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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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@ -1,95 +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 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
|
||||
Distributed under the Boost Software License, Version 1.0.
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(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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[/ Macros will be used for links so we have a central place to change them ]
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|
||||
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[/ Cited Boost resources ]
|
||||
|
||||
[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()`]]
|
||||
[def __FUNCTION_LESS_POINTEES__ [@../../../utility/OptionalPointee.html#less `less_pointees()`]]
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|
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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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|
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[/ Other web resources ]
|
||||
|
||||
[def __HASKELL__ [@http://www.haskell.org/ Haskell]]
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[def __SGI_DEFAULT_CONSTRUCTIBLE__ [@http://www.sgi.com/tech/stl/DefaultConstructible.html `DefaultConstructible`]]
|
||||
[def __SGI_LESS_THAN_COMPARABLE__ [@http://www.sgi.com/tech/stl/LessThanComparable.html `LessThanComparable`]]
|
||||
[def __SGI_EQUALITY_COMPARABLE__ [@http://www.sgi.com/tech/stl/EqualityComparable.html `EqualityComparable`]]
|
||||
[def __SGI_GENERATOR__ [@http://www.sgi.com/tech/stl/Generator.html `Generator`]]
|
||||
|
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|
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[/ Icons ]
|
||||
|
||||
[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 different. Also suppose that any possible value of type `int` is a valid value for `"MaxValue"`, so we cannot jut 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();
|
||||
}
|
||||
|
||||
[endsect]
|
||||
|
||||
[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_optional_references.qbk]
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[include 15_in_place_factories.qbk]
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[include 16_optional_bool.qbk]
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[include 17_exception_safety.qbk]
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[include 18_type_requirements.qbk]
|
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[endsect]
|
||||
[section Reference]
|
||||
[include 20_reference.qbk]
|
||||
[endsect]
|
||||
[include 90_dependencies.qbk]
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||||
[include 91_acknowledgments.qbk]
|
||||
|
||||
|
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@ -1,158 +0,0 @@
|
||||
[/
|
||||
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)
|
||||
]
|
||||
|
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|
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[section Quick Start]
|
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|
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[section Optional return values]
|
||||
|
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Let's write and use a converter function that converts an a `std::string` to an `int`. It is possible that for a given string (e.g. `"cat"`) there exist 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:
|
||||
|
||||
#include <boost/optional.hpp>
|
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boost::optional<int> convert(const std::string& text);
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|
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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:
|
||||
|
||||
const std::string& text = /*... */;
|
||||
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:
|
||||
|
||||
if (boost::optional<int> oi = convert(text))
|
||||
int i = *oi;
|
||||
|
||||
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:
|
||||
|
||||
int i = *convert("100");
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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 {
|
||||
int j = convert(text).value();
|
||||
}
|
||||
catch (const boost::bad_optional_access&) {
|
||||
// deal with it
|
||||
}
|
||||
|
||||
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:
|
||||
|
||||
int k = convert(text).value_or(0);
|
||||
|
||||
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:
|
||||
|
||||
int fallback_to_default()
|
||||
{
|
||||
cerr << "could not convert; using -1 instead" << endl;
|
||||
return -1;
|
||||
}
|
||||
|
||||
int l = convert(text).value_or_eval(fallback_to_default);
|
||||
|
||||
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.
|
||||
|
||||
Now, let's consider how function `convert` can be implemented.
|
||||
|
||||
boost::optional<int> convert(const std::string& text)
|
||||
{
|
||||
std::stringstream s(text);
|
||||
int i;
|
||||
if ((s >> i) && s.get() == std::char_traits<char>::eof())
|
||||
return i;
|
||||
else
|
||||
return boost::none;
|
||||
}
|
||||
|
||||
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.
|
||||
|
||||
[endsect]
|
||||
|
||||
[section Optional automatic variables]
|
||||
|
||||
We could write function `convert` in a slightly different manner, so that it has a single `return`-statement:
|
||||
|
||||
boost::optional<int> convert(const std::string& text)
|
||||
{
|
||||
boost::optional<int> ans;
|
||||
std::stringstream s(text);
|
||||
int i;
|
||||
if ((s >> i) && s.get() == std::char_traits<char>::eof())
|
||||
ans = i;
|
||||
|
||||
return ans;
|
||||
}
|
||||
|
||||
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 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 it with `T`'s copy constructor. If the optional object is already initialized, it assigns the new value to it using `T`'s copy assignment.
|
||||
[endsect]
|
||||
|
||||
[section Optional data members]
|
||||
|
||||
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:
|
||||
|
||||
class Widget
|
||||
{
|
||||
mutable boost::optional<const Resource> resource_;
|
||||
|
||||
public:
|
||||
Widget() {}
|
||||
|
||||
const Resource& getResource() const // not thread-safe
|
||||
{
|
||||
if (resource_ == boost::none)
|
||||
resource_.emplace("resource", "arguments");
|
||||
|
||||
return *resource_;
|
||||
}
|
||||
};
|
||||
|
||||
`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`.
|
||||
|
||||
[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]
|
||||
|
||||
[section Bypassing unnecessary default construction]
|
||||
|
||||
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`:
|
||||
|
||||
boost::tuple<Date, Date> getPeriod();
|
||||
|
||||
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':
|
||||
|
||||
Date begin, end; // Error: no default ctor!
|
||||
boost::tie(begin, end) = getPeriod();
|
||||
|
||||
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:
|
||||
|
||||
boost::optional<Date> begin, end;
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||||
boost::tie(begin, end) = getPeriod();
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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]
|
||||
|
||||
[section Storage in containers]
|
||||
|
||||
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`:
|
||||
|
||||
std::map<boost::optional<int>, int> choices;
|
||||
|
||||
for (int i = 0; i < LIMIT; ++i) {
|
||||
boost::optional<int> choice = readChoice();
|
||||
++choices[choice];
|
||||
}
|
||||
|
||||
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`.
|
||||
[endsect]
|
||||
|
||||
[endsect]
|
||||
|
||||
|
||||
@ -1,80 +0,0 @@
|
||||
[/
|
||||
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)
|
||||
]
|
||||
|
||||
[section Motivation]
|
||||
|
||||
Consider these functions which should return a value but which might not have
|
||||
a value to return:
|
||||
|
||||
* (A) `double sqrt(double n );`
|
||||
* (B) `char get_async_input();`
|
||||
* (C) `point polygon::get_any_point_effectively_inside();`
|
||||
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
|
||||
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.
|
||||
|
||||
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.
|
||||
|
||||
A similar situation occurs with function (C): it is conceptually an error to
|
||||
ask a ['null-area] 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.
|
||||
|
||||
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 `EOF`,
|
||||
`string::npos`, points at infinity, etc...
|
||||
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
['plus] the ['signal] 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 `int` in place of
|
||||
`char`; or a compound type, such as `std::pair<point,bool>`.
|
||||
|
||||
Returning a `std::pair<T,bool>`, 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:
|
||||
|
||||
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
|
||||
results:
|
||||
|
||||
std::pair<point,bool> p = poly.get_any_point_effectively_inside();
|
||||
if ( p.second )
|
||||
flood_fill(p.first);
|
||||
|
||||
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.
|
||||
|
||||
Clearly, we need a better idiom.
|
||||
|
||||
[endsect]
|
||||
@ -1,251 +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 Design Overview]
|
||||
|
||||
[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)).
|
||||
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).
|
||||
`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
|
||||
is, when a variable is declared as `optional<T>` and no initial value is given,
|
||||
the variable is ['formally] uninitialized. A formally uninitialized optional object
|
||||
has conceptually no value at all and this situation can be tested at runtime. It
|
||||
is formally ['undefined behavior] 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 ['uninitialized].
|
||||
|
||||
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: `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
|
||||
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`.
|
||||
Using the __BOOST_VARIANT__ library, this model can be implemented in terms of `boost::variant<T,nil_t>`.
|
||||
There is precedent for a discriminated union as a model for an optional value:
|
||||
the __HASKELL__ [*Maybe] built-in type constructor. Thus, a discriminated union
|
||||
`T+nil_t` serves as a conceptual foundation.
|
||||
|
||||
A `variant<T,nil_t>` follows naturally from the traditional idiom of extending
|
||||
the range of possible values adding an additional sentinel value with the
|
||||
special meaning of ['Nothing]. However, this additional ['Nothing] 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.
|
||||
|
||||
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
|
||||
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.
|
||||
|
||||
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 ['exact] semantics required for a wrapper
|
||||
of optional values:
|
||||
|
||||
Discriminated-union:
|
||||
|
||||
* [*deep-copy] semantics: copies of the variant implies copies of the value.
|
||||
* [*deep-relational] semantics: comparisons between variants matches both
|
||||
current types and values
|
||||
* If the variant's current type is `T`, it is modeling an ['initialized] optional.
|
||||
* If the variant's current type is not `T`, it is modeling an ['uninitialized]
|
||||
optional.
|
||||
* Testing if the variant's current type is `T` models testing if the optional
|
||||
is initialized
|
||||
* Trying to extract a `T` from a variant when its current type is not `T`, models
|
||||
the undefined behavior of trying to access the value of an uninitialized optional
|
||||
|
||||
Single-element container:
|
||||
|
||||
* [*deep-copy] semantics: copies of the container implies copies of the value.
|
||||
* [*deep-relational] semantics: comparisons between containers compare container
|
||||
size and if match, contained value
|
||||
* If the container is not empty (contains an object of type `T`), it is modeling
|
||||
an ['initialized] optional.
|
||||
* If the container is empty, it is modeling an ['uninitialized] optional.
|
||||
* Testing if the container is empty models testing if the optional is
|
||||
initialized
|
||||
* Trying to extract a `T` from an empty container models the undefined behavior
|
||||
of trying to access the value of an uninitialized optional
|
||||
|
||||
[endsect]
|
||||
|
||||
[section The semantics]
|
||||
|
||||
Objects of type `optional<T>` are intended to be used in places where objects of
|
||||
type `T` would but which might be uninitialized. Hence, `optional<T>`'s purpose is
|
||||
to formalize the additional possibly uninitialized state.
|
||||
From the perspective of this role, `optional<T>` can have the same operational
|
||||
semantics of `T` plus the additional semantics corresponding to this special
|
||||
state.
|
||||
As such, `optional<T>` could be thought of as a ['supertype] of `T`. 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 `optional<T>` a ['subtype] of `T`
|
||||
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.
|
||||
|
||||
We can draw from the purpose of `optional<T>` the required basic semantics:
|
||||
|
||||
* [*Default Construction:] To introduce a formally uninitialized wrapped
|
||||
object.
|
||||
* [*Direct Value Construction via copy:] To introduce a formally initialized
|
||||
wrapped object whose value is obtained as a copy of some object.
|
||||
* [*Deep Copy Construction:] To obtain a new yet equivalent wrapped object.
|
||||
* [*Direct Value Assignment (upon initialized):] To assign a value to the
|
||||
wrapped object.
|
||||
* [*Direct Value Assignment (upon uninitialized):] To initialize the wrapped
|
||||
object with a value obtained as a copy of some object.
|
||||
* [*Assignment (upon initialized):] To assign to the wrapped object the value
|
||||
of another wrapped object.
|
||||
* [*Assignment (upon uninitialized):] To initialize the wrapped object with
|
||||
value of another wrapped object.
|
||||
* [*Deep Relational Operations (when supported by the type T):] To compare
|
||||
wrapped object values taking into account the presence of uninitialized states.
|
||||
* [*Value access:] To unwrap the wrapped object.
|
||||
* [*Initialization state query:] To determine if the object is formally
|
||||
initialized or not.
|
||||
* [*Swap:] To exchange wrapped objects. (with whatever exception safety
|
||||
guarantees are provided by `T`'s swap).
|
||||
* [*De-initialization:] To release the wrapped object (if any) and leave the
|
||||
wrapper in the uninitialized state.
|
||||
|
||||
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.
|
||||
|
||||
[endsect]
|
||||
|
||||
[section The Interface]
|
||||
|
||||
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 `T` type as much as possible. In order to choose
|
||||
the proper degree of adoption of the native `T` interface, the following must
|
||||
be noted: Even if all the operations supported by an instance of type `T` are
|
||||
defined for the entire range of values for such a type, an `optional<T>`
|
||||
extends such a set of values with a new value for which most
|
||||
(otherwise valid) operations are not defined in terms of `T`.
|
||||
|
||||
Furthermore, since `optional<T>` itself is merely a `T` wrapper (modeling a `T`
|
||||
supertype), any attempt to define such operations upon uninitialized optionals
|
||||
will be totally artificial w.r.t. `T`.
|
||||
|
||||
This library chooses an interface which follows from `T`'s interface only for
|
||||
those operations which are well defined (w.r.t the type `T`) even if any of the
|
||||
operands are uninitialized. These operations include: construction,
|
||||
copy-construction, assignment, swap and relational operations.
|
||||
|
||||
For the value access operations, which are undefined (w.r.t the type `T`) when
|
||||
the operand is uninitialized, a different interface is chosen (which will be
|
||||
explained next).
|
||||
|
||||
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
|
||||
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.
|
||||
|
||||
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
|
||||
function, but also quite often as optional data members.
|
||||
|
||||
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: `( *p = 2 )` and `( p->foo() )`,
|
||||
implicitly convey the notion of optionality, and this information is tied to
|
||||
the ['syntax] of the expressions. That is, the presence of operators `*` and `->`
|
||||
tell by themselves —without any additional context— that the expression will
|
||||
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.
|
||||
|
||||
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
|
||||
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.
|
||||
|
||||
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 `X`, it can
|
||||
use `shared_ptr<X>` as the return value. However, this requires dynamic allocation
|
||||
of `X`. If `X` 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
|
||||
__OPTIONAL_POINTEE__ concept incarnated by pointers.
|
||||
|
||||
|
||||
[heading Optional<T> as a model of OptionalPointee]
|
||||
|
||||
For value access operations `optional<>` uses operators `*` and `->` to
|
||||
lexically warn about the possibly uninitialized state appealing to the
|
||||
familiar pointer semantics w.r.t. to null pointers.
|
||||
|
||||
[warning
|
||||
However, it is particularly important to note that `optional<>` objects
|
||||
are not pointers. [_`optional<>` is not, and does not model, a pointer].
|
||||
]
|
||||
|
||||
For instance, `optional<>` does not have shallow-copy so does not alias:
|
||||
two different optionals never refer to the ['same] value unless `T` itself is
|
||||
a reference (but may have ['equivalent] values).
|
||||
The difference between an `optional<T>` and a pointer must be kept in mind,
|
||||
particularly because the semantics of relational operators are different:
|
||||
since `optional<T>` 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 `optional<T>` by `T*` 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 __FUNCTION_EQUAL_POINTEES__ and __FUNCTION_LESS_POINTEES__ instead.
|
||||
|
||||
[endsect]
|
||||
|
||||
[endsect]
|
||||
@ -1,37 +0,0 @@
|
||||
|
||||
[section When to use Optional]
|
||||
|
||||
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'.
|
||||
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:
|
||||
|
||||
template <typename T>
|
||||
optional<size_t> find_smallest_elem(const std::vector<T>& vec);
|
||||
|
||||
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.
|
||||
|
||||
`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.
|
||||
|
||||
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:
|
||||
|
||||
if(boost::optional<std::string> name = ask_user_name()) {
|
||||
assert(*name != "");
|
||||
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`).
|
||||
|
||||
[heading Not recommended usages]
|
||||
|
||||
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 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;
|
||||
boost::optional<int> o1 = 1;
|
||||
|
||||
assert(oN != o0);
|
||||
assert(o1 != oN);
|
||||
assert(o2 != 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(o2 != 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]
|
||||
@ -1,120 +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]
|
||||
1288
doc/20_reference.qbk
1288
doc/20_reference.qbk
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]
|
||||
@ -1,62 +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 Acknowledgements]
|
||||
|
||||
[heading Pre-formal review]
|
||||
|
||||
* Peter Dimov suggested the name 'optional', and was the first to point out
|
||||
the need for aligned storage.
|
||||
* Douglas Gregor developed 'type_with_alignment', and later Eric Friedman
|
||||
coded 'aligned_storage', which are the core of the optional class
|
||||
implementation.
|
||||
* Andrei Alexandrescu and Brian Parker also worked with aligned storage
|
||||
techniques and their work influenced the current implementation.
|
||||
* Gennadiy Rozental made extensive and important comments which shaped the
|
||||
design.
|
||||
* Vesa Karvonen and Douglas Gregor made quite useful comparisons between
|
||||
optional, variant and any; and made other relevant comments.
|
||||
* Douglas Gregor and Peter Dimov commented on comparisons and evaluation
|
||||
in boolean contexts.
|
||||
* Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
* Many others have participated with useful comments: Aleksey Gurotov,
|
||||
Kevlin Henney, David Abrahams, and others I can't recall.
|
||||
|
||||
[heading Post-formal review]
|
||||
|
||||
* 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.
|
||||
* 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.
|
||||
* 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.
|
||||
* Joel de Guzman noticed that optional<> can be seen as an API on top of
|
||||
variant<T,nil_t>.
|
||||
* Dave Gomboc explained the meaning and usage of the Haskell analog to
|
||||
optional<>: the Maybe type constructor (analogy originally pointed out by
|
||||
David Sankel).
|
||||
* Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey,
|
||||
Rob Stewart, and others.
|
||||
* Joel de Guzman made the case for the support of references and helped
|
||||
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]
|
||||
|
||||
@ -1,39 +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)
|
||||
|
||||
|
||||
# Quickbook
|
||||
# -----------------------------------------------------------------------------
|
||||
|
||||
using boostbook ;
|
||||
import quickbook ;
|
||||
|
||||
path-constant images : html ;
|
||||
|
||||
xml optional
|
||||
:
|
||||
00_optional.qbk
|
||||
;
|
||||
|
||||
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
|
||||
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<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>
|
||||
</h2></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 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>
|
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</tr></table>
|
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|
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<div class="spirit-nav">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<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 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>
|
||||
<hr>
|
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<a name="boost_optional.acknowledgments"></a><a class="link" href="acknowledgments.html" title="Acknowledgments">Acknowledgments</a>
|
||||
</h2></div></div></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgments.h0"></a>
|
||||
<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
|
||||
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.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
|
||||
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>
|
||||
</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>
|
||||
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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|
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|
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and Portability</a>
|
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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>
|
||||
</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>
|
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<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 class="listitem">
|
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detail
|
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|
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<li class="listitem">
|
||||
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>
|
||||
</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 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 class="section">
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<div class="titlepage"><div><div><h3 class="title">
|
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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>
|
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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 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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|
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||||
<div class="section">
|
||||
<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>
|
||||
<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><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>
|
||||
<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><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>
|
||||
<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>
|
||||
<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
|
||||
Value Access in the presence of possibly untitialized optional objects: The
|
||||
operators * and -></a>
|
||||
</h5>
|
||||
<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>
|
||||
<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>
|
||||
as a model of OptionalPointee</a>
|
||||
</h5>
|
||||
<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>
|
||||
<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>
|
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<div class="spirit-nav">
|
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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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|
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||||
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|
||||
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|
||||
<a name="boost_optional.examples"></a><a class="link" href="examples.html" title="Examples">Examples</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_return_values">Optional
|
||||
return values</a></span></dt>
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_local_variables">Optional
|
||||
local variables</a></span></dt>
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_data_members">Optional
|
||||
data members</a></span></dt>
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction">Bypassing
|
||||
expensive unnecessary default construction</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<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>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">()</span>
|
||||
<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">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="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="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>
|
||||
</h3></div></div></div>
|
||||
<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="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="special">}</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">name</span> <span class="special">)</span>
|
||||
<span class="identifier">print</span><span class="special">(*</span><span class="identifier">name</span><span class="special">);</span>
|
||||
<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="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>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">figure</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<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="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="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>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">m_clipping_rect</span> <span class="special">)</span>
|
||||
<span class="identifier">do_clipping</span><span class="special">(*</span><span class="identifier">m_clipping_rect</span><span class="special">);</span>
|
||||
|
||||
<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="keyword">private</span> <span class="special">:</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">rect</span><span class="special">></span> <span class="identifier">m_clipping_rect</span> <span class="special">;</span>
|
||||
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
</div>
|
||||
<div class="section">
|
||||
<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>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">ExpensiveCtor</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">Fred</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">Fred</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">mLargeVector</span><span class="special">(</span><span class="number">10000</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">ExpensiveCtor</span><span class="special">></span> <span class="special">></span> <span class="identifier">mLargeVector</span> <span class="special">;</span>
|
||||
<span class="special">}</span> <span class="special">;</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 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="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>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
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@ -1,170 +0,0 @@
|
||||
<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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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<td align="center"><a href="../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../more/index.htm">More</a></td>
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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.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.
|
||||
</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>
|
||||
<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>
|
||||
</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
|
||||
</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="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 name="boost_optional.implementation_notes"></a><a class="link" href="implementation_notes.html" title="Implementation Notes">Implementation Notes</a>
|
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</h2></div></div></div>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
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>.
|
||||
</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 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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||||
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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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>
|
||||
</h2></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">Copy Constructible</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 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 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>
|
||||
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|
||||
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|
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@ -1,114 +0,0 @@
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||||
<html>
|
||||
<head>
|
||||
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|
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|
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|
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|
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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>
|
||||
<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>
|
||||
<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>
|
||||
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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|
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<a name="boost_optional.quick_start"></a><a class="link" href="quick_start.html" title="Quick Start">Quick Start</a>
|
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</h2></div></div></div>
|
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<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 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">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 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="../index.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="quick_start/optional_automatic_variables.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="titlepage"><div><div><h3 class="title">
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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 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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<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 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 it with <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 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="../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_data_members.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -1,94 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Optional data members</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="home" href="../../index.html" title="Boost.Optional">
|
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|
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|
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<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 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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</body>
|
||||
</html>
|
||||
@ -1,135 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>Optional return values</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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<link rel="home" href="../../index.html" title="Chapter 1. Boost.Optional">
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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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<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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<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="../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>
|
||||
</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>
|
||||
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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.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>
|
||||
<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 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>
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</tr></table>
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<hr>
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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>Rebinding semantics for assignment of optional references</title>
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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.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>
|
||||
</h2></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>
|
||||
<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>
|
||||
</h4>
|
||||
<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 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_references.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="in_place_factories.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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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>Synopsis</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="Chapter 1. Boost.Optional">
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<link rel="prev" href="../../optional/reference.html" title="Reference">
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<table cellpadding="2" width="100%"><tr>
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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="detailed_semantics.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.reference.synopsis"></a><a class="link" href="synopsis.html" title="Synopsis">Synopsis</a>
|
||||
</h3></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>
|
||||
|
||||
<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="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">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="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">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="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="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="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="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">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="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="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">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>
|
||||
|
||||
<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_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="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="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="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="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="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="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="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>
|
||||
|
||||
<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="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="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="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="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="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="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>
|
||||
<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/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="detailed_semantics.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -1,174 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Synopsis</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="home" href="../index.html" title="Chapter 1. Boost.Optional">
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<link rel="up" href="../index.html" title="Chapter 1. Boost.Optional">
|
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<link rel="prev" href="tutorial/type_requirements.html" title="Type requirements">
|
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<link rel="next" href="detailed_semantics.html" title="Detailed Semantics">
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</head>
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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="http://www.boost.org/users/faq.html">FAQ</a></td>
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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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</div>
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||||
<div class="section">
|
||||
<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>
|
||||
|
||||
<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="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">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="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">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="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="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="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="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">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="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="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">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>
|
||||
|
||||
<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_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="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="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="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="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="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="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="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>
|
||||
|
||||
<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="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="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="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="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="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="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>
|
||||
<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="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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<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="../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>
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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">
|
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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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||||
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|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
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<title>A note about optional<bool></title>
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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 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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@ -1,184 +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>Design Overview</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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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/the_semantics.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.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 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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</div>
|
||||
</body>
|
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|
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@ -1,187 +0,0 @@
|
||||
<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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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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|
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|
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|
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</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_untitialized_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_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"><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 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="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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|
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|
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<html>
|
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<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 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="spirit-nav">
|
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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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<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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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<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 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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|
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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>
|
||||
</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 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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||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -1,129 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Motivation</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="Chapter 1. Boost.Optional">
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||||
<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="prev" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="next" href="design_overview.html" title="Design Overview">
|
||||
</head>
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||||
<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>
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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>
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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>
|
||||
</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>
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||||
</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">
|
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<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="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 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>
|
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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="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="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 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="rebinding_semantics_for_assignment_of_optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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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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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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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="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="in_place_factories.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.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 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_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="in_place_factories.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>Relational operators</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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<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="optional_references.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.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 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">o2</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">o2</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 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="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="optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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@ -1,109 +0,0 @@
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||||
<html>
|
||||
<head>
|
||||
<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="../../optional/reference.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.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 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="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="../../optional/reference.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -1,142 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>When to use Optional</title>
|
||||
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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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<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="prev" href="design_overview/the_interface.html" title="The Interface">
|
||||
<link rel="next" href="relational_operators.html" title="Relational operators">
|
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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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|
||||
</div>
|
||||
<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 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>
|
||||
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|
||||
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|
||||
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|
||||
@ -1,61 +0,0 @@
|
||||
<html>
|
||||
<head>
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||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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|
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<link rel="home" href="../index.html" title="Chapter 1. Boost.Optional">
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|
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<div class="section">
|
||||
<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>.
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> <span class="underline">is
|
||||
not</span> required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top">Default
|
||||
Constructible</a>.
|
||||
</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 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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<div><h2 class="title">
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<a name="optional"></a>Boost.Optional</h2></div>
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<div><div class="author"><h3 class="author">
|
||||
<span class="firstname">Fernando Luis</span> <span class="surname">Cacciola Carballal</span>
|
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</h3></div></div>
|
||||
<div><p class="copyright">Copyright © 2003-2007 Fernando Luis Cacciola Carballal</p></div>
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||||
<div><p class="copyright">Copyright © 2014 Andrzej Krzemieński</p></div>
|
||||
<div><div class="legalnotice">
|
||||
<a name="optional.legal"></a><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></div>
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||||
</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>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start.html#boost_optional.quick_start.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
|
||||
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/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>
|
||||
</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.synopsis">Synopsis</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/detailed_semantics.html">Detailed
|
||||
Semantics</a></span></dt>
|
||||
</dl></dd>
|
||||
<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/acknowledgements.html">Acknowledgements</a></span></dt>
|
||||
</dl>
|
||||
</div>
|
||||
<div class="section">
|
||||
<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>
|
||||
</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.
|
||||
</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>
|
||||
<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 different. 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 jut 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.
|
||||
</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>:
|
||||
</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>
|
||||
</pre>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"><p><small>Last revised: July 10, 2014 at 11:41:49 GMT</small></p></td>
|
||||
<td align="right"><div class="copyright-footer"></div></td>
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<dt><span class="section"><a href="reference.html#boost_optional.reference.synopsis">Synopsis</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/reference/detailed_semantics.html">Detailed
|
||||
Semantics</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.reference.synopsis"></a><a class="link" href="reference.html#boost_optional.reference.synopsis" title="Synopsis">Synopsis</a>
|
||||
</h3></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>
|
||||
|
||||
<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="identifier">optional</span> <span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../boost_optional/reference/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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/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 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/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="../boost_optional/reference/detailed_semantics.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
|
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</div>
|
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</body>
|
||||
</html>
|
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@ -1,153 +0,0 @@
|
||||
<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>Tutorial</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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<link rel="home" href="../index.html" title="Boost.Optional">
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<link rel="up" href="../index.html" title="Boost.Optional">
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<link rel="prev" href="../boost_optional/quick_start/storage_in_containers.html" title="Storage in containers">
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<link rel="next" href="../boost_optional/tutorial/design_overview.html" title="Design Overview">
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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="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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<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>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<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/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>
|
||||
</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 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>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -9,17 +9,20 @@
|
||||
// You are welcome to contact the author at:
|
||||
// fernando_cacciola@hotmail.com
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
#ifndef BOOST_DETAIL_NONE_T_17SEP2003_HPP
|
||||
#define BOOST_DETAIL_NONE_T_17SEP2003_HPP
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void test_deep_constantness()
|
||||
{
|
||||
boost::optional<int> opt ;
|
||||
boost::optional<int> const copt ;
|
||||
namespace boost {
|
||||
|
||||
*copt = opt ; // Cannot assign to "int const&"
|
||||
}
|
||||
namespace detail {
|
||||
|
||||
struct none_helper{};
|
||||
|
||||
typedef int none_helper::*none_t ;
|
||||
|
||||
} // namespace detail
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif
|
||||
|
||||
@ -1,28 +0,0 @@
|
||||
// Copyright (C) 2003, 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)
|
||||
//
|
||||
// See http://www.boost.org/libs/optional for documentation.
|
||||
//
|
||||
// You are welcome to contact the author at:
|
||||
// fernando_cacciola@hotmail.com
|
||||
//
|
||||
#ifndef BOOST_NONE_17SEP2003_HPP
|
||||
#define BOOST_NONE_17SEP2003_HPP
|
||||
|
||||
#include "boost/none_t.hpp"
|
||||
|
||||
// NOTE: Borland users have to include this header outside any precompiled headers
|
||||
// (bcc<=5.64 cannot include instance data in a precompiled header)
|
||||
// -- * To be verified, now that there's no unnamed namespace
|
||||
|
||||
namespace boost {
|
||||
|
||||
none_t const none = (static_cast<none_t>(0)) ;
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif
|
||||
|
||||
@ -1,24 +0,0 @@
|
||||
// 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/libs/optional for documentation.
|
||||
//
|
||||
// You are welcome to contact the author at:
|
||||
// fernando_cacciola@hotmail.com
|
||||
//
|
||||
#ifndef BOOST_NONE_T_17SEP2003_HPP
|
||||
#define BOOST_NONE_T_17SEP2003_HPP
|
||||
|
||||
namespace boost {
|
||||
|
||||
namespace detail { struct none_helper{}; }
|
||||
|
||||
typedef int detail::none_helper::*none_t ;
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif
|
||||
|
||||
@ -1,18 +0,0 @@
|
||||
// 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/libs/optional for documentation.
|
||||
//
|
||||
// You are welcome to contact the author at:
|
||||
// fernando_cacciola@hotmail.com
|
||||
//
|
||||
#ifndef BOOST_OPTIONAL_FLC_19NOV2002_HPP
|
||||
#define BOOST_OPTIONAL_FLC_19NOV2002_HPP
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#endif
|
||||
|
||||
@ -1,29 +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>
|
||||
|
||||
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
@ -1,30 +0,0 @@
|
||||
// Copyright (C) 2003, 2008 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/libs/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> struct optional_swap_should_use_default_constructor ;
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif
|
||||
|
||||
@ -1,78 +0,0 @@
|
||||
// Copyright (C) 2005, 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/libs/optional for documentation.
|
||||
//
|
||||
// You are welcome to contact the author at:
|
||||
// fernando_cacciola@hotmail.com
|
||||
//
|
||||
#ifndef BOOST_OPTIONAL_OPTIONAL_IO_FLC_19NOV2002_HPP
|
||||
#define BOOST_OPTIONAL_OPTIONAL_IO_FLC_19NOV2002_HPP
|
||||
|
||||
#include <istream>
|
||||
#include <ostream>
|
||||
|
||||
#include <boost/none.hpp>
|
||||
#include <boost/assert.hpp>
|
||||
#include "boost/optional/optional.hpp"
|
||||
#include "boost/utility/value_init.hpp"
|
||||
|
||||
namespace boost
|
||||
{
|
||||
|
||||
template<class CharType, class CharTrait, class T>
|
||||
inline
|
||||
std::basic_ostream<CharType, CharTrait>&
|
||||
operator<<(std::basic_ostream<CharType, CharTrait>& out, optional<T> const& v)
|
||||
{
|
||||
if ( out.good() )
|
||||
{
|
||||
if ( !v )
|
||||
out << "--" ;
|
||||
else out << ' ' << *v ;
|
||||
}
|
||||
|
||||
return out;
|
||||
}
|
||||
|
||||
template<class CharType, class CharTrait, class T>
|
||||
inline
|
||||
std::basic_istream<CharType, CharTrait>&
|
||||
operator>>(std::basic_istream<CharType, CharTrait>& in, optional<T>& v)
|
||||
{
|
||||
if (in.good())
|
||||
{
|
||||
int d = in.get();
|
||||
if (d == ' ')
|
||||
{
|
||||
T x;
|
||||
in >> x;
|
||||
v = x;
|
||||
}
|
||||
else
|
||||
{
|
||||
if (d == '-')
|
||||
{
|
||||
d = in.get();
|
||||
|
||||
if (d == '-')
|
||||
{
|
||||
v = none;
|
||||
return in;
|
||||
}
|
||||
}
|
||||
|
||||
in.setstate( std::ios::failbit );
|
||||
}
|
||||
}
|
||||
|
||||
return in;
|
||||
}
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif
|
||||
|
||||
13
index.html
13
index.html
@ -1,13 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="refresh" content="0; URL=doc/html/index.html">
|
||||
</head>
|
||||
<body>
|
||||
Automatic redirection failed, please go to
|
||||
<a href="doc/html/index.html">doc/html/index.html</a>. <hr>
|
||||
<p><EFBFBD> Copyright Beman Dawes, 2001</p>
|
||||
<p>Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or copy
|
||||
at <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</p>
|
||||
</body>
|
||||
</html>
|
||||
@ -1,47 +0,0 @@
|
||||
# Boost.Optional Library test Jamfile
|
||||
#
|
||||
# Copyright (C) 2003, Fernando Luis Cacciola Carballal.
|
||||
#
|
||||
# This material is provided "as is", with absolutely no warranty expressed
|
||||
# or implied. Any use is at your own risk.
|
||||
#
|
||||
# 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 ;
|
||||
|
||||
{
|
||||
test-suite optional :
|
||||
[ run optional_test.cpp ]
|
||||
[ run optional_test_tie.cpp ]
|
||||
[ run optional_test_ref.cpp ]
|
||||
[ run optional_test_inplace.cpp ]
|
||||
[ run optional_test_io.cpp ]
|
||||
[ run optional_test_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 ]
|
||||
[ compile-fail optional_test_fail1.cpp ]
|
||||
[ compile-fail optional_test_fail3a.cpp ]
|
||||
[ compile-fail optional_test_fail3b.cpp ]
|
||||
[ compile-fail optional_test_ref_fail1.cpp ]
|
||||
[ compile-fail optional_test_ref_fail3.cpp ]
|
||||
[ 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_explicit_convert_in_value_or.cpp ]
|
||||
[ compile-fail optional_test_fail_explicit_convert_in_value_or_call.cpp ]
|
||||
;
|
||||
}
|
||||
File diff suppressed because it is too large
Load Diff
@ -1,269 +0,0 @@
|
||||
// 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
|
||||
// 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
|
||||
//
|
||||
#include <boost/core/ignore_unused.hpp>
|
||||
|
||||
#ifdef ENABLE_TRACE
|
||||
#define TRACE(msg) std::cout << msg << std::endl ;
|
||||
#else
|
||||
#define TRACE(msg)
|
||||
#endif
|
||||
|
||||
namespace boost {
|
||||
|
||||
void assertion_failed (char const * expr, char const * func, char const * file, long )
|
||||
{
|
||||
using std::string ;
|
||||
string msg = string("Boost assertion failure for \"")
|
||||
+ string(expr)
|
||||
+ string("\" at file \"")
|
||||
+ string(file)
|
||||
+ string("\" function \"")
|
||||
+ string(func)
|
||||
+ string("\"") ;
|
||||
|
||||
TRACE(msg);
|
||||
|
||||
throw std::logic_error(msg);
|
||||
}
|
||||
|
||||
}
|
||||
|
||||
using boost::optional ;
|
||||
|
||||
|
||||
#ifdef BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP
|
||||
using boost::swap ;
|
||||
using boost::get ;
|
||||
using boost::get_pointer ;
|
||||
#endif
|
||||
|
||||
// MSVC6.0 does not support comparisons of optional against a literal null pointer value (0)
|
||||
// via the safe_bool operator.
|
||||
#if BOOST_WORKAROUND(BOOST_MSVC, BOOST_TESTED_AT(1300) ) // 1300 == VC++ 7.1
|
||||
#define BOOST_OPTIONAL_NO_NULL_COMPARE
|
||||
#endif
|
||||
|
||||
#define ARG(T) (static_cast< T const* >(0))
|
||||
|
||||
//
|
||||
// Helper class used to verify the lifetime managment of the values held by optional
|
||||
//
|
||||
class X
|
||||
{
|
||||
public :
|
||||
|
||||
X ( int av ) : v(av)
|
||||
{
|
||||
++ count ;
|
||||
|
||||
TRACE ( "X::X(" << av << "). this=" << this ) ;
|
||||
}
|
||||
|
||||
X ( X const& rhs ) : v(rhs.v)
|
||||
{
|
||||
pending_copy = false ;
|
||||
|
||||
TRACE ( "X::X( X const& rhs). this=" << this << " rhs.v=" << rhs.v ) ;
|
||||
|
||||
if ( throw_on_copy )
|
||||
{
|
||||
TRACE ( "throwing exception in X's copy ctor" ) ;
|
||||
throw 0 ;
|
||||
}
|
||||
|
||||
++ count ;
|
||||
}
|
||||
|
||||
~X()
|
||||
{
|
||||
pending_dtor = false ;
|
||||
|
||||
-- count ;
|
||||
|
||||
TRACE ( "X::~X(). v=" << v << " this=" << this );
|
||||
}
|
||||
|
||||
X& operator= ( X const& rhs )
|
||||
{
|
||||
pending_assign = false ;
|
||||
|
||||
if ( throw_on_assign )
|
||||
{
|
||||
TRACE ( "throwing exception in X's assignment" ) ;
|
||||
|
||||
v = -1 ;
|
||||
|
||||
throw 0 ;
|
||||
}
|
||||
else
|
||||
{
|
||||
v = rhs.v ;
|
||||
|
||||
TRACE ( "X::operator =( X const& rhs). this=" << this << " rhs.v=" << rhs.v ) ;
|
||||
}
|
||||
return *this ;
|
||||
}
|
||||
|
||||
friend bool operator == ( X const& a, X const& b )
|
||||
{ return a.v == b.v ; }
|
||||
|
||||
friend bool operator != ( X const& a, X const& b )
|
||||
{ return a.v != b.v ; }
|
||||
|
||||
friend bool operator < ( X const& a, X const& b )
|
||||
{ return a.v < b.v ; }
|
||||
|
||||
int V() const { return v ; }
|
||||
int& V() { return v ; }
|
||||
|
||||
static int count ;
|
||||
static bool pending_copy ;
|
||||
static bool pending_dtor ;
|
||||
static bool pending_assign ;
|
||||
static bool throw_on_copy ;
|
||||
static bool throw_on_assign ;
|
||||
|
||||
private :
|
||||
|
||||
int v ;
|
||||
|
||||
private :
|
||||
|
||||
X() ;
|
||||
} ;
|
||||
|
||||
|
||||
int X::count = 0 ;
|
||||
bool X::pending_copy = false ;
|
||||
bool X::pending_dtor = false ;
|
||||
bool X::pending_assign = false ;
|
||||
bool X::throw_on_copy = false ;
|
||||
bool X::throw_on_assign = false ;
|
||||
|
||||
inline void set_pending_copy ( X const* x ) { X::pending_copy = true ; }
|
||||
inline void set_pending_dtor ( X const* x ) { X::pending_dtor = true ; }
|
||||
inline void set_pending_assign ( X const* x ) { X::pending_assign = true ; }
|
||||
inline void set_throw_on_copy ( X const* x ) { X::throw_on_copy = true ; }
|
||||
inline void set_throw_on_assign ( X const* x ) { X::throw_on_assign = true ; }
|
||||
inline void reset_throw_on_copy ( X const* x ) { X::throw_on_copy = false ; }
|
||||
inline void reset_throw_on_assign ( X const* x ) { X::throw_on_assign = false ; }
|
||||
inline void check_is_pending_copy ( X const* x ) { BOOST_CHECK( X::pending_copy ) ; }
|
||||
inline void check_is_pending_dtor ( X const* x ) { BOOST_CHECK( X::pending_dtor ) ; }
|
||||
inline void check_is_pending_assign ( X const* x ) { BOOST_CHECK( X::pending_assign ) ; }
|
||||
inline void check_is_not_pending_copy ( X const* x ) { BOOST_CHECK( !X::pending_copy ) ; }
|
||||
inline void check_is_not_pending_dtor ( X const* x ) { BOOST_CHECK( !X::pending_dtor ) ; }
|
||||
inline void check_is_not_pending_assign( X const* x ) { BOOST_CHECK( !X::pending_assign ) ; }
|
||||
inline void check_instance_count ( int c, X const* x ) { BOOST_CHECK( X::count == c ) ; }
|
||||
inline int get_instance_count ( X const* x ) { return X::count ; }
|
||||
|
||||
inline void set_pending_copy (...) {}
|
||||
inline void set_pending_dtor (...) {}
|
||||
inline void set_pending_assign (...) {}
|
||||
inline void set_throw_on_copy (...) {}
|
||||
inline void set_throw_on_assign (...) {}
|
||||
inline void reset_throw_on_copy (...) {}
|
||||
inline void reset_throw_on_assign (...) {}
|
||||
inline void check_is_pending_copy (...) {}
|
||||
inline void check_is_pending_dtor (...) {}
|
||||
inline void check_is_pending_assign (...) {}
|
||||
inline void check_is_not_pending_copy (...) {}
|
||||
inline void check_is_not_pending_dtor (...) {}
|
||||
inline void check_is_not_pending_assign(...) {}
|
||||
inline void check_instance_count (...) {}
|
||||
inline int get_instance_count (...) { return 0 ; }
|
||||
|
||||
|
||||
template<class T>
|
||||
inline void check_uninitialized_const ( optional<T> const& opt )
|
||||
{
|
||||
#ifndef BOOST_OPTIONAL_NO_NULL_COMPARE
|
||||
BOOST_CHECK( opt == 0 ) ;
|
||||
#endif
|
||||
BOOST_CHECK( !opt ) ;
|
||||
BOOST_CHECK( !get_pointer(opt) ) ;
|
||||
BOOST_CHECK( !opt.get_ptr() ) ;
|
||||
}
|
||||
template<class T>
|
||||
inline void check_uninitialized ( optional<T>& opt )
|
||||
{
|
||||
#ifndef BOOST_OPTIONAL_NO_NULL_COMPARE
|
||||
BOOST_CHECK( opt == 0 ) ;
|
||||
#endif
|
||||
BOOST_CHECK( !opt ) ;
|
||||
BOOST_CHECK( !get_pointer(opt) ) ;
|
||||
BOOST_CHECK( !opt.get_ptr() ) ;
|
||||
|
||||
check_uninitialized_const(opt);
|
||||
}
|
||||
|
||||
template<class T>
|
||||
inline void check_initialized_const ( optional<T> const& opt )
|
||||
{
|
||||
BOOST_CHECK( opt ) ;
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_NULL_COMPARE
|
||||
BOOST_CHECK( opt != 0 ) ;
|
||||
#endif
|
||||
|
||||
BOOST_CHECK ( !!opt ) ;
|
||||
BOOST_CHECK ( get_pointer(opt) ) ;
|
||||
BOOST_CHECK ( opt.get_ptr() ) ;
|
||||
}
|
||||
|
||||
template<class T>
|
||||
inline void check_initialized ( optional<T>& opt )
|
||||
{
|
||||
BOOST_CHECK( opt ) ;
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_NULL_COMPARE
|
||||
BOOST_CHECK( opt != 0 ) ;
|
||||
#endif
|
||||
|
||||
BOOST_CHECK ( !!opt ) ;
|
||||
BOOST_CHECK ( get_pointer(opt) ) ;
|
||||
BOOST_CHECK ( opt.get_ptr() ) ;
|
||||
|
||||
check_initialized_const(opt);
|
||||
}
|
||||
|
||||
template<class T>
|
||||
inline void check_value_const ( optional<T> const& opt, T const& v, T const& z )
|
||||
{
|
||||
BOOST_CHECK( *opt == v ) ;
|
||||
BOOST_CHECK( *opt != z ) ;
|
||||
BOOST_CHECK( opt.get() == v ) ;
|
||||
BOOST_CHECK( opt.get() != z ) ;
|
||||
BOOST_CHECK( (*(opt.operator->()) == v) ) ;
|
||||
BOOST_CHECK( *get_pointer(opt) == v ) ;
|
||||
}
|
||||
|
||||
template<class T>
|
||||
inline void check_value ( optional<T>& opt, T const& v, T const& z )
|
||||
{
|
||||
#if BOOST_WORKAROUND(BOOST_MSVC, <= 1200) // 1200 == VC++ 6.0
|
||||
// For some reason, VC6.0 is creating a temporary while evaluating (*opt == v),
|
||||
// so we need to turn throw on copy off first.
|
||||
reset_throw_on_copy( ARG(T) ) ;
|
||||
#endif
|
||||
|
||||
BOOST_CHECK( *opt == v ) ;
|
||||
BOOST_CHECK( *opt != z ) ;
|
||||
BOOST_CHECK( opt.get() == v ) ;
|
||||
BOOST_CHECK( opt.get() != z ) ;
|
||||
BOOST_CHECK( (*(opt.operator->()) == v) ) ;
|
||||
BOOST_CHECK( *get_pointer(opt) == v ) ;
|
||||
|
||||
check_value_const(opt,v,z);
|
||||
}
|
||||
|
||||
|
||||
@ -1,189 +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
|
||||
//
|
||||
// Revisions:
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/bind/apply.hpp" // Included just to test proper interaction with boost::apply<> as reported by Daniel Wallin
|
||||
#include "boost/mpl/bool.hpp"
|
||||
#include "boost/mpl/bool_fwd.hpp" // For mpl::true_ and mpl::false_
|
||||
#include "boost/static_assert.hpp"
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/none.hpp"
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
//#ifndef BOOST_OPTIONAL_NO_CONVERTING_ASSIGNMENT
|
||||
//#ifndef BOOST_OPTIONAL_NO_CONVERTING_COPY_CTOR
|
||||
|
||||
#if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES) && (!defined BOOST_NO_CXX11_VARIADIC_TEMPLATES)
|
||||
|
||||
class Guard
|
||||
{
|
||||
public:
|
||||
int which_ctor;
|
||||
Guard () : which_ctor(0) { }
|
||||
Guard (int&, double&&) : which_ctor(1) { }
|
||||
Guard (int&&, double&) : which_ctor(2) { }
|
||||
Guard (int&&, double&&) : which_ctor(3) { }
|
||||
Guard (int&, double&) : which_ctor(4) { }
|
||||
Guard (std::string const&) : which_ctor(5) { }
|
||||
Guard (std::string &) : which_ctor(6) { }
|
||||
Guard (std::string &&) : which_ctor(7) { }
|
||||
private:
|
||||
Guard(Guard&&);
|
||||
Guard(Guard const&);
|
||||
void operator=(Guard &&);
|
||||
void operator=(Guard const&);
|
||||
};
|
||||
|
||||
|
||||
void test_emplace()
|
||||
{
|
||||
int i = 0;
|
||||
double d = 0.0;
|
||||
const std::string cs;
|
||||
std::string ms;
|
||||
optional<Guard> o;
|
||||
|
||||
o.emplace();
|
||||
BOOST_CHECK(o);
|
||||
BOOST_CHECK(0 == o->which_ctor);
|
||||
|
||||
o.emplace(i, 2.0);
|
||||
BOOST_CHECK(o);
|
||||
BOOST_CHECK(1 == o->which_ctor);
|
||||
|
||||
o.emplace(1, d);
|
||||
BOOST_CHECK(o);
|
||||
BOOST_CHECK(2 == o->which_ctor);
|
||||
|
||||
o.emplace(1, 2.0);
|
||||
BOOST_CHECK(o);
|
||||
BOOST_CHECK(3 == o->which_ctor);
|
||||
|
||||
o.emplace(i, d);
|
||||
BOOST_CHECK(o);
|
||||
BOOST_CHECK(4 == o->which_ctor);
|
||||
|
||||
o.emplace(cs);
|
||||
BOOST_CHECK(o);
|
||||
BOOST_CHECK(5 == o->which_ctor);
|
||||
|
||||
o.emplace(ms);
|
||||
BOOST_CHECK(o);
|
||||
BOOST_CHECK(6 == o->which_ctor);
|
||||
|
||||
o.emplace(std::string());
|
||||
BOOST_CHECK(o);
|
||||
BOOST_CHECK(7 == o->which_ctor);
|
||||
}
|
||||
|
||||
|
||||
#endif
|
||||
|
||||
#if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES)
|
||||
|
||||
|
||||
struct ThrowOnMove
|
||||
{
|
||||
ThrowOnMove(ThrowOnMove&&) { throw int(); }
|
||||
ThrowOnMove(ThrowOnMove const&) { throw int(); }
|
||||
ThrowOnMove(int){}
|
||||
};
|
||||
|
||||
|
||||
void test_no_moves_on_emplacement()
|
||||
{
|
||||
try {
|
||||
optional<ThrowOnMove> o;
|
||||
o.emplace(1);
|
||||
BOOST_CHECK(o);
|
||||
}
|
||||
catch (...) {
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
}
|
||||
#endif
|
||||
|
||||
struct Thrower
|
||||
{
|
||||
Thrower(bool throw_) { if (throw_) throw int(); }
|
||||
|
||||
private:
|
||||
Thrower(Thrower const&);
|
||||
#if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES)
|
||||
Thrower(Thrower&&);
|
||||
#endif
|
||||
};
|
||||
|
||||
void test_clear_on_throw()
|
||||
{
|
||||
optional<Thrower> ot;
|
||||
try {
|
||||
ot.emplace(false);
|
||||
BOOST_CHECK(ot);
|
||||
} catch(...) {
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
|
||||
try {
|
||||
ot.emplace(true);
|
||||
BOOST_CHECK(false);
|
||||
} catch(...) {
|
||||
BOOST_CHECK(!ot);
|
||||
}
|
||||
}
|
||||
|
||||
void test_no_assignment_on_emplacement()
|
||||
{
|
||||
optional<const std::string> os;
|
||||
BOOST_CHECK(!os);
|
||||
os.emplace("wow");
|
||||
BOOST_CHECK(os);
|
||||
BOOST_CHECK(*os == "wow");
|
||||
}
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
try
|
||||
{
|
||||
#if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES) && (!defined BOOST_NO_CXX11_VARIADIC_TEMPLATES)
|
||||
test_emplace();
|
||||
#endif
|
||||
#if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES)
|
||||
test_no_moves_on_emplacement();
|
||||
#endif
|
||||
test_clear_on_throw();
|
||||
test_no_assignment_on_emplacement();
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
BOOST_ERROR("Unexpected Exception caught!");
|
||||
}
|
||||
|
||||
return 0;
|
||||
}
|
||||
|
||||
|
||||
@ -1,68 +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
|
||||
//
|
||||
// Revisions:
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/bind/apply.hpp" // Included just to test proper interaction with boost::apply<> as reported by Daniel Wallin
|
||||
#include "boost/mpl/bool.hpp"
|
||||
#include "boost/mpl/bool_fwd.hpp" // For mpl::true_ and mpl::false_
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/none.hpp"
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
struct SemiRegular // no operator==
|
||||
{
|
||||
private: void operator==(SemiRegular const&) const {}
|
||||
private: void operator!=(SemiRegular const&) const {}
|
||||
};
|
||||
|
||||
void test_equal_to_none_of_noncomparable_T()
|
||||
{
|
||||
optional<SemiRegular> i = SemiRegular();
|
||||
optional<SemiRegular> o;
|
||||
|
||||
BOOST_CHECK(i != boost::none);
|
||||
BOOST_CHECK(boost::none != i);
|
||||
BOOST_CHECK(o == boost::none);
|
||||
BOOST_CHECK(boost::none == o);
|
||||
}
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
try
|
||||
{
|
||||
test_equal_to_none_of_noncomparable_T();
|
||||
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
BOOST_ERROR("Unexpected Exception caught!");
|
||||
}
|
||||
|
||||
return 0;
|
||||
}
|
||||
|
||||
|
||||
@ -1,26 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void test_no_implicit_conversion()
|
||||
{
|
||||
boost::optional<int> opt(1) ;
|
||||
|
||||
// You can compare against 0 or against another optional<>,
|
||||
// but not against another value
|
||||
if ( opt == 1 ) ;
|
||||
}
|
||||
|
||||
|
||||
@ -1,33 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
#if BOOST_WORKAROUND( BOOST_INTEL_CXX_VERSION, <= 700) // Intel C++ 7.0
|
||||
// Interl C++ 7.0 incorrectly accepts the initialization "boost::optional<int> opt = 3"
|
||||
// even though the ctor is explicit (c.f. 12.3.1.2), so the test uses another form of
|
||||
// copy-initialization: argument-passing (8.5.12)
|
||||
void helper ( boost::optional<int> ) ;
|
||||
void test_explicit_constructor()
|
||||
{
|
||||
helper(3) ; // ERROR: Ctor is explicit.
|
||||
}
|
||||
#else
|
||||
void test_explicit_constructor()
|
||||
{
|
||||
boost::optional<int> opt = 3 ; // ERROR: Ctor is explicit.
|
||||
}
|
||||
#endif
|
||||
|
||||
|
||||
@ -1,25 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include<string>
|
||||
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void test_no_unsupported_conversion()
|
||||
{
|
||||
boost::optional<int> opt1(1) ;
|
||||
boost::optional< std::string > opt2( opt1 ) ; // Cannot convert from "int" to "std::string"
|
||||
}
|
||||
|
||||
|
||||
@ -1,28 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include<string>
|
||||
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
struct A {} ;
|
||||
struct B {} ;
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void test_no_unsupported_conversion()
|
||||
{
|
||||
boost::optional<A> opt1;
|
||||
boost::optional<B> opt2;
|
||||
opt2 = opt1 ; // Cannot convert from "A" to "B"
|
||||
}
|
||||
|
||||
|
||||
@ -1,36 +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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
class MoveOnly
|
||||
{
|
||||
public:
|
||||
int val;
|
||||
MoveOnly(int v) : val(v) {}
|
||||
MoveOnly(MoveOnly&& rhs) : val(rhs.val) { rhs.val = 0; }
|
||||
void operator=(MoveOnly&& rhs) {val = rhs.val; rhs.val = 0; }
|
||||
|
||||
private:
|
||||
MoveOnly(MoveOnly const&);
|
||||
void operator=(MoveOnly const&);
|
||||
};
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void test_copying_optional_with_noncopyable_T()
|
||||
{
|
||||
boost::optional<MoveOnly> opt1 ;
|
||||
boost::optional<MoveOnly> opt2(opt1) ;
|
||||
}
|
||||
|
||||
|
||||
@ -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/lib/optional for documentation.
|
||||
//
|
||||
// You are welcome to contact the author at:
|
||||
// akrzemi1@gmail.com
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
|
||||
struct U
|
||||
{};
|
||||
|
||||
struct T
|
||||
{
|
||||
explicit T(U const&) {}
|
||||
};
|
||||
|
||||
|
||||
void test_implicit_conversion_to_bool()
|
||||
{
|
||||
boost::optional<T> opt;
|
||||
opt.value_or(U());
|
||||
}
|
||||
|
||||
@ -1,34 +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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
|
||||
struct U
|
||||
{};
|
||||
|
||||
struct T
|
||||
{
|
||||
explicit T(U const&) {}
|
||||
};
|
||||
|
||||
U get_U() { return U(); }
|
||||
|
||||
|
||||
void test_verifying_the_implicit_conversion_to_bool()
|
||||
{
|
||||
boost::optional<T> opt;
|
||||
opt.value_or_eval(get_U);
|
||||
}
|
||||
|
||||
@ -1,28 +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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
|
||||
#if !defined(BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS)
|
||||
|
||||
bool test_implicit_conversion_to_bool()
|
||||
{
|
||||
boost::optional<int> opt;
|
||||
return opt;
|
||||
}
|
||||
|
||||
#else
|
||||
# error "Test skipped: this compiler does not support explicit conversion operators."
|
||||
#endif
|
||||
@ -1,19 +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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
|
||||
boost::optional<int&&> oi;
|
||||
|
||||
|
||||
@ -1,84 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
|
||||
#include "boost/utility/in_place_factory.hpp"
|
||||
#include "boost/utility/typed_in_place_factory.hpp"
|
||||
#endif
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
|
||||
struct A
|
||||
{
|
||||
A ( double a0, std::string a1 ) : m_a0(a0), m_a1(a1) {}
|
||||
|
||||
friend bool operator == ( A const& x, A const& y )
|
||||
{ return x.m_a0 == y.m_a0 && x.m_a1 == y.m_a1 ; }
|
||||
|
||||
double m_a0 ;
|
||||
std::string m_a1 ;
|
||||
} ;
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
double a00 = 3.14, a10 = 6.02e-23;
|
||||
std::string a01("pi"), a11("mol");
|
||||
|
||||
A a0(a00,a01);
|
||||
A a1(a10,a11);
|
||||
|
||||
boost::optional<A> opt1(a0);
|
||||
|
||||
boost::optional<A> opt2 ( boost::in_place(a00,a01) ) ;
|
||||
|
||||
boost::optional<A> opt3 ( boost::in_place<A>(a00,a01) ) ;
|
||||
|
||||
BOOST_CHECK( opt1 == opt2 ) ;
|
||||
BOOST_CHECK( opt2 == opt2 ) ;
|
||||
BOOST_CHECK( *opt2 == a0 ) ;
|
||||
|
||||
#ifndef BOOST_OPTIONAL_WEAK_OVERLOAD_RESOLUTION
|
||||
|
||||
opt2 = boost::in_place(a10,a11);
|
||||
BOOST_CHECK( *opt2 == a1 ) ;
|
||||
|
||||
opt3 = boost::in_place<A>(a10,a11);
|
||||
BOOST_CHECK( *opt3 == a1 ) ;
|
||||
|
||||
#endif
|
||||
|
||||
return 0;
|
||||
}
|
||||
#else
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
// If in-place factories are not supported there is nothing to test
|
||||
return 0 ;
|
||||
}
|
||||
#endif
|
||||
|
||||
|
||||
|
||||
@ -1,60 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
|
||||
#include "boost/utility/in_place_factory.hpp"
|
||||
#endif
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
|
||||
struct A
|
||||
{
|
||||
A ( double a0, std::string a1 ) : m_a0(a0), m_a1(a1) {}
|
||||
|
||||
friend bool operator == ( A const& x, A const& y )
|
||||
{ return x.m_a0 == y.m_a0 && x.m_a1 == y.m_a1 ; }
|
||||
|
||||
double m_a0 ;
|
||||
std::string m_a1 ;
|
||||
} ;
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
int invalid_extra_parameter ;
|
||||
boost::optional<A> opt2 ( boost::in_place(3.14,"pi",invalid_extra_parameter) ) ;
|
||||
|
||||
return 0;
|
||||
}
|
||||
#else
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
int invalid_extra_parameter ;
|
||||
boost::optional<A> opt2 ( A(3.14,"pi",invalid_extra_parameter) ) ;
|
||||
|
||||
return 0;
|
||||
}
|
||||
#endif
|
||||
|
||||
@ -1,62 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
|
||||
#include "boost/utility/typed_in_place_factory.hpp"
|
||||
#endif
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
|
||||
struct A
|
||||
{
|
||||
A ( double a0, std::string a1 ) : m_a0(a0), m_a1(a1) {}
|
||||
|
||||
friend bool operator == ( A const& x, A const& y )
|
||||
{ return x.m_a0 == y.m_a0 && x.m_a1 == y.m_a1 ; }
|
||||
|
||||
double m_a0 ;
|
||||
std::string m_a1 ;
|
||||
} ;
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
// This must fail to compile.
|
||||
// The first template argument to in_place<> is the target-type,
|
||||
// not the first constructor parameter type.
|
||||
boost::optional<A> opt2 ( boost::in_place<int>(3.14,"pi") ) ;
|
||||
|
||||
return 0;
|
||||
}
|
||||
#else
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
boost::optional<A> opt2 ( int(3.14) ) ;
|
||||
|
||||
return 0;
|
||||
}
|
||||
#endif
|
||||
|
||||
|
||||
@ -1,92 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
#include<sstream>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
#include "boost/optional/optional_io.hpp"
|
||||
#include "boost/none.hpp"
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#ifdef ENABLE_TRACE
|
||||
#define TRACE(msg) std::cout << msg << std::endl ;
|
||||
#else
|
||||
#define TRACE(msg)
|
||||
#endif
|
||||
|
||||
namespace boost {
|
||||
|
||||
void assertion_failed (char const * expr, char const * func, char const * file, long )
|
||||
{
|
||||
using std::string ;
|
||||
string msg = string("Boost assertion failure for \"")
|
||||
+ string(expr)
|
||||
+ string("\" at file \"")
|
||||
+ string(file)
|
||||
+ string("\" function \"")
|
||||
+ string(func)
|
||||
+ string("\"") ;
|
||||
|
||||
TRACE(msg);
|
||||
|
||||
throw std::logic_error(msg);
|
||||
}
|
||||
|
||||
}
|
||||
|
||||
using namespace std ;
|
||||
using namespace boost ;
|
||||
|
||||
template<class Opt>
|
||||
void test2( Opt o, Opt buff )
|
||||
{
|
||||
stringstream s ;
|
||||
|
||||
const int markv = 123 ;
|
||||
int mark = 0 ;
|
||||
|
||||
s << o << " " << markv ;
|
||||
s >> buff >> mark ;
|
||||
|
||||
BOOST_ASSERT( buff == o ) ;
|
||||
BOOST_ASSERT( mark == markv ) ;
|
||||
}
|
||||
|
||||
|
||||
template<class T>
|
||||
void test( T v, T w )
|
||||
{
|
||||
test2( make_optional(v), optional<T> ());
|
||||
test2( make_optional(v), make_optional(w));
|
||||
test2( optional<T> () , optional<T> ());
|
||||
test2( optional<T> () , make_optional(w));
|
||||
}
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
try
|
||||
{
|
||||
test(1,2);
|
||||
test(string("hello"),string("buffer"));
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
BOOST_ERROR("Unexpected Exception caught!");
|
||||
}
|
||||
|
||||
return 0;
|
||||
}
|
||||
|
||||
@ -1,104 +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
|
||||
//
|
||||
// Revisions:
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/bind/apply.hpp" // Included just to test proper interaction with boost::apply<> as reported by Daniel Wallin
|
||||
#include "boost/mpl/bool.hpp"
|
||||
#include "boost/mpl/bool_fwd.hpp" // For mpl::true_ and mpl::false_
|
||||
#include "boost/static_assert.hpp"
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/none.hpp"
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
class NonConstructible
|
||||
{
|
||||
private:
|
||||
NonConstructible();
|
||||
NonConstructible(NonConstructible const&);
|
||||
#if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES)
|
||||
NonConstructible(NonConstructible &&);
|
||||
#endif
|
||||
};
|
||||
|
||||
void test_non_constructible()
|
||||
{
|
||||
optional<NonConstructible> o;
|
||||
BOOST_CHECK(!o);
|
||||
BOOST_CHECK(o == boost::none);
|
||||
try {
|
||||
o.value();
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
catch(...) {
|
||||
BOOST_CHECK(true);
|
||||
}
|
||||
}
|
||||
|
||||
class Guard
|
||||
{
|
||||
public:
|
||||
explicit Guard(int) {}
|
||||
private:
|
||||
Guard();
|
||||
Guard(Guard const&);
|
||||
#if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES)
|
||||
Guard(Guard &&);
|
||||
#endif
|
||||
};
|
||||
|
||||
void test_guard()
|
||||
{
|
||||
optional<Guard> o;
|
||||
o.emplace(1);
|
||||
BOOST_CHECK(o);
|
||||
}
|
||||
|
||||
void test_non_assignable()
|
||||
{
|
||||
optional<const std::string> o;
|
||||
o.emplace("cat");
|
||||
BOOST_CHECK(o);
|
||||
BOOST_CHECK(*o == "cat");
|
||||
}
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
try
|
||||
{
|
||||
test_non_constructible();
|
||||
test_guard();
|
||||
test_non_assignable();
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
BOOST_ERROR("Unexpected Exception caught!");
|
||||
}
|
||||
|
||||
return 0;
|
||||
}
|
||||
|
||||
|
||||
@ -1,414 +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
|
||||
//
|
||||
// Revisions:
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/bind/apply.hpp" // Included just to test proper interaction with boost::apply<> as reported by Daniel Wallin
|
||||
#include "boost/mpl/bool.hpp"
|
||||
#include "boost/mpl/bool_fwd.hpp" // For mpl::true_ and mpl::false_
|
||||
#include "boost/static_assert.hpp"
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/none.hpp"
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
//#ifndef BOOST_OPTIONAL_NO_CONVERTING_ASSIGNMENT
|
||||
//#ifndef BOOST_OPTIONAL_NO_CONVERTING_COPY_CTOR
|
||||
|
||||
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
|
||||
|
||||
enum State
|
||||
{
|
||||
sDefaultConstructed,
|
||||
sValueCopyConstructed,
|
||||
sValueMoveConstructed,
|
||||
sCopyConstructed,
|
||||
sMoveConstructed,
|
||||
sMoveAssigned,
|
||||
sCopyAssigned,
|
||||
sValueCopyAssigned,
|
||||
sValueMoveAssigned,
|
||||
sMovedFrom,
|
||||
sIntConstructed
|
||||
};
|
||||
|
||||
struct OracleVal
|
||||
{
|
||||
State s;
|
||||
int i;
|
||||
OracleVal(int i = 0) : s(sIntConstructed), i(i) {}
|
||||
};
|
||||
|
||||
|
||||
struct Oracle
|
||||
{
|
||||
State s;
|
||||
OracleVal val;
|
||||
|
||||
Oracle() : s(sDefaultConstructed) {}
|
||||
Oracle(const OracleVal& v) : s(sValueCopyConstructed), val(v) {}
|
||||
Oracle(OracleVal&& v) : s(sValueMoveConstructed), val(std::move(v)) {v.s = sMovedFrom;}
|
||||
Oracle(const Oracle& o) : s(sCopyConstructed), val(o.val) {}
|
||||
Oracle(Oracle&& o) : s(sMoveConstructed), val(std::move(o.val)) {o.s = sMovedFrom;}
|
||||
|
||||
Oracle& operator=(const OracleVal& v) { s = sValueCopyAssigned; val = v; return *this; }
|
||||
Oracle& operator=(OracleVal&& v) { s = sValueMoveAssigned; val = std::move(v); v.s = sMovedFrom; return *this; }
|
||||
Oracle& operator=(const Oracle& o) { s = sCopyAssigned; val = o.val; return *this; }
|
||||
Oracle& operator=(Oracle&& o) { s = sMoveAssigned; val = std::move(o.val); o.s = sMovedFrom; return *this; }
|
||||
};
|
||||
|
||||
bool operator==( Oracle const& a, Oracle const& b ) { return a.val.i == b.val.i; }
|
||||
bool operator!=( Oracle const& a, Oracle const& b ) { return a.val.i != b.val.i; }
|
||||
|
||||
|
||||
void test_move_ctor_from_U()
|
||||
{
|
||||
optional<Oracle> o1 ((OracleVal()));
|
||||
BOOST_CHECK(o1);
|
||||
BOOST_CHECK(o1->s == sValueMoveConstructed || o1->s == sMoveConstructed);
|
||||
|
||||
OracleVal v1;
|
||||
optional<Oracle> o2 (v1);
|
||||
BOOST_CHECK(o2);
|
||||
BOOST_CHECK(o2->s == sValueCopyConstructed || o2->s == sCopyConstructed || o2->s == sMoveConstructed );
|
||||
BOOST_CHECK(v1.s == sIntConstructed);
|
||||
|
||||
optional<Oracle> o3 (boost::move(v1));
|
||||
BOOST_CHECK(o3);
|
||||
BOOST_CHECK(o3->s == sValueMoveConstructed || o3->s == sMoveConstructed);
|
||||
BOOST_CHECK(v1.s == sMovedFrom);
|
||||
}
|
||||
|
||||
void test_move_ctor_form_T()
|
||||
{
|
||||
optional<Oracle> o1 ((Oracle()));
|
||||
BOOST_CHECK(o1);
|
||||
BOOST_CHECK(o1->s == sMoveConstructed);
|
||||
|
||||
Oracle v1;
|
||||
optional<Oracle> o2 (v1);
|
||||
BOOST_CHECK(o2);
|
||||
BOOST_CHECK(o2->s == sCopyConstructed);
|
||||
BOOST_CHECK(v1.s == sDefaultConstructed);
|
||||
|
||||
optional<Oracle> o3 (boost::move(v1));
|
||||
BOOST_CHECK(o3);
|
||||
BOOST_CHECK(o3->s == sMoveConstructed);
|
||||
BOOST_CHECK(v1.s == sMovedFrom);
|
||||
}
|
||||
|
||||
void test_move_ctor_from_optional_T()
|
||||
{
|
||||
optional<Oracle> o1;
|
||||
optional<Oracle> o2(boost::move(o1));
|
||||
|
||||
BOOST_CHECK(!o1);
|
||||
BOOST_CHECK(!o2);
|
||||
|
||||
optional<Oracle> o3((Oracle()));
|
||||
optional<Oracle> o4(boost::move(o3));
|
||||
BOOST_CHECK(o3);
|
||||
BOOST_CHECK(o4);
|
||||
BOOST_CHECK(o3->s == sMovedFrom);
|
||||
BOOST_CHECK(o4->s == sMoveConstructed);
|
||||
|
||||
optional<Oracle> o5((optional<Oracle>()));
|
||||
BOOST_CHECK(!o5);
|
||||
|
||||
optional<Oracle> o6((optional<Oracle>(Oracle())));
|
||||
BOOST_CHECK(o6);
|
||||
BOOST_CHECK(o6->s == sMoveConstructed);
|
||||
|
||||
optional<Oracle> o7(o6); // does copy ctor from non-const lvalue compile?
|
||||
}
|
||||
|
||||
void test_move_assign_from_U()
|
||||
{
|
||||
optional<Oracle> o1 = boost::none; // test if additional ctors didn't break it
|
||||
o1 = boost::none; // test if additional assignments didn't break it
|
||||
o1 = OracleVal();
|
||||
BOOST_CHECK(o1);
|
||||
|
||||
BOOST_CHECK(o1->s == sValueMoveConstructed);
|
||||
|
||||
o1 = OracleVal();
|
||||
BOOST_CHECK(o1);
|
||||
BOOST_CHECK(o1->s == sMoveAssigned);
|
||||
|
||||
OracleVal v1;
|
||||
optional<Oracle> o2;
|
||||
o2 = v1;
|
||||
BOOST_CHECK(o2);
|
||||
BOOST_CHECK(o2->s == sValueCopyConstructed);
|
||||
BOOST_CHECK(v1.s == sIntConstructed);
|
||||
o2 = v1;
|
||||
BOOST_CHECK(o2);
|
||||
BOOST_CHECK(o2->s == sCopyAssigned || o2->s == sMoveAssigned);
|
||||
BOOST_CHECK(v1.s == sIntConstructed);
|
||||
|
||||
optional<Oracle> o3;
|
||||
o3 = boost::move(v1);
|
||||
BOOST_CHECK(o3);
|
||||
BOOST_CHECK(o3->s == sValueMoveConstructed);
|
||||
BOOST_CHECK(v1.s == sMovedFrom);
|
||||
}
|
||||
|
||||
void test_move_assign_from_T()
|
||||
{
|
||||
optional<Oracle> o1;
|
||||
o1 = Oracle();
|
||||
BOOST_CHECK(o1);
|
||||
BOOST_CHECK(o1->s == sMoveConstructed);
|
||||
|
||||
o1 = Oracle();
|
||||
BOOST_CHECK(o1);
|
||||
BOOST_CHECK(o1->s == sMoveAssigned);
|
||||
|
||||
Oracle v1;
|
||||
optional<Oracle> o2;
|
||||
o2 = v1;
|
||||
BOOST_CHECK(o2);
|
||||
BOOST_CHECK(o2->s == sCopyConstructed);
|
||||
BOOST_CHECK(v1.s == sDefaultConstructed);
|
||||
o2 = v1;
|
||||
BOOST_CHECK(o2);
|
||||
BOOST_CHECK(o2->s == sCopyAssigned);
|
||||
BOOST_CHECK(v1.s == sDefaultConstructed);
|
||||
|
||||
optional<Oracle> o3;
|
||||
o3 = boost::move(v1);
|
||||
BOOST_CHECK(o3);
|
||||
BOOST_CHECK(o3->s == sMoveConstructed);
|
||||
BOOST_CHECK(v1.s == sMovedFrom);
|
||||
}
|
||||
|
||||
void test_move_assign_from_optional_T()
|
||||
{
|
||||
optional<Oracle> o1;
|
||||
optional<Oracle> o2;
|
||||
o1 = optional<Oracle>();
|
||||
BOOST_CHECK(!o1);
|
||||
optional<Oracle> o3((Oracle()));
|
||||
o1 = o3;
|
||||
BOOST_CHECK(o3);
|
||||
BOOST_CHECK(o3->s == sMoveConstructed);
|
||||
BOOST_CHECK(o1);
|
||||
BOOST_CHECK(o1->s == sCopyConstructed);
|
||||
|
||||
o2 = boost::move(o3);
|
||||
BOOST_CHECK(o3);
|
||||
BOOST_CHECK(o3->s == sMovedFrom);
|
||||
BOOST_CHECK(o2);
|
||||
BOOST_CHECK(o2->s == sMoveConstructed);
|
||||
|
||||
o2 = optional<Oracle>((Oracle()));
|
||||
BOOST_CHECK(o2);
|
||||
BOOST_CHECK(o2->s == sMoveAssigned);
|
||||
}
|
||||
|
||||
class MoveOnly
|
||||
{
|
||||
public:
|
||||
int val;
|
||||
MoveOnly(int v) : val(v) {}
|
||||
MoveOnly(MoveOnly&& rhs) : val(rhs.val) { rhs.val = 0; }
|
||||
void operator=(MoveOnly&& rhs) {val = rhs.val; rhs.val = 0; }
|
||||
|
||||
private:
|
||||
MoveOnly(MoveOnly const&);
|
||||
void operator=(MoveOnly const&);
|
||||
|
||||
friend class MoveOnlyB;
|
||||
};
|
||||
|
||||
void test_with_move_only()
|
||||
{
|
||||
optional<MoveOnly> o1;
|
||||
optional<MoveOnly> o2((MoveOnly(1)));
|
||||
BOOST_CHECK(o2);
|
||||
BOOST_CHECK(o2->val == 1);
|
||||
optional<MoveOnly> o3 (boost::move(o1));
|
||||
BOOST_CHECK(!o3);
|
||||
optional<MoveOnly> o4 (boost::move(o2));
|
||||
BOOST_CHECK(o4);
|
||||
BOOST_CHECK(o4->val == 1);
|
||||
BOOST_CHECK(o2);
|
||||
BOOST_CHECK(o2->val == 0);
|
||||
|
||||
o3 = boost::move(o4);
|
||||
BOOST_CHECK(o3);
|
||||
BOOST_CHECK(o3->val == 1);
|
||||
BOOST_CHECK(o4);
|
||||
BOOST_CHECK(o4->val == 0);
|
||||
}
|
||||
|
||||
class MoveOnlyB
|
||||
{
|
||||
public:
|
||||
int val;
|
||||
MoveOnlyB(int v) : val(v) {}
|
||||
MoveOnlyB(MoveOnlyB&& rhs) : val(rhs.val) { rhs.val = 0; }
|
||||
void operator=(MoveOnlyB&& rhs) {val = rhs.val; rhs.val = 0; }
|
||||
MoveOnlyB(MoveOnly&& rhs) : val(rhs.val) { rhs.val = 0; }
|
||||
void operator=(MoveOnly&& rhs) {val = rhs.val; rhs.val = 0; }
|
||||
|
||||
private:
|
||||
MoveOnlyB(MoveOnlyB const&);
|
||||
void operator=(MoveOnlyB const&);
|
||||
MoveOnlyB(MoveOnly const&);
|
||||
void operator=(MoveOnly const&);
|
||||
};
|
||||
|
||||
void test_move_assign_from_optional_U()
|
||||
{
|
||||
optional<MoveOnly> a((MoveOnly(2)));
|
||||
optional<MoveOnlyB> b1;
|
||||
b1 = boost::move(a);
|
||||
|
||||
BOOST_CHECK(b1);
|
||||
BOOST_CHECK(b1->val == 2);
|
||||
BOOST_CHECK(a);
|
||||
BOOST_CHECK(a->val == 0);
|
||||
|
||||
b1 = MoveOnly(4);
|
||||
|
||||
BOOST_CHECK(b1);
|
||||
BOOST_CHECK(b1->val == 4);
|
||||
}
|
||||
|
||||
void test_move_ctor_from_optional_U()
|
||||
{
|
||||
optional<MoveOnly> a((MoveOnly(2)));
|
||||
optional<MoveOnlyB> b1(boost::move(a));
|
||||
|
||||
BOOST_CHECK(b1);
|
||||
BOOST_CHECK(b1->val == 2);
|
||||
BOOST_CHECK(a);
|
||||
BOOST_CHECK(a->val == 0);
|
||||
|
||||
optional<MoveOnlyB> b2(( optional<MoveOnly>(( MoveOnly(4) )) ));
|
||||
|
||||
BOOST_CHECK(b2);
|
||||
BOOST_CHECK(b2->val == 4);
|
||||
}
|
||||
|
||||
void test_swap()
|
||||
{
|
||||
optional<MoveOnly> a((MoveOnly(2)));
|
||||
optional<MoveOnly> b((MoveOnly(3)));
|
||||
swap(a, b);
|
||||
|
||||
BOOST_CHECK(a->val == 3);
|
||||
BOOST_CHECK(b->val == 2);
|
||||
}
|
||||
|
||||
void test_optional_ref_to_movables()
|
||||
{
|
||||
MoveOnly m(3);
|
||||
optional<MoveOnly&> orm = m;
|
||||
orm->val = 2;
|
||||
BOOST_CHECK(m.val == 2);
|
||||
|
||||
optional<MoveOnly&> orm2 = orm;
|
||||
orm2->val = 1;
|
||||
BOOST_CHECK(m.val == 1);
|
||||
BOOST_CHECK(orm->val == 1);
|
||||
|
||||
optional<MoveOnly&> orm3 = boost::move(orm);
|
||||
orm3->val = 4;
|
||||
BOOST_CHECK(m.val == 4);
|
||||
BOOST_CHECK(orm->val == 4);
|
||||
BOOST_CHECK(orm2->val == 4);
|
||||
}
|
||||
|
||||
// these 4 classes have different noexcept signatures in move operations
|
||||
struct NothrowBoth {
|
||||
NothrowBoth(NothrowBoth&&) BOOST_NOEXCEPT_IF(true) {};
|
||||
void operator=(NothrowBoth&&) BOOST_NOEXCEPT_IF(true) {};
|
||||
};
|
||||
struct NothrowCtor {
|
||||
NothrowCtor(NothrowCtor&&) BOOST_NOEXCEPT_IF(true) {};
|
||||
void operator=(NothrowCtor&&) BOOST_NOEXCEPT_IF(false) {};
|
||||
};
|
||||
struct NothrowAssign {
|
||||
NothrowAssign(NothrowAssign&&) BOOST_NOEXCEPT_IF(false) {};
|
||||
void operator=(NothrowAssign&&) BOOST_NOEXCEPT_IF(true) {};
|
||||
};
|
||||
struct NothrowNone {
|
||||
NothrowNone(NothrowNone&&) BOOST_NOEXCEPT_IF(false) {};
|
||||
void operator=(NothrowNone&&) BOOST_NOEXCEPT_IF(false) {};
|
||||
};
|
||||
|
||||
#ifndef BOOST_NO_NOEXCEPT
|
||||
|
||||
void test_noexcept() // this is a compile-time test
|
||||
{
|
||||
BOOST_STATIC_ASSERT(::boost::is_nothrow_move_constructible<optional<NothrowBoth> >::value);
|
||||
BOOST_STATIC_ASSERT(::boost::is_nothrow_move_assignable<optional<NothrowBoth> >::value);
|
||||
BOOST_STATIC_ASSERT(BOOST_NOEXCEPT_EXPR(optional<NothrowBoth>()));
|
||||
|
||||
BOOST_STATIC_ASSERT(::boost::is_nothrow_move_constructible<optional<NothrowCtor> >::value);
|
||||
BOOST_STATIC_ASSERT(!::boost::is_nothrow_move_assignable<optional<NothrowCtor> >::value);
|
||||
BOOST_STATIC_ASSERT(BOOST_NOEXCEPT_EXPR(optional<NothrowCtor>()));
|
||||
|
||||
BOOST_STATIC_ASSERT(!::boost::is_nothrow_move_constructible<optional<NothrowAssign> >::value);
|
||||
BOOST_STATIC_ASSERT(!::boost::is_nothrow_move_assignable<optional<NothrowAssign> >::value);
|
||||
BOOST_STATIC_ASSERT(BOOST_NOEXCEPT_EXPR(optional<NothrowAssign>()));
|
||||
|
||||
BOOST_STATIC_ASSERT(!::boost::is_nothrow_move_constructible<optional<NothrowNone> >::value);
|
||||
BOOST_STATIC_ASSERT(!::boost::is_nothrow_move_assignable<optional<NothrowNone> >::value);
|
||||
BOOST_STATIC_ASSERT(BOOST_NOEXCEPT_EXPR(optional<NothrowNone>()));
|
||||
}
|
||||
#endif // !defned BOOST_NO_NOEXCEPT
|
||||
|
||||
#endif
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
try
|
||||
{
|
||||
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
|
||||
test_move_ctor_from_U();
|
||||
test_move_ctor_form_T();
|
||||
test_move_ctor_from_optional_T();
|
||||
test_move_ctor_from_optional_U();
|
||||
test_move_assign_from_U();
|
||||
test_move_assign_from_T();
|
||||
test_move_assign_from_optional_T();
|
||||
test_move_assign_from_optional_U();
|
||||
test_with_move_only();
|
||||
test_optional_ref_to_movables();
|
||||
test_swap();
|
||||
#endif
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
BOOST_ERROR("Unexpected Exception caught!");
|
||||
}
|
||||
|
||||
return 0;
|
||||
}
|
||||
|
||||
|
||||
@ -1,373 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/none.hpp"
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
template<class T>
|
||||
inline void check_ref_uninitialized_const ( optional<T&> const& opt )
|
||||
{
|
||||
#ifndef BOOST_OPTIONAL_NO_NULL_COMPARE
|
||||
BOOST_CHECK( opt == 0 ) ;
|
||||
#endif
|
||||
BOOST_CHECK( !opt ) ;
|
||||
}
|
||||
template<class T>
|
||||
inline void check_ref_uninitialized ( optional<T&>& opt )
|
||||
{
|
||||
#ifndef BOOST_OPTIONAL_NO_NULL_COMPARE
|
||||
BOOST_CHECK( opt == 0 ) ;
|
||||
#endif
|
||||
BOOST_CHECK( !opt ) ;
|
||||
|
||||
check_ref_uninitialized_const(opt);
|
||||
}
|
||||
|
||||
template<class T>
|
||||
inline void check_ref_initialized_const ( optional<T&> const& opt )
|
||||
{
|
||||
BOOST_CHECK( opt ) ;
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_NULL_COMPARE
|
||||
BOOST_CHECK( opt != 0 ) ;
|
||||
#endif
|
||||
|
||||
BOOST_CHECK ( !!opt ) ;
|
||||
}
|
||||
|
||||
template<class T>
|
||||
inline void check_ref_initialized ( optional<T&>& opt )
|
||||
{
|
||||
BOOST_CHECK( opt ) ;
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_NULL_COMPARE
|
||||
BOOST_CHECK( opt != 0 ) ;
|
||||
#endif
|
||||
|
||||
BOOST_CHECK ( !!opt ) ;
|
||||
|
||||
check_ref_initialized_const(opt);
|
||||
}
|
||||
|
||||
template<class T>
|
||||
inline void check_ref_value_const ( optional<T&> const& opt, T const& v, T const& z )
|
||||
{
|
||||
BOOST_CHECK( *opt == v ) ;
|
||||
BOOST_CHECK( *opt != z ) ;
|
||||
BOOST_CHECK( opt.get() == v ) ;
|
||||
BOOST_CHECK( opt.get() != z ) ;
|
||||
}
|
||||
|
||||
template<class T>
|
||||
inline void check_ref_value ( optional<T&>& opt, T const& v, T const& z )
|
||||
{
|
||||
BOOST_CHECK( *opt == v ) ;
|
||||
BOOST_CHECK( *opt != z ) ;
|
||||
BOOST_CHECK( opt.get() == v ) ;
|
||||
BOOST_CHECK( opt.get() != z ) ;
|
||||
|
||||
check_ref_value_const(opt,v,z);
|
||||
}
|
||||
|
||||
//
|
||||
// Basic test.
|
||||
// Check ordinary functionality:
|
||||
// Initialization, assignment, comparison and value-accessing.
|
||||
//
|
||||
template<class T>
|
||||
void test_basics( T const* )
|
||||
{
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
T z(0);
|
||||
|
||||
T original_a(1);
|
||||
|
||||
T a(1);
|
||||
|
||||
T b(2);
|
||||
|
||||
T c(10);
|
||||
|
||||
T& aref = a ;
|
||||
T& bref = b ;
|
||||
|
||||
// Default construction.
|
||||
// 'def' state is Uninitialized.
|
||||
// T::T() is not called
|
||||
optional<T&> def ;
|
||||
check_ref_uninitialized(def);
|
||||
|
||||
// Direct initialization.
|
||||
// 'oa' state is Initialized and binds to 'a'
|
||||
// T::T( T const& x ) is NOT used becasue the optional holds a reference.
|
||||
set_pending_copy( ARG(T) ) ;
|
||||
optional<T&> oa ( aref ) ;
|
||||
check_is_pending_copy( ARG(T) );
|
||||
check_ref_initialized(oa);
|
||||
check_ref_value(oa,a,z);
|
||||
*oa = b ; // changes the value of 'a' through the reference
|
||||
BOOST_CHECK( a == b ) ;
|
||||
|
||||
|
||||
// Copy initialization.
|
||||
// T::T ( T const& x ) is NOT used becasue the optional holds a reference.
|
||||
set_pending_copy( ARG(T) ) ;
|
||||
optional<T&> const oa2 ( oa ) ;
|
||||
check_is_pending_copy( ARG(T) ) ;
|
||||
check_ref_initialized_const(oa2);
|
||||
check_ref_value_const(oa2,a,z);
|
||||
*oa2 = original_a ; // restores the value of 'a' through the reference
|
||||
BOOST_CHECK( a == original_a ) ;
|
||||
|
||||
optional<T&> ob ;
|
||||
|
||||
// Value-Assignment upon Uninitialized optional.
|
||||
// T::T ( T const& x ) is NOT used becasue the optional holds a reference.
|
||||
set_pending_copy( ARG(T) ) ;
|
||||
ob = a ; // Binds ob to a temporary non-const refererence to 'a'
|
||||
check_is_pending_copy( ARG(T) ) ;
|
||||
check_ref_initialized(ob);
|
||||
check_ref_value(ob,a,z);
|
||||
a = c;
|
||||
check_ref_value(ob,a,z);
|
||||
|
||||
// Value-Assignment upon Initialized optional.
|
||||
// T::operator= ( T const& x ) is used.
|
||||
set_pending_assign( ARG(T) ) ;
|
||||
ob = b ; // Rebinds 'ob' to 'b' (without changing 'a')
|
||||
check_is_pending_assign( ARG(T) ) ;
|
||||
check_ref_initialized(ob);
|
||||
check_ref_value(ob,b,z);
|
||||
BOOST_CHECK(a == c); // From a=c in previous test
|
||||
b = c;
|
||||
check_ref_value(ob,b,z);
|
||||
|
||||
|
||||
// Assignment initialization.
|
||||
// T::T ( T const& x ) is NOT used becasue the optional holds a reference.
|
||||
set_pending_copy( ARG(T) ) ;
|
||||
optional<T&> const oa3 = b ;
|
||||
check_is_pending_copy( ARG(T) ) ;
|
||||
check_ref_initialized_const(oa3);
|
||||
check_ref_value_const(oa3,b,z);
|
||||
|
||||
|
||||
// Assignment
|
||||
// T::operator=( T const& x ) is used.
|
||||
set_pending_assign( ARG(T) ) ;
|
||||
oa = ob ; // Rebinds 'a' to 'b'
|
||||
check_is_pending_assign( ARG(T) ) ;
|
||||
check_ref_initialized(oa);
|
||||
a = original_a ;
|
||||
check_ref_value(oa,b,z);
|
||||
|
||||
// Uninitializing Assignment upon Initialized Optional
|
||||
// T::~T() is NOT used becasue the optional holds a reference.
|
||||
set_pending_dtor( ARG(T) ) ;
|
||||
set_pending_copy( ARG(T) ) ;
|
||||
oa = def ;
|
||||
check_is_pending_dtor( ARG(T) ) ;
|
||||
check_is_pending_copy( ARG(T) ) ;
|
||||
check_ref_uninitialized(oa);
|
||||
|
||||
// Uninitializing Assignment upon Uninitialized Optional
|
||||
// (Dtor is not called this time)
|
||||
set_pending_dtor( ARG(T) ) ;
|
||||
set_pending_copy( ARG(T) ) ;
|
||||
oa = def ;
|
||||
check_is_pending_dtor( ARG(T) ) ;
|
||||
check_is_pending_copy( ARG(T) ) ;
|
||||
check_ref_uninitialized(oa);
|
||||
|
||||
|
||||
// Deinitialization of Initialized Optional
|
||||
// T::~T() is NOT used becasue the optional holds a reference.
|
||||
set_pending_dtor( ARG(T) ) ;
|
||||
ob.reset();
|
||||
check_is_pending_dtor( ARG(T) ) ;
|
||||
check_ref_uninitialized(ob);
|
||||
|
||||
// Deinitialization of Uninitialized Optional
|
||||
// T::~T() is not called this time
|
||||
set_pending_dtor( ARG(T) ) ;
|
||||
ob.reset();
|
||||
check_is_pending_dtor( ARG(T) ) ;
|
||||
check_ref_uninitialized(ob);
|
||||
}
|
||||
|
||||
//
|
||||
// This verifies relational operators.
|
||||
//
|
||||
template<class T>
|
||||
void test_relops( T const* )
|
||||
{
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
reset_throw_on_copy( ARG(T) ) ;
|
||||
|
||||
T v0(18);
|
||||
T v1(19);
|
||||
T v2(19);
|
||||
|
||||
optional<T&> def0 ;
|
||||
optional<T&> def1 ;
|
||||
optional<T&> opt0(v0);
|
||||
optional<T&> opt1(v1);
|
||||
optional<T&> opt2(v2);
|
||||
|
||||
// Check identity
|
||||
BOOST_CHECK ( def0 == def0 ) ;
|
||||
BOOST_CHECK ( opt0 == opt0 ) ;
|
||||
BOOST_CHECK ( !(def0 != def0) ) ;
|
||||
BOOST_CHECK ( !(opt0 != opt0) ) ;
|
||||
|
||||
// Check when both are uininitalized.
|
||||
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_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_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_CHECK ( opt0 != opt1 ) ;
|
||||
BOOST_CHECK ( opt1 == opt2 ) ;
|
||||
BOOST_CHECK ( opt0 < opt1 ) ;
|
||||
BOOST_CHECK ( opt1 > opt0 ) ;
|
||||
BOOST_CHECK ( opt1 <= opt2 ) ;
|
||||
BOOST_CHECK ( opt1 >= opt0 ) ;
|
||||
}
|
||||
|
||||
template<class T>
|
||||
void test_none( T const* )
|
||||
{
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
using boost::none ;
|
||||
|
||||
T a(1234);
|
||||
|
||||
optional<T&> def0 ;
|
||||
optional<T&> def1(none) ;
|
||||
optional<T&> non_def(a) ;
|
||||
|
||||
BOOST_CHECK ( def0 == none ) ;
|
||||
BOOST_CHECK ( non_def != none ) ;
|
||||
BOOST_CHECK ( !def1 ) ;
|
||||
|
||||
non_def = none ;
|
||||
BOOST_CHECK ( !non_def ) ;
|
||||
}
|
||||
|
||||
template<class T>
|
||||
void test_arrow( T const* )
|
||||
{
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
T a(1234);
|
||||
|
||||
optional<T&> oa(a) ;
|
||||
optional<T&> const coa(a) ;
|
||||
|
||||
BOOST_CHECK ( coa->V() == 1234 ) ;
|
||||
|
||||
oa->V() = 4321 ;
|
||||
|
||||
BOOST_CHECK ( a.V() = 4321 ) ;
|
||||
}
|
||||
|
||||
void test_with_builtin_types()
|
||||
{
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
test_basics( ARG(double) );
|
||||
test_relops( ARG(double) ) ;
|
||||
test_none ( ARG(double) ) ;
|
||||
}
|
||||
|
||||
void test_with_class_type()
|
||||
{
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
test_basics( ARG(X) );
|
||||
test_relops( ARG(X) ) ;
|
||||
test_none ( ARG(X) ) ;
|
||||
test_arrow ( ARG(X) ) ;
|
||||
|
||||
BOOST_CHECK ( X::count == 0 ) ;
|
||||
}
|
||||
|
||||
void test_binding()
|
||||
{
|
||||
int i = 0 ;
|
||||
optional<int&> ori1 = i ;
|
||||
BOOST_CHECK( &(*ori1) == &i ) ;
|
||||
|
||||
optional<int&> ori2(i) ;
|
||||
BOOST_CHECK( &(*ori2) == &i ) ;
|
||||
|
||||
int const ci = 0 ;
|
||||
optional<int const&> orci1 = ci ;
|
||||
BOOST_CHECK( &(*orci1) == &ci ) ;
|
||||
|
||||
optional<int const&> orci2(ci) ;
|
||||
BOOST_CHECK( &(*orci2) == &ci ) ;
|
||||
}
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
try
|
||||
{
|
||||
test_with_class_type();
|
||||
test_with_builtin_types();
|
||||
test_binding();
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
BOOST_ERROR("Unexpected Exception caught!");
|
||||
}
|
||||
|
||||
return 0;
|
||||
}
|
||||
|
||||
|
||||
@ -1,25 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void optional_reference__test_no_converting_assignment()
|
||||
{
|
||||
boost::optional<int&> opt ;
|
||||
double v = 1 ;
|
||||
double& r = v ;
|
||||
opt = r ;
|
||||
}
|
||||
|
||||
|
||||
@ -1,23 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void optional_reference__test_no_converting_ctor()
|
||||
{
|
||||
boost::optional<short&> opt1 ;
|
||||
boost::optional<int&> opt2 = opt1 ;
|
||||
}
|
||||
|
||||
|
||||
@ -1,24 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void optional_reference__test_no_converting_initialization()
|
||||
{
|
||||
short v = 1 ;
|
||||
short& r = v;
|
||||
boost::optional<int&> opt(r) ;
|
||||
}
|
||||
|
||||
|
||||
@ -1,26 +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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void optional_reference__test_no_assign_from_Trefref()
|
||||
{
|
||||
boost::optional<const int&> opt;
|
||||
opt = int(3);
|
||||
}
|
||||
|
||||
#else
|
||||
# error "Test skipped. This cannot be implemented w/o rvalue references."
|
||||
#endif
|
||||
@ -1,26 +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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void optional_reference__test_no_assign_from_Urefref()
|
||||
{
|
||||
boost::optional<const int&> opt;
|
||||
opt = long(3);
|
||||
}
|
||||
|
||||
#else
|
||||
# error "Test skipped. This cannot be implemented w/o rvalue references."
|
||||
#endif
|
||||
@ -1,25 +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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void optional_reference__test_no_init_from_Trefref()
|
||||
{
|
||||
boost::optional<const int&> opt = int(3);
|
||||
}
|
||||
|
||||
#else
|
||||
# error "Test skipped. This cannot be implemented w/o rvalue references."
|
||||
#endif
|
||||
@ -1,25 +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
|
||||
//
|
||||
#include "boost/optional.hpp"
|
||||
|
||||
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
|
||||
//
|
||||
// THIS TEST SHOULD FAIL TO COMPILE
|
||||
//
|
||||
void optional_reference__test_no_init_from_Urefref()
|
||||
{
|
||||
boost::optional<const int&> opt = long(3);
|
||||
}
|
||||
|
||||
#else
|
||||
# error "Test skipped. This cannot be implemented w/o rvalue references."
|
||||
#endif
|
||||
@ -1,108 +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
|
||||
//
|
||||
// Revisions:
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
//#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
//#include "boost/bind/apply.hpp" // Included just to test proper interaction with boost::apply<> as reported by Daniel Wallin
|
||||
#include "boost/mpl/bool.hpp"
|
||||
#include "boost/mpl/bool_fwd.hpp" // For mpl::true_ and mpl::false_
|
||||
#include "boost/static_assert.hpp"
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
|
||||
const int global_i = 0;
|
||||
|
||||
class TestingReferenceBinding
|
||||
{
|
||||
public:
|
||||
TestingReferenceBinding(const int& ii)
|
||||
{
|
||||
BOOST_CHECK(&ii == &global_i);
|
||||
}
|
||||
|
||||
void operator=(const int& ii)
|
||||
{
|
||||
BOOST_CHECK(&ii == &global_i);
|
||||
}
|
||||
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
|
||||
void operator=(int&&)
|
||||
{
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
#endif
|
||||
};
|
||||
|
||||
class TestingReferenceBinding2 // same definition as above, I need a different type
|
||||
{
|
||||
public:
|
||||
TestingReferenceBinding2(const int& ii)
|
||||
{
|
||||
BOOST_CHECK(&ii == &global_i);
|
||||
}
|
||||
|
||||
void operator=(const int& ii)
|
||||
{
|
||||
BOOST_CHECK(&ii == &global_i);
|
||||
}
|
||||
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
|
||||
void operator=(int&&)
|
||||
{
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
#endif
|
||||
};
|
||||
|
||||
|
||||
void test_broken_compiler()
|
||||
{
|
||||
// we are not testing boost::optional here, but the c++ compiler
|
||||
// if this test fails, optional references will obviously fail too
|
||||
|
||||
const int& iref = global_i;
|
||||
BOOST_CHECK(&iref == &global_i);
|
||||
|
||||
TestingReferenceBinding ttt = global_i;
|
||||
ttt = global_i;
|
||||
|
||||
TestingReferenceBinding2 ttt2 = iref;
|
||||
ttt2 = iref;
|
||||
}
|
||||
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
try
|
||||
{
|
||||
test_broken_compiler();
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
BOOST_ERROR("Unexpected Exception caught!");
|
||||
}
|
||||
|
||||
return 0;
|
||||
}
|
||||
|
||||
|
||||
@ -1,64 +0,0 @@
|
||||
// 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
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/optional.hpp"
|
||||
#include "boost/tuple/tuple.hpp"
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
// Test boost::tie() interoperabiliy.
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
typedef X T ;
|
||||
|
||||
try
|
||||
{
|
||||
TRACE( std::endl << BOOST_CURRENT_FUNCTION );
|
||||
|
||||
T z(0);
|
||||
T a(1);
|
||||
T b(2);
|
||||
|
||||
optional<T> oa, ob ;
|
||||
|
||||
// T::T( T const& x ) is used
|
||||
set_pending_dtor( ARG(T) ) ;
|
||||
set_pending_copy( ARG(T) ) ;
|
||||
boost::tie(oa,ob) = std::make_pair(a,b) ;
|
||||
check_is_not_pending_dtor( ARG(T) ) ;
|
||||
check_is_not_pending_copy( ARG(T) ) ;
|
||||
check_initialized(oa);
|
||||
check_initialized(ob);
|
||||
check_value(oa,a,z);
|
||||
check_value(ob,b,z);
|
||||
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
BOOST_ERROR("Unexpected Exception caught!");
|
||||
}
|
||||
|
||||
return 0;
|
||||
}
|
||||
|
||||
|
||||
@ -1,264 +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
|
||||
//
|
||||
// Revisions:
|
||||
//
|
||||
#include<iostream>
|
||||
#include<stdexcept>
|
||||
#include<string>
|
||||
|
||||
#define BOOST_ENABLE_ASSERT_HANDLER
|
||||
|
||||
#include "boost/bind/apply.hpp" // Included just to test proper interaction with boost::apply<> as reported by Daniel Wallin
|
||||
#include "boost/mpl/bool.hpp"
|
||||
#include "boost/mpl/bool_fwd.hpp" // For mpl::true_ and mpl::false_
|
||||
|
||||
#include "boost/optional/optional.hpp"
|
||||
|
||||
#ifdef __BORLANDC__
|
||||
#pragma hdrstop
|
||||
#endif
|
||||
|
||||
#include "boost/none.hpp"
|
||||
|
||||
#include "boost/test/minimal.hpp"
|
||||
|
||||
#include "optional_test_common.cpp"
|
||||
|
||||
struct IntWrapper
|
||||
{
|
||||
int _i;
|
||||
IntWrapper(int i) : _i(i) {}
|
||||
bool operator==(IntWrapper const& rhs) const { return _i == rhs._i; }
|
||||
};
|
||||
|
||||
template <typename T>
|
||||
void test_function_value_or_for()
|
||||
{
|
||||
optional<T> oM0;
|
||||
const optional<T> oC0;
|
||||
optional<T> oM1(1);
|
||||
const optional<T> oC2(2);
|
||||
|
||||
BOOST_CHECK(oM0.value_or(5) == 5);
|
||||
BOOST_CHECK(oC0.value_or(5) == 5);
|
||||
BOOST_CHECK(oM1.value_or(5) == 1);
|
||||
BOOST_CHECK(oC2.value_or(5) == 2);
|
||||
}
|
||||
|
||||
template <typename T>
|
||||
void test_function_value_for()
|
||||
{
|
||||
optional<T> o0;
|
||||
optional<T> o1(1);
|
||||
const optional<T> oC(2);
|
||||
|
||||
try
|
||||
{
|
||||
T& v = o1.value();
|
||||
BOOST_CHECK(v == 1);
|
||||
}
|
||||
catch(...)
|
||||
{
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
|
||||
try
|
||||
{
|
||||
T const& v = oC.value();
|
||||
BOOST_CHECK(v == 2);
|
||||
}
|
||||
catch(...)
|
||||
{
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
|
||||
try
|
||||
{
|
||||
T& v = o0.value();
|
||||
BOOST_CHECK(false);
|
||||
boost::ignore_unused(v);
|
||||
}
|
||||
catch(boost::bad_optional_access const&)
|
||||
{
|
||||
}
|
||||
catch(...)
|
||||
{
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
}
|
||||
|
||||
void test_function_value()
|
||||
{
|
||||
test_function_value_for<int>();
|
||||
test_function_value_for<double>();
|
||||
test_function_value_for<IntWrapper>();
|
||||
}
|
||||
|
||||
struct FatToIntConverter
|
||||
{
|
||||
static int conversions;
|
||||
int _val;
|
||||
FatToIntConverter(int val) : _val(val) {}
|
||||
operator int() const { conversions += 1; return _val; }
|
||||
};
|
||||
|
||||
int FatToIntConverter::conversions = 0;
|
||||
|
||||
void test_function_value_or()
|
||||
{
|
||||
test_function_value_or_for<int>();
|
||||
test_function_value_or_for<double>();
|
||||
test_function_value_or_for<IntWrapper>();
|
||||
|
||||
optional<int> oi(1);
|
||||
BOOST_CHECK(oi.value_or(FatToIntConverter(2)) == 1);
|
||||
BOOST_CHECK(FatToIntConverter::conversions == 0);
|
||||
|
||||
oi = boost::none;
|
||||
BOOST_CHECK(oi.value_or(FatToIntConverter(2)) == 2);
|
||||
BOOST_CHECK(FatToIntConverter::conversions == 1);
|
||||
}
|
||||
|
||||
|
||||
struct FunM
|
||||
{
|
||||
int operator()() { return 5; }
|
||||
};
|
||||
|
||||
struct FunC
|
||||
{
|
||||
int operator()() const { return 6; }
|
||||
};
|
||||
|
||||
int funP ()
|
||||
{
|
||||
return 7;
|
||||
}
|
||||
|
||||
int throw_()
|
||||
{
|
||||
throw int();
|
||||
}
|
||||
|
||||
void test_function_value_or_eval()
|
||||
{
|
||||
optional<int> o1 = 1;
|
||||
optional<int> oN;
|
||||
FunM funM;
|
||||
FunC funC;
|
||||
|
||||
BOOST_CHECK(o1.value_or_eval(funM) == 1);
|
||||
BOOST_CHECK(oN.value_or_eval(funM) == 5);
|
||||
BOOST_CHECK(o1.value_or_eval(FunM()) == 1);
|
||||
BOOST_CHECK(oN.value_or_eval(FunM()) == 5);
|
||||
|
||||
BOOST_CHECK(o1.value_or_eval(funC) == 1);
|
||||
BOOST_CHECK(oN.value_or_eval(funC) == 6);
|
||||
BOOST_CHECK(o1.value_or_eval(FunC()) == 1);
|
||||
BOOST_CHECK(oN.value_or_eval(FunC()) == 6);
|
||||
|
||||
BOOST_CHECK(o1.value_or_eval(funP) == 1);
|
||||
BOOST_CHECK(oN.value_or_eval(funP) == 7);
|
||||
|
||||
#ifndef BOOST_NO_CXX11_LAMBDAS
|
||||
BOOST_CHECK(o1.value_or_eval([](){return 8;}) == 1);
|
||||
BOOST_CHECK(oN.value_or_eval([](){return 8;}) == 8);
|
||||
#endif
|
||||
|
||||
try
|
||||
{
|
||||
BOOST_CHECK(o1.value_or_eval(throw_) == 1);
|
||||
}
|
||||
catch(...)
|
||||
{
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
|
||||
try
|
||||
{
|
||||
BOOST_CHECK(oN.value_or_eval(throw_) == 1);
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
catch(...)
|
||||
{
|
||||
BOOST_CHECK(true);
|
||||
}
|
||||
}
|
||||
|
||||
const optional<std::string> makeConstOptVal()
|
||||
{
|
||||
return std::string("something");
|
||||
}
|
||||
|
||||
void test_const_move()
|
||||
{
|
||||
std::string s5 = *makeConstOptVal();
|
||||
std::string s6 = makeConstOptVal().value();
|
||||
boost::ignore_unused(s5);
|
||||
boost::ignore_unused(s6);
|
||||
}
|
||||
|
||||
|
||||
#ifndef BOOST_NO_CXX11_REF_QUALIFIERS
|
||||
struct MoveOnly
|
||||
{
|
||||
explicit MoveOnly(int){}
|
||||
MoveOnly(MoveOnly &&){}
|
||||
void operator=(MoveOnly &&);
|
||||
private:
|
||||
MoveOnly(MoveOnly const&);
|
||||
void operator=(MoveOnly const&);
|
||||
};
|
||||
|
||||
optional<MoveOnly> makeMoveOnly()
|
||||
{
|
||||
return MoveOnly(1);
|
||||
}
|
||||
|
||||
MoveOnly moveOnlyDefault()
|
||||
{
|
||||
return MoveOnly(1);
|
||||
}
|
||||
|
||||
// compile-time test
|
||||
void test_move_only_getters()
|
||||
{
|
||||
MoveOnly m1 = *makeMoveOnly();
|
||||
MoveOnly m2 = makeMoveOnly().value();
|
||||
MoveOnly m3 = makeMoveOnly().value_or(MoveOnly(1));
|
||||
MoveOnly m4 = makeMoveOnly().value_or_eval(moveOnlyDefault);
|
||||
boost::ignore_unused(m1);
|
||||
boost::ignore_unused(m2);
|
||||
boost::ignore_unused(m3);
|
||||
boost::ignore_unused(m4);
|
||||
}
|
||||
|
||||
#endif // !defined BOOST_NO_CXX11_REF_QUALIFIERS
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
try
|
||||
{
|
||||
test_function_value();
|
||||
test_function_value_or();
|
||||
test_function_value_or_eval();
|
||||
test_const_move();
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
BOOST_ERROR("Unexpected Exception caught!");
|
||||
}
|
||||
|
||||
return 0;
|
||||
}
|
||||
|
||||
|
||||
Reference in New Issue
Block a user