mirror of
https://github.com/boostorg/optional.git
synced 2025-07-29 20:17:21 +02:00
Merge branch 'develop'
This commit is contained in:
Andrzej Krzemienski
@ -23,6 +23,7 @@ Distributed under the Boost Software License, Version 1.0.
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[/ Cited Boost resources ]
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[def __BOOST_VARIANT__ [@../../../variant/index.html Boost.Variant]]
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[def __BOOST_TUPLE__ [@../../../tuple/index.html Boost.Tuple]]
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[def __BOOST_TRIBOOL__ [@../../../../doc/html/tribool.html boost::tribool]]
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[def __OPTIONAL_POINTEE__ [@../../../utility/OptionalPointee.html OptionalPointee]]
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@ -36,7 +37,9 @@ Distributed under the Boost Software License, Version 1.0.
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[/ Other web resources ]
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[def __HASKELL__ [@http://www.haskell.org/ Haskell]]
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[def __SGI_DEFAULT_CONSTRUCTIBLE__ [@http://www.sgi.com/tech/stl/DefaultConstructible.html Default Constructible]]
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[def __SGI_DEFAULT_CONSTRUCTIBLE__ [@http://www.sgi.com/tech/stl/DefaultConstructible.html `DefaultConstructible`]]
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[def __SGI_LESS_THAN_COMPARABLE__ [@http://www.sgi.com/tech/stl/LessThanComparable.html `LessThanComparable`]]
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[def __SGI_EQUALITY_COMPARABLE__ [@http://www.sgi.com/tech/stl/EqualityComparable.html `EqualityComparable`]]
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[/ Icons ]
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@ -71,8 +74,10 @@ This is how you solve it with `boost::optional`:
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[include 01_quick_start.qbk]
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[section Tutorial]
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[include 12_motivation.qbk]
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[include 13_development.qbk]
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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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@ -88,12 +88,12 @@ Suppose we want to implement a ['lazy load] optimization. This is because we do
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class Widget
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{
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boost::optional<Resource> resource_;
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mutable boost::optional<const Resource> resource_;
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public:
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Widget() {}
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Resource& getResource() // not thread-safe
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const Resource& getResource() const // not thread-safe
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{
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if (resource_ == boost::none)
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resource_.emplace("resource", "arguments");
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@ -119,7 +119,7 @@ In other place we want to use the result of `getPeriod`, but want the two dates
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Date begin, end; // Error: no default ctor!
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boost::tie(begin, end) = getPeriod();
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The second line works already, this is the capability of Boost.Tuple library, but the first line won't work. We could set some invented initial dates, but it is confusing and may be an unacceptable cost, given that these values will be overwritten in the next line anyway. This is where `optional` can help:
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The second line works already, this is the capability of __BOOST_TUPLE__ library, but the first line won't work. We could set some invented initial dates, but it is confusing and may be an unacceptable cost, given that these values will be overwritten in the next line anyway. This is where `optional` can help:
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boost::optional<Date> begin, end;
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boost::tie(begin, end) = getPeriod();
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@ -138,7 +138,7 @@ Suppose you want to ask users to choose some number (an `int`). One of the valid
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++choices[choice];
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}
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This works because `optional<T>` is `LessThanComparable` whenever `T` is `LessThanComparable`. 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`. So the set of values that type `optional<T>` can assume is {`boost::none`, -2147483648, -2147483647, ..., -1, 0, 1, ..., 2147483647} (assuming a 32-bit `int`).
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This works because `optional<T>` is __SGI_LESS_THAN_COMPARABLE__ whenever `T` is __SGI_LESS_THAN_COMPARABLE__. In this case the state of being uninitialized is treated as a yet another value of `T`, which is compared less than any value of `T`.
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[endsect]
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||||
|
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[endsect]
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||||
|
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37
doc/12_when_to_use.qbk
Normal file
37
doc/12_when_to_use.qbk
Normal file
@ -0,0 +1,37 @@
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[section When to use Optional]
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It is recommended to use `optional<T>` in situations where there is exactly one, clear (to all parties) reason for having no value of type `T`, and where the lack of value is as natural as having any regular value of `T`. One example of such situation is asking the user in some GUI form to optionally specify some limit on an `int` value, but the user is allowed to say 'I want the number not to be constrained by the maximum'.
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For another example, consider a config parameter specifying how many threads the application should launch. Leaving this parameter unspecified means that the application should decide itself. For yet another example, consider a function returning the index of the smallest element in a `vector`. We need to be prepared for the situation, where the `vector` is empty. Therefore a natural signature for such function would be:
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template <typename T>
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optional<size_t> find_smallest_elem(const std::vector<T>& vec);
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Here, having received an empty `vec` and having no `size_t` to return is not a ['failure] but a ['normal], albeit irregular, situation.
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||||
|
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Another typical situation is to indicate that we do not have a value yet, but we expect to have it later. This notion can be used in implementing solutions like lazy initialization or a two-phase initialization.
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||||
|
||||
`optional` can be used to take a non-__SGI_DEFAULT_CONSTRUCTIBLE__ type `T` and create a sibling type with a default constructor. This is a way to add a ['null-state] to any type that doesn't have it already.
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||||
|
||||
Sometimes type `T` already provides a built-in null-state, but it may still be useful to wrap it into `optional`. Consider `std::string`. When you read a piece of text from a GUI form or a DB table, it is hardly ever that the empty string indicates anything else but a missing text. And some data bases do not even distinguish between a null string entry and a non-null string of length 0. Still, it may be practical to use `optional<string>` to indicate in the returned type that we want to treat the empty string in a special dedicated program path:
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||||
|
||||
if(boost::optional<std::string> name = ask_user_name()) {
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assert(*name != "");
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logon_as(*name);
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||||
}
|
||||
else {
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||||
skip_logon();
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||||
}
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|
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In the example above, the assertion indicates that if we choose to use this technique, we must translate the empty string state to an optional object with no contained value (inside function `ask_user_name`).
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[heading Not recommended usages]
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It is not recommended to use `optional` to indicate that we were not able to compute a value because of a ['failure]. It is difficult to define what a failure is, but it usually has one common characteristic: an associated information on the cause of the failure. This can be the type and member data of an exception object, or an error code. It is a bad design to signal a failure and not inform about the cause. If you do not want to use exceptions, and do not like the fact that by returning error codes you cannot return the computed value, you can use [@https://github.com/ptal/Boost.Expected Expected] library. It is sort of __BOOST_VARIANT__ that contains either a computed value or a reason why the computation failed.
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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
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||||
expected<ErrorT, optional<int>> convert2(const string& str); // return either optional or error
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||||
|
||||
[endsect]
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||||
37
doc/13_relational_operators.qbk
Normal file
37
doc/13_relational_operators.qbk
Normal file
@ -0,0 +1,37 @@
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||||
|
||||
[section Relational operators]
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||||
|
||||
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`:
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||||
|
||||
boost::optional<int> oN = boost::none;
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||||
boost::optional<int> o0 = 0;
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||||
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();
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||||
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`).
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||||
[endsect]
|
||||
@ -1,10 +1,38 @@
|
||||
|
||||
[section Type requirements]
|
||||
|
||||
At the very minimum for `optional<T>` to work with a minimum interface it is required that `T` has a publicly accessible no-throw destructor. In that case you need to initialize the optional object with function `emplace()` or use [*InPlaceFactories].
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Additionally, if `T` is `Moveable`, `optional<T>` is also `Moveable` and can be easily initialized from an rvalue of type `T` and be passed by value.
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||||
Additionally, if `T` is `Copyable`, `optional<T>` is also `Copyable` and can be easily initialized from an lvalue of type `T`.
|
||||
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:
|
||||
|
||||
`T` [_is not] required to be __SGI_DEFAULT_CONSTRUCTIBLE__.
|
||||
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 `MoveConstructible`, `optional<T>` is also `MoveConstructible` 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 `CopyConstructible`, `optional<T>` is also `CopyConstructible` 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 `MoveConstructible` 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 `CopyConstructible` 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]
|
||||
|
||||
@ -761,8 +761,16 @@ __SPACE__
|
||||
of type `T` with `std::forward<Args>(args)...`.
|
||||
* [*Postconditions: ] `*this` is [_initialized].
|
||||
* [*Throws:] Whatever the selected `T`'s constructor throws.
|
||||
* [*Notes:] `T` need not be `MoveConstructible` or `MoveAssignable`.
|
||||
* [*Notes:] `T` need not be `MoveConstructible` or `MoveAssignable`. On compilers that do not support variadic templates, the signature falls back to single-argument: `template<class Arg> void emplace(Arg&& arg)`. On compilers that do not support rvalue references, the signature falls back to two overloads: taking `const` and non-`const` lvalue reference.
|
||||
* [*Exception Safety:] If an exception is thrown during the initialization of `T`, `*this` is ['uninitialized].
|
||||
* [*Example:]
|
||||
``
|
||||
T v;
|
||||
optional<const T> opt;
|
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opt.emplace(0); // create in-place using ctor T(int)
|
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opt.emplace(); // destroy previous and default-construct another T
|
||||
opt.emplace(v); // destroy and copy-construct in-place (no assignment called)
|
||||
``
|
||||
|
||||
__SPACE__
|
||||
|
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|
||||
@ -44,10 +44,10 @@
|
||||
<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 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 <code class="computeroutput"><span class="identifier">optional</span></code> can help:
|
||||
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>
|
||||
|
||||
@ -35,12 +35,12 @@
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Widget</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">Resource</span><span class="special">></span> <span class="identifier">resource_</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="identifier">Resource</span><span class="special">&</span> <span class="identifier">getResource</span><span class="special">()</span> <span class="comment">// not thread-safe</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>
|
||||
@ -53,13 +53,15 @@
|
||||
<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">Default
|
||||
Constructible</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 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">
|
||||
|
||||
@ -43,14 +43,9 @@
|
||||
</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 <code class="computeroutput"><span class="identifier">LessThanComparable</span></code> whenever
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">LessThanComparable</span></code>.
|
||||
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>.
|
||||
So the set of values that type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
can assume is {<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>, -2147483648, -2147483647, ..., -1,
|
||||
0, 1, ..., 2147483647} (assuming a 32-bit <code class="computeroutput"><span class="keyword">int</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>
|
||||
|
||||
@ -1094,7 +1094,11 @@
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
need not be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
|
||||
or <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>.
|
||||
or <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>. On
|
||||
compilers that do not support variadic templates, the signature falls
|
||||
back to single-argument: <code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">(</span><span class="identifier">Arg</span><span class="special">&&</span> <span class="identifier">arg</span><span class="special">)</span></code>. On compilers that do not support rvalue
|
||||
references, the signature falls back to two overloads: taking <code class="computeroutput"><span class="keyword">const</span></code> and non-<code class="computeroutput"><span class="keyword">const</span></code>
|
||||
lvalue reference.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Exception Safety:</strong></span> If an exception is
|
||||
@ -1102,6 +1106,15 @@
|
||||
<code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
|
||||
is <span class="emphasis"><em>uninitialized</em></span>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<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">opt</span><span class="special">;</span>
|
||||
<span class="identifier">opt</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> <span class="comment">// create in-place using ctor T(int)</span>
|
||||
<span class="identifier">opt</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">();</span> <span class="comment">// destroy previous and default-construct another T</span>
|
||||
<span class="identifier">opt</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// destroy and copy-construct in-place (no assignment called)</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/space.png" alt="space"></span>
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@ -7,7 +7,7 @@
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<link rel="home" href="../../../index.html" title="Boost.Optional">
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113
doc/html/boost_optional/tutorial/relational_operators.html
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<a name="boost_optional.tutorial.relational_operators"></a><a class="link" href="relational_operators.html" title="Relational operators">Relational
|
||||
operators</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
<a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>. Two optional
|
||||
objects containing a value compare in the same 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>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -27,22 +27,73 @@
|
||||
<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>
|
||||
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>.
|
||||
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 <code class="computeroutput"><span class="identifier">CopyConstructible</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">CopyConstructible</span></code> 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>
|
||||
<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>.
|
||||
Similarly, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Copyable</span></code> (both <code class="computeroutput"><span class="identifier">CopyConstructible</span></code>
|
||||
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>
|
||||
|
||||
142
doc/html/boost_optional/tutorial/when_to_use_optional.html
Normal file
142
doc/html/boost_optional/tutorial/when_to_use_optional.html
Normal file
@ -0,0 +1,142 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>When to use Optional</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
|
||||
<link rel="home" href="../../index.html" title="Boost.Optional">
|
||||
<link rel="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">
|
||||
</head>
|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
|
||||
<table cellpadding="2" width="100%"><tr>
|
||||
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
|
||||
<td align="center"><a href="../../../../../../index.html">Home</a></td>
|
||||
<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="design_overview/the_interface.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="relational_operators.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<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>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@ -55,6 +55,10 @@
|
||||
<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
|
||||
@ -129,7 +133,7 @@
|
||||
</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: June 05, 2014 at 22:46:34 GMT</small></p></td>
|
||||
<td align="left"><p><small>Last revised: June 08, 2014 at 18:14:37 GMT</small></p></td>
|
||||
<td align="right"><div class="copyright-footer"></div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
|
||||
184
doc/html/optional/reference.html
Normal file
184
doc/html/optional/reference.html
Normal file
@ -0,0 +1,184 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Reference</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="prev" href="../boost_optional/tutorial/type_requirements.html" title="Type requirements">
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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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<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 class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="optional.reference"></a><a class="link" href="reference.html" title="Reference">Reference</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<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> <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> <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="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"><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"><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="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>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
153
doc/html/optional/tutorial.html
Normal file
153
doc/html/optional/tutorial.html
Normal file
@ -0,0 +1,153 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Tutorial</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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<link rel="up" href="../index.html" title="Boost.Optional">
|
||||
<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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||||
</head>
|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
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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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<hr>
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<div class="spirit-nav">
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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>
|
||||
@ -475,6 +475,7 @@ class optional_base : public optional_tag
|
||||
}
|
||||
#endif
|
||||
|
||||
|
||||
#if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES) && (!defined BOOST_NO_CXX11_VARIADIC_TEMPLATES)
|
||||
// Constructs in-place
|
||||
// upon exception *this is always uninitialized
|
||||
@ -485,6 +486,30 @@ class optional_base : public optional_tag
|
||||
::new (m_storage.address()) internal_type( boost::forward<Args>(args)... );
|
||||
m_initialized = true ;
|
||||
}
|
||||
#elif (!defined BOOST_NO_CXX11_RVALUE_REFERENCES)
|
||||
template<class Arg>
|
||||
void emplace_assign ( Arg&& arg )
|
||||
{
|
||||
destroy();
|
||||
::new (m_storage.address()) internal_type( boost::forward<Arg>(arg) );
|
||||
m_initialized = true ;
|
||||
}
|
||||
#else
|
||||
template<class Arg>
|
||||
void emplace_assign ( const Arg& arg )
|
||||
{
|
||||
destroy();
|
||||
::new (m_storage.address()) internal_type( arg );
|
||||
m_initialized = true ;
|
||||
}
|
||||
|
||||
template<class Arg>
|
||||
void emplace_assign ( Arg& arg )
|
||||
{
|
||||
destroy();
|
||||
::new (m_storage.address()) internal_type( arg );
|
||||
m_initialized = true ;
|
||||
}
|
||||
#endif
|
||||
|
||||
#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
|
||||
@ -933,6 +958,24 @@ class optional : public optional_detail::optional_base<T>
|
||||
{
|
||||
this->emplace_assign( boost::forward<Args>(args)... );
|
||||
}
|
||||
#elif (!defined BOOST_NO_CXX11_RVALUE_REFERENCES)
|
||||
template<class Arg>
|
||||
void emplace ( Arg&& arg )
|
||||
{
|
||||
this->emplace_assign( boost::forward<Arg>(arg) );
|
||||
}
|
||||
#else
|
||||
template<class Arg>
|
||||
void emplace ( const Arg& arg )
|
||||
{
|
||||
this->emplace_assign( arg );
|
||||
}
|
||||
|
||||
template<class Arg>
|
||||
void emplace ( Arg& arg )
|
||||
{
|
||||
this->emplace_assign( arg );
|
||||
}
|
||||
#endif
|
||||
|
||||
void swap( optional & arg )
|
||||
|
||||
@ -25,6 +25,7 @@ import testing ;
|
||||
[ 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 ]
|
||||
|
||||
@ -58,21 +58,6 @@ private:
|
||||
void operator=(Guard const&);
|
||||
};
|
||||
|
||||
struct Thrower
|
||||
{
|
||||
Thrower(bool throw_) { if (throw_) throw int(); }
|
||||
|
||||
private:
|
||||
Thrower(Thrower const&);
|
||||
Thrower(Thrower&&);
|
||||
};
|
||||
|
||||
struct ThrowOnMove
|
||||
{
|
||||
ThrowOnMove(ThrowOnMove&&) { throw int(); }
|
||||
ThrowOnMove(ThrowOnMove const&) { throw int(); }
|
||||
ThrowOnMove(int){}
|
||||
};
|
||||
|
||||
void test_emplace()
|
||||
{
|
||||
@ -115,6 +100,44 @@ void test_emplace()
|
||||
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;
|
||||
@ -133,30 +156,27 @@ void test_clear_on_throw()
|
||||
}
|
||||
}
|
||||
|
||||
void test_no_moves_on_emplacement()
|
||||
void test_no_assignment_on_emplacement()
|
||||
{
|
||||
try {
|
||||
optional<ThrowOnMove> o;
|
||||
o.emplace(1);
|
||||
BOOST_CHECK(o);
|
||||
}
|
||||
catch (...) {
|
||||
BOOST_CHECK(false);
|
||||
}
|
||||
optional<const std::string> os;
|
||||
BOOST_CHECK(!os);
|
||||
os.emplace("wow");
|
||||
BOOST_CHECK(os);
|
||||
BOOST_CHECK(*os == "wow");
|
||||
}
|
||||
|
||||
#endif
|
||||
|
||||
|
||||
int test_main( int, char* [] )
|
||||
{
|
||||
try
|
||||
{
|
||||
#if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES) && (!defined BOOST_NO_CXX11_VARIADIC_TEMPLATES)
|
||||
test_emplace();
|
||||
test_clear_on_throw();
|
||||
#endif
|
||||
#if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES)
|
||||
test_no_moves_on_emplacement();
|
||||
#endif
|
||||
test_clear_on_throw();
|
||||
test_no_assignment_on_emplacement();
|
||||
}
|
||||
catch ( ... )
|
||||
{
|
||||
|
||||
104
test/optional_test_minimum_requirements.cpp
Normal file
104
test/optional_test_minimum_requirements.cpp
Normal file
@ -0,0 +1,104 @@
|
||||
// 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;
|
||||
}
|
||||
|
||||
|
||||
Reference in New Issue
Block a user