std::pmr::polymorphic_allocator<T>::construct - cppreference.com

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)

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).

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