std:: uses_allocator_construction_args

From cppreference.com

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)
template < class T, class Alloc, class NonPair > constexpr auto uses_allocator_construction_args ( const Alloc & alloc, NonPair && non_pair ) noexcept ;	(9)	(since C++20)

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

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

is a specialization of std::pair . For

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

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

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

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

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)))
);

9) This overload participates in overload resolution only if

is a specialization of std::pair , and given the exposition-only function template

template<class A, class B>
void /*deduce-as-pair*/(const std::pair<A, B>&);

, /*deduce-as-pair*/ ( non_pair ) is ill-formed when considered as an unevaluated operand.
Let the exposition-only class pair-constructor be defined as

class /*pair-constructor*/
{
    const Alloc& alloc_; // exposition only
    NonPair&     u_;     // exposition only
 
    constexpr reconstruct(const std::remove_cv<T>& p) const // exposition only
    {
        return std::make_obj_using_allocator<std::remove_cv<T>>(alloc_, p);
    }
 
    constexpr reconstruct(std::remove_cv<T>&& p) const // exposition only
    {
        return std::make_obj_using_allocator<std::remove_cv<T>>(alloc_, std::move(p));
    }
 
public:
    constexpr operator std::remove_cv<T>() const
    {
        return reconstruct(std::forward<NonPair>(u_));
    }
};

This overload is equivalent to return std:: make_tuple ( pair_construction ) ; , where


        pair_construction

is a value of type


        
         pair-constructor

whose


        
         alloc_

and

u_

members are


        alloc

and


        non_pair

respectively.

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` data member
y	-	tuple of arguments to pass to the constructors of `T` 's `second` data member
u	-	single argument to pass to the constructor of `T` 's `first` data member
v	-	single argument to pass to the constructor of `T` 's `second` data member
pr	-	a pair whose `first` data member will be passed to the constructor of `T` 's `first` data member and `second` data member will be passed to the constructor of `T` 's `second` data member
non_pair	-	single argument to convert to a std::pair for further construction

Return value

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

Notes

The overloads (2-9) 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).

When used in uses-allocator construction, the conversion function of pair-constructor converts the provided argument to std::pair at first, and then constructs the result from that std::pair by uses-allocator construction.

Example

This section is incomplete
Reason: no example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3525	C++20	no overload could handle non- `pair` types convertible to `pair`	reconstructing overload added

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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)

std:: uses_allocator_construction_args

Parameters

Return value

Notes

Example

Defect reports

See also

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