||
CustomAddressOfClass(CustomAddressOfClass const& that) : n(that.n) {}
|
||||
explicit CustomAddressOfClass(int m) : n(m) {}
|
||||
int* operator& () { return &n; }
|
||||
bool operator== (CustomAddressOfClass const& that) const { return n == that.n; }
|
||||
};
|
||||
#ifndef BOOST_OPTIONAL_NO_CONVERTING_COPY_CTOR
|
||||
char c = 20 ;
|
||||
optional<char> opt0(c);
|
||||
optional<int> opt1(opt0);
|
||||
BOOST_CHECK(*opt1 == static_cast<int>(c));
|
||||
#endif
|
||||
|
||||
void test_custom_addressof_operator()
|
||||
{
|
||||
boost::optional< CustomAddressOfClass > o1(CustomAddressOfClass(10));
|
||||
BOOST_TEST(!!o1);
|
||||
BOOST_TEST(o1.get() == CustomAddressOfClass(10));
|
||||
|
||||
o1 = CustomAddressOfClass(20);
|
||||
BOOST_TEST(!!o1);
|
||||
BOOST_TEST(o1.get() == CustomAddressOfClass(20));
|
||||
|
||||
o1 = boost::none;
|
||||
BOOST_TEST(!o1);
|
||||
#ifndef BOOST_OPTIONAL_NO_CONVERTING_ASSIGNMENT
|
||||
float f = 21.22f ;
|
||||
double d = f ;
|
||||
optional<float> opt2(f) ;
|
||||
optional<double> opt3 ;
|
||||
opt3 = opt2 ;
|
||||
BOOST_CHECK(*opt3 == d);
|
||||
#endif
|
||||
}
|
||||
|
||||
int main()
|
||||
void test_conversions2()
|
||||
{
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
char c = 20 ;
|
||||
optional<int> opt(c);
|
||||
BOOST_CHECK( get(opt) == static_cast<int>(c));
|
||||
|
||||
float f = 21.22f ;
|
||||
optional<double> opt1;
|
||||
opt1 = f ;
|
||||
BOOST_CHECK(*get(&opt1) == static_cast<double>(f));
|
||||
}
|
||||
|
||||
|
||||
namespace optional_swap_test
|
||||
{
|
||||
class default_ctor_exception : public std::exception {} ;
|
||||
class copy_ctor_exception : public std::exception {} ;
|
||||
class assignment_exception : public std::exception {} ;
|
||||
|
||||
//
|
||||
// Base class for swap test classes. Its assignment should not be called, when swapping
|
||||
// optional<T> objects. (The default std::swap would do so.)
|
||||
//
|
||||
class base_class_with_forbidden_assignment
|
||||
{
|
||||
public:
|
||||
base_class_with_forbidden_assignment & operator=(const base_class_with_forbidden_assignment &)
|
||||
{
|
||||
BOOST_CHECK(!"The assignment should not be used while swapping!");
|
||||
throw assignment_exception();
|
||||
}
|
||||
|
||||
virtual ~base_class_with_forbidden_assignment() {}
|
||||
};
|
||||
|
||||
//
|
||||
// Class without default constructor
|
||||
//
|
||||
class class_without_default_ctor : public base_class_with_forbidden_assignment
|
||||
{
|
||||
public:
|
||||
char data;
|
||||
explicit class_without_default_ctor(char arg) : data(arg) {}
|
||||
};
|
||||
|
||||
//
|
||||
// Class whose default constructor should not be used by optional::swap!
|
||||
//
|
||||
class class_whose_default_ctor_should_not_be_used : public base_class_with_forbidden_assignment
|
||||
{
|
||||
public:
|
||||
char data;
|
||||
explicit class_whose_default_ctor_should_not_be_used(char arg) : data(arg) {}
|
||||
|
||||
class_whose_default_ctor_should_not_be_used()
|
||||
{
|
||||
BOOST_CHECK(!"This default constructor should not be used while swapping!");
|
||||
throw default_ctor_exception();
|
||||
}
|
||||
};
|
||||
|
||||
//
|
||||
// Class whose default constructor should be used by optional::swap.
|
||||
// Its copy constructor should be avoided!
|
||||
//
|
||||
class class_whose_default_ctor_should_be_used : public base_class_with_forbidden_assignment
|
||||
{
|
||||
public:
|
||||
char data;
|
||||
explicit class_whose_default_ctor_should_be_used(char arg) : data(arg) { }
|
||||
|
||||
class_whose_default_ctor_should_be_used() : data('\0') { }
|
||||
|
||||
class_whose_default_ctor_should_be_used(const class_whose_default_ctor_should_be_used &)
|
||||
{
|
||||
BOOST_CHECK(!"This copy constructor should not be used while swapping!");
|
||||
throw copy_ctor_exception();
|
||||
}
|
||||
};
|
||||
|
||||
//
|
||||
// Class template whose default constructor should be used by optional::swap.
|
||||
// Its copy constructor should be avoided!
|
||||
//
|
||||
template <class T>
|
||||
class template_whose_default_ctor_should_be_used : public base_class_with_forbidden_assignment
|
||||
{
|
||||
public:
|
||||
T data;
|
||||
explicit template_whose_default_ctor_should_be_used(T arg) : data(arg) { }
|
||||
|
||||
template_whose_default_ctor_should_be_used() : data('\0') { }
|
||||
|
||||
template_whose_default_ctor_should_be_used(const template_whose_default_ctor_should_be_used &)
|
||||
{
|
||||
BOOST_CHECK(!"This copy constructor should not be used while swapping!");
|
||||
throw copy_ctor_exception();
|
||||
}
|
||||
};
|
||||
|
||||
//
|
||||
// Class whose explicit constructor should be used by optional::swap.
|
||||
// Its other constructors should be avoided!
|
||||
//
|
||||
class class_whose_explicit_ctor_should_be_used : public base_class_with_forbidden_assignment
|
||||
{
|
||||
public:
|
||||
char data;
|
||||
explicit class_whose_explicit_ctor_should_be_used(char arg) : data(arg) { }
|
||||
|
||||
class_whose_explicit_ctor_should_be_used()
|
||||
{
|
||||
BOOST_CHECK(!"This default constructor should not be used while swapping!");
|
||||
throw default_ctor_exception();
|
||||
}
|
||||
|
||||
class_whose_explicit_ctor_should_be_used(const class_whose_explicit_ctor_should_be_used &)
|
||||
{
|
||||
BOOST_CHECK(!"This copy constructor should not be used while swapping!");
|
||||
throw copy_ctor_exception();
|
||||
}
|
||||
};
|
||||
|
||||
void swap(class_whose_default_ctor_should_not_be_used & lhs, class_whose_default_ctor_should_not_be_used & rhs)
|
||||
{
|
||||
std::swap(lhs.data, rhs.data);
|
||||
}
|
||||
|
||||
void swap(class_whose_default_ctor_should_be_used & lhs, class_whose_default_ctor_should_be_used & rhs)
|
||||
{
|
||||
std::swap(lhs.data, rhs.data);
|
||||
}
|
||||
|
||||
void swap(class_without_default_ctor & lhs, class_without_default_ctor & rhs)
|
||||
{
|
||||
std::swap(lhs.data, rhs.data);
|
||||
}
|
||||
|
||||
void swap(class_whose_explicit_ctor_should_be_used & lhs, class_whose_explicit_ctor_should_be_used & rhs)
|
||||
{
|
||||
std::swap(lhs.data, rhs.data);
|
||||
}
|
||||
|
||||
template <class T>
|
||||
void swap(template_whose_default_ctor_should_be_used<T> & lhs, template_whose_default_ctor_should_be_used<T> & rhs)
|
||||
{
|
||||
std::swap(lhs.data, rhs.data);
|
||||
}
|
||||
|
||||
//
|
||||
// optional<T>::swap should be customized when neither the copy constructor
|
||||
// nor the default constructor of T are supposed to be used when swapping, e.g.,
|
||||
// for the following type T = class_whose_explicit_ctor_should_be_used.
|
||||
//
|
||||
void swap(boost::optional<class_whose_explicit_ctor_should_be_used> & x, boost::optional<class_whose_explicit_ctor_should_be_used> & y)
|
||||
{
|
||||
bool hasX = x;
|
||||
bool hasY = y;
|
||||
|
||||
if ( !hasX && !hasY )
|
||||
return;
|
||||
|
||||
if( !hasX )
|
||||
x = boost::in_place('\0');
|
||||
else if ( !hasY )
|
||||
y = boost::in_place('\0');
|
||||
|
||||
optional_swap_test::swap(*x,*y);
|
||||
|
||||
if( !hasX )
|
||||
y = boost::none ;
|
||||
else if( !hasY )
|
||||
x = boost::none ;
|
||||
}
|
||||
|
||||
|
||||
} // End of namespace optional_swap_test.
|
||||
|
||||
|
||||
namespace boost {
|
||||
|
||||
//
|
||||
// Compile time tweaking on whether or not swap should use the default constructor:
|
||||
//
|
||||
|
||||
template <> struct optional_swap_should_use_default_constructor<
|
||||
optional_swap_test::class_whose_default_ctor_should_be_used> : mpl::true_ {} ;
|
||||
|
||||
template <> struct optional_swap_should_use_default_constructor<
|
||||
optional_swap_test::class_whose_default_ctor_should_not_be_used> : mpl::false_ {} ;
|
||||
|
||||
template <class T> struct optional_swap_should_use_default_constructor<
|
||||
optional_swap_test::template_whose_default_ctor_should_be_used<T> > : mpl::true_ {} ;
|
||||
|
||||
|
||||
//
|
||||
// Specialization of boost::swap:
|
||||
//
|
||||
template <>
|
||||
void swap(optional<optional_swap_test::class_whose_explicit_ctor_should_be_used> & x, optional<optional_swap_test::class_whose_explicit_ctor_should_be_used> & y)
|
||||
{
|
||||
optional_swap_test::swap(x, y);
|
||||
}
|
||||
|
||||
} // namespace boost
|
||||
|
||||
|
||||
namespace std {
|
||||
|
||||
//
|
||||
// Specializations of std::swap:
|
||||
//
|
||||
|
||||
template <>
|
||||
void swap(optional_swap_test::class_whose_default_ctor_should_be_used & x, optional_swap_test::class_whose_default_ctor_should_be_used & y)
|
||||
{
|
||||
optional_swap_test::swap(x, y);
|
||||
}
|
||||
|
||||
template <>
|
||||
void swap(optional_swap_test::class_whose_default_ctor_should_not_be_used & x, optional_swap_test::class_whose_default_ctor_should_not_be_used & y)
|
||||
{
|
||||
optional_swap_test::swap(x, y);
|
||||
}
|
||||
|
||||
template <>
|
||||
void swap(optional_swap_test::class_without_default_ctor & x, optional_swap_test::class_without_default_ctor & y)
|
||||
{
|
||||
optional_swap_test::swap(x, y);
|
||||
}
|
||||
|
||||
template <>
|
||||
void swap(optional_swap_test::class_whose_explicit_ctor_should_be_used & x, optional_swap_test::class_whose_explicit_ctor_should_be_used & y)
|
||||
{
|
||||
optional_swap_test::swap(x, y);
|
||||
}
|
||||
|
||||
} // namespace std
|
||||
|
||||
|
||||
//
|
||||
// Tests whether the swap function works properly for optional<T>.
|
||||
// Assumes that T has one data member, of type char.
|
||||
// Returns true iff the test is passed.
|
||||
//
|
||||
template <class T>
|
||||
bool test_swap_function( T const* )
|
||||
{
|
||||
const boost::unit_test::counter_t counter_before_test = boost::minimal_test::errors_counter();
|
||||
try
|
||||
{
|
||||
optional<T> obj1;
|
||||
optional<T> obj2('a');
|
||||
|
||||
// Self-swap should not have any effect.
|
||||
swap(obj1, obj1);
|
||||
swap(obj2, obj2);
|
||||
BOOST_CHECK(!obj1);
|
||||
BOOST_CHECK(!!obj2 && obj2->data == 'a');
|
||||
|
||||
// Call non-member swap.
|
||||
swap(obj1, obj2);
|
||||
|
||||
// Test if obj1 and obj2 are really swapped.
|
||||
BOOST_CHECK(!!obj1 && obj1->data == 'a');
|
||||
BOOST_CHECK(!obj2);
|
||||
|
||||
// Call non-member swap one more time.
|
||||
swap(obj1, obj2);
|
||||
|
||||
// Test if obj1 and obj2 are swapped back.
|
||||
BOOST_CHECK(!obj1);
|
||||
BOOST_CHECK(!!obj2 && obj2->data == 'a');
|
||||
}
|
||||
catch(const std::exception &)
|
||||
{
|
||||
// The swap function should not throw, for our test cases.
|
||||
return false ;
|
||||
}
|
||||
return boost::minimal_test::errors_counter() == counter_before_test ;
|
||||
}
|
||||
|
||||
//
|
||||
// Tests whether the optional<T>::swap member function works properly.
|
||||
// Assumes that T has one data member, of type char.
|
||||
// Returns true iff the test is passed.
|
||||
//
|
||||
template <class T>
|
||||
bool test_swap_member_function( T const* )
|
||||
{
|
||||
const boost::unit_test::counter_t counter_before_test = boost::minimal_test::errors_counter();
|
||||
try
|
||||
{
|
||||
optional<T> obj1;
|
||||
optional<T> obj2('a');
|
||||
|
||||
// Self-swap should not have any effect.
|
||||
obj1.swap(obj1);
|
||||
obj2.swap(obj2);
|
||||
BOOST_CHECK(!obj1);
|
||||
BOOST_CHECK(!!obj2 && obj2->data == 'a');
|
||||
|
||||
// Call member swap.
|
||||
obj1.swap(obj2);
|
||||
|
||||
// Test if obj1 and obj2 are really swapped.
|
||||
BOOST_CHECK(!!obj1 && obj1->data == 'a');
|
||||
BOOST_CHECK(!obj2);
|
||||
|
||||
// Call member swap one more time.
|
||||
obj1.swap(obj2);
|
||||
|
||||
// Test if obj1 and obj2 are swapped back.
|
||||
BOOST_CHECK(!obj1);
|
||||
BOOST_CHECK(!!obj2 && obj2->data == 'a');
|
||||
}
|
||||
catch(const std::exception &)
|
||||
{
|
||||
// The optional<T>::swap member function should not throw, for our test cases.
|
||||
return false ;
|
||||
}
|
||||
return boost::minimal_test::errors_counter() == counter_before_test ;
|
||||
}
|
||||
|
||||
|
||||
//
|
||||
// Tests compile time tweaking of swap, by means of
|
||||
// optional_swap_should_use_default_constructor.
|
||||
//
|
||||
void test_swap_tweaking()
|
||||
{
|
||||
BOOST_CHECK( test_swap_function( ARG(optional_swap_test::class_without_default_ctor) ) );
|
||||
BOOST_CHECK( test_swap_function( ARG(optional_swap_test::class_whose_default_ctor_should_be_used) ) );
|
||||
BOOST_CHECK( test_swap_function( ARG(optional_swap_test::class_whose_default_ctor_should_not_be_used) ) );
|
||||
BOOST_CHECK( test_swap_function( ARG(optional_swap_test::class_whose_explicit_ctor_should_be_used) ) );
|
||||
BOOST_CHECK( test_swap_function( ARG(optional_swap_test::template_whose_default_ctor_should_be_used<char>) ) );
|
||||
BOOST_CHECK( test_swap_member_function( ARG(optional_swap_test::class_without_default_ctor) ) );
|
||||
BOOST_CHECK( test_swap_member_function( ARG(optional_swap_test::class_whose_default_ctor_should_be_used) ) );
|
||||
BOOST_CHECK( test_swap_member_function( ARG(optional_swap_test::class_whose_default_ctor_should_not_be_used) ) );
|
||||
BOOST_CHECK( test_swap_member_function( ARG(optional_swap_test::class_whose_explicit_ctor_should_be_used) ) );
|
||||
BOOST_CHECK( test_swap_member_function( ARG(optional_swap_test::template_whose_default_ctor_should_be_used<char>) ) );
|
||||
}
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
try
|
||||
{
|
||||
test_with_class_type();
|
||||
test_with_builtin_types();
|
||||
test_no_implicit_conversions();
|
||||
test_custom_addressof_operator();
|
||||
test_conversions1();
|
||||
test_conversions2();
|
||||
test_swap_tweaking();
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
BOOST_ERROR("Unexpected Exception caught!");
|
||||
}
|
||||
|
||||
return boost::report_errors();
|
||||
return 0;
|
||||
}
|
||||
|
||||
|
||||
|
||||
Some files were not shown because too many files have changed in this diff Show More
Reference in New Issue
Block a user