NAME¶

std::uses_allocator_construction_args - std::uses_allocator_construction_args

Synopsis¶

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& (1) (since C++20)
alloc,

Args&&... args ) noexcept;
T is a specialization of std::pair
template< class T, class Alloc, class Tuple1, class Tuple2 >

constexpr auto uses_allocator_construction_args( const Alloc& (2) (since C++20)
alloc,

std::piecewise_construct_t, Tuple1&& x, Tuple2&& y ) noexcept;
template< class T, class Alloc >
constexpr auto uses_allocator_construction_args( const Alloc& (3) (since C++20)
alloc ) noexcept;
template< class T, class Alloc, class U, class V >

constexpr auto uses_allocator_construction_args( const Alloc& (4) (since C++20)
alloc,

U&& u, V&& v ) noexcept;
template< class T, class Alloc, class U, class V >

constexpr auto uses_allocator_construction_args( const Alloc& (5) (since C++23)
alloc,

std::pair<U, V>& pr ) noexcept;
template< class T, class Alloc, class U, class V >

constexpr auto uses_allocator_construction_args( const Alloc& (6) (since C++20)
alloc,

const std::pair<U, V>& pr ) noexcept;
template< class T, class Alloc, class U, class V >

constexpr auto uses_allocator_construction_args( const Alloc& (7) (since C++20)
alloc,

std::pair<U, V>&& pr ) noexcept;
template< class T, class Alloc, class U, class V >

constexpr auto uses_allocator_construction_args( const Alloc& (8) (since C++23)
alloc,

const std::pair<U, V>&& pr ) noexcept;
template< class T, class Alloc, class NonPair >

constexpr auto uses_allocator_construction_args( const Alloc& (9) (since C++20)
alloc,

NonPair&& non_pair ) noexcept;

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
a pair whose first data member will be passed to the constructor of T's
pr - 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 reconstructing overload
convertible to pair added

See also¶

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

Category:¶

* Todo no example