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std::pmr::polymorphic_allocator<T>::construct

template< class U, class... Args > void construct( U* p, Args&&... args );	(1)	(since C++17)
template< class T1, class T2, class... Args1, class... Args2 > void construct( std::pair<T1, T2>* p, std::piecewise_construct_t, std::tuple<Args1...> x, std::tuple<Args2...> y );	(2)	(since C++17) (until C++20)
template< class T1, class T2 > void construct( std::pair<T1, T2>* p );	(3)	(since C++17) (until C++20)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, U&& x, V&& y );	(4)	(since C++17) (until C++20)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy );	(5)	(since C++17) (until C++20)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy );	(6)	(since C++17) (until C++20)
template< class T1, class T2, class NonPair > void construct( std::pair<T1, T2>* p, NonPair&& non_pair );	(7)	(since C++17) (until C++20)

Constructs an object in allocated, but not initialized storage pointed to by p the provided constructor arguments. If the object is of type that itself uses allocators, or if it is std::pair, passes *this down to the constructed object.

1) Creates an object of the given type U by means of uses-allocator construction at the uninitialized memory location indicated by p, using *this as the allocator. This overload participates in overload resolution only if U is not a specialization of std::pair. (until C++20)

2) First, if either T1 or T2 is allocator-aware, modifies the tuples x and y to include this->resource(), resulting in the two new tuples xprime and yprime, according to the following three rules: 2a) if T1 is not allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==false) and std::is_constructible<T1, Args1...>::value==true, then xprime is x, unmodified. 2b) if T1 is allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==true), and its constructor takes an allocator tag (std::is_constructible<T1, std::allocator_arg_t, polymorphic_allocator, Args1...>::value==true, then xprime is std::tuple_cat(std::make_tuple(std::allocator_arg, *this), std::move(x)) 2c) if T1 is allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==true), and its constructor takes the allocator as the last argument (std::is_constructible<T1, Args1..., polymorphic_allocator>::value==true), then xprime is std::tuple_cat(std::move(x), std::make_tuple(*this)). 2d) Otherwise, the program is ill-formed. Same rules apply to T2 and the replacement of y with yprime. Once xprime and yprime are constructed, constructs the pair p in allocated storage as if by ::new((void *) p) pair<T1, T2>(std::piecewise_construct, std::move(xprime), std::move(yprime)); 3) Equivalent to construct(p, std::piecewise_construct, std::tuple<>(), std::tuple<>()), that is, passes the memory resource on to the pair's member types if they accept them. 4) Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(x)),
                                       std::forward_as_tuple(std::forward<V>(y)))

5) Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first),
                                       std::forward_as_tuple(xy.second))

6) Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(xy.first)),
                                       std::forward_as_tuple(std::forward<V>(xy.second)))

7) This overload participates in overload resolution only if 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. Equivalent to.

construct<T1, T2, T1, T2>(p, std::forward<NonPair>(non_pair));

(until C++20)

Parameters

p	-	pointer to allocated, but not initialized storage
args...	-	the constructor arguments to pass to the constructor of `T`
x	-	the constructor arguments to pass to the constructor of `T1`
y	-	the constructor arguments to pass to the constructor of `T2`
xy	-	the pair whose two members are the constructor arguments for `T1` and `T2`
non_pair	-	non-`pair` argument to convert to `pair` for further construction

Return value

(none).

Notes

This function is called (through std::allocator_traits) by any allocator-aware object, such as std::pmr::vector (or another std::vector that was given a std::pmr::polymorphic_allocator as the allocator to use).

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 2969	C++17	uses-allocator construction passed `resource()`	passes `*this`
LWG 2975	C++17	first overload is mistakenly used for pair construction in some cases	constrained to not accept pairs
LWG 3525	C++17	no overload could handle non-`pair` types convertible to `pair`	reconstructing overload added

construct [static]	constructs an object in the allocated storage (function template)
construct (until C++20)	constructs an object in allocated storage (public member function of `std::allocator<T>`)

std::pmr::polymorphic_allocator<T>::construct

Parameters

Return value

Notes

Defect reports

See also