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)
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 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)))
);

9) This overload participates in overload resolution only if T 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.

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

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)