std::uses_allocator_construction_args

 

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Defined in header <memory>
T is not a specialization of std::pair
template< class T, class Alloc, class... Args > constexpr auto uses_allocator_construction_args( const Alloc& alloc,     Args&&... args) noexcept;	(1)	(since C++20)
T is a specialization of std::pair
template< class T, class Alloc, class Tuple1, class Tuple2 > constexpr auto uses_allocator_construction_args( const Alloc& alloc,     std::piecewise_construct_t, Tuple1&& x, Tuple2&& y) noexcept;	(2)	(since C++20)
template< class T, class Alloc > constexpr auto uses_allocator_construction_args( const Alloc& alloc ) noexcept;	(3)	(since C++20)
template< class T, class Alloc, class U, class V > constexpr auto uses_allocator_construction_args( const Alloc& alloc,     U&& u, V&& v) noexcept;	(4)	(since C++20)
template< class T, class Alloc, class U, class V > constexpr auto uses_allocator_construction_args( const Alloc& alloc,     std::pair<U, V>& pr ) noexcept;	(5)	(since C++23)
template< class T, class Alloc, class U, class V > constexpr auto uses_allocator_construction_args( const Alloc& alloc,     const std::pair<U, V>& pr ) noexcept;	(6)	(since C++20)
template< class T, class Alloc, class U, class V > constexpr auto uses_allocator_construction_args( const Alloc& alloc,     std::pair<U, V>&& pr ) noexcept;	(7)	(since C++20)
template< class T, class Alloc, class U, class V > constexpr auto uses_allocator_construction_args( const Alloc& alloc,     const std::pair<U, V>&& pr ) noexcept;	(8)	(since C++23)

Prepares the argument list needed to create an object of the given type T by means of uses-allocator construction.

1) This overload participates in overload resolution only if T is not a specialization of std::pair. Returns std::tuple determined as follows:

• If std::uses_allocator_v<T, Alloc> is false and std::is_constructible_v<T, Args...> is true, returns std::forward_as_tuple(std::forward<Args>(args)...)
• Otherwise, if std::uses_allocator_v<T, Alloc> is true and std::is_constructible_v<T, std::allocator_arg_t, const Alloc&, Args...> is true, returns std::tuple<std::allocator_arg_t, const Alloc&, Args&&...>(std::allocator_arg, alloc, std::forward<Args>(args)...)
• Otherwise, if std::uses_allocator_v<T, Alloc> is true and std::is_constructible_v<T, Args..., const Alloc&> is true, returns std::forward_as_tuple(std::forward<Args>(args)..., alloc)
• Otherwise, the program is ill-formed

2) This overload participates in overload resolution only if T is a specialization of std::pair. For T that is std::pair<T1, T2>, equivalent to

return std::make_tuple( std::piecewise_construct,
    std::apply( [&alloc](auto&&... args1) {
            return std::uses_allocator_construction_args<T1>( alloc,
                       std::forward<decltype(args1)>(args1)...);
        }, std::forward<Tuple1>(x)),
    std::apply( [&alloc](auto&&... args2) {
            return std::uses_allocator_construction_args<T2>( alloc,
                    std::forward<decltype(args2)>(args2)...);
        }, std::forward<Tuple2>(y))
    );

3) This overload participates in overload resolution only if T is a specialization of std::pair. Equivalent to

return std::uses_allocator_construction_args<T>(alloc,
    std::piecewise_construct, std::tuple<>{}, std::tuple<>{}
);

4) This overload participates in overload resolution only if T is a specialization of std::pair. Equivalent to

return std::uses_allocator_construction_args<T>( alloc,
    std::piecewise_construct,
    std::forward_as_tuple(std::forward<U>(u)),
    std::forward_as_tuple(std::forward<V>(v))
);

5-6) This overload participates in overload resolution only if T is a specialization of std::pair. Equivalent to

return std::uses_allocator_construction_args<T>( alloc,
    std::piecewise_construct,
    std::forward_as_tuple(pr.first),
    std::forward_as_tuple(pr.second)
);

7-8) This overload participates in overload resolution only if T is a specialization of std::pair. Equivalent to

return std::uses_allocator_construction_args<T>( alloc,
    std::piecewise_construct,
    std::forward_as_tuple(std::get<0>(std::move(pr))),
    std::forward_as_tuple(std::get<1>(std::move(pr))));

Parameters

alloc	-	the allocator to use.
args	-	the arguments to pass to T's constructor
x	-	tuple of arguments to pass to the constructors of T's .first
y	-	tuple of arguments to pass to the constructors of T's .second
u	-	single argument to pass to the constructor of T's .first
v	-	single argument to pass to the constructor of T's .second
pr	-	a pair whose .first will be passed to the constructor of T's .first and .second will be passed to the constructor of T's .second

Return value

std::tuple of arguments suitable for passing to the constructor of T.

Notes

The overloads (2-8) provide allocator propagation into std::pair, which supports neither leading-allocator nor trailing-allocator calling conventions (unlike, e.g. std::tuple, which uses leading-allocator convention).

See also

uses_allocator (C++11)	checks if the specified type supports uses-allocator construction (class template)
make_obj_using_allocator (C++20)	creates an object of the given type by means of uses-allocator construction (function template)
uninitialized_construct_using_allocator (C++20)	creates an object of the given type at specified memory location by means of uses-allocator construction (function template)