std::pmr::polymorphic_allocator<T>::construct (3) - Linux Manuals

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

NAME

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

Synopsis

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

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)))

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

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::std::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