std::pmr::polymorphic_allocator::construct - cppreference.com (original) (raw)

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. 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));. 7) This overload participates in overload resolution only if given the exposition-only function templatetemplate< 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)

[edit] 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

[edit] Return value

(none)

[edit] 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).

[edit] 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

[edit] See also

| | constructs an object in the allocated storage (function template) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | | constructs an object in allocated storage (public member function of std::allocator) [edit] |