std::uses_allocator_construction_args

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.

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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