std::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>::construct - cppreference.com (original) (raw)

Defined in header <scoped_allocator>
template< class T, class... Args > void construct( T* p, Args&&... args ); (1)
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) (until C++20)
template< class T1, class T2 > void construct( std::pair<T1, T2>* p ); (3) (until C++20)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, U&& x, V&& y ); (4) (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) (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) (until C++20)
Helper function templates
template < class T, class... Args > std::tuple</* see below */> /*concat-args*/( std::tuple<Args...>&& tup ); (7) (exposition only*) (until C++20)

Constructs an object in allocated, but not initialized storage pointed to by p using the outer allocator and the provided constructor arguments. If the object is of a type that itself uses allocators, or if it is std::pair(until C++20), passes the inner allocator down to the constructed object.

  1. Constructs an object of type T by uses-allocator construction at the uninitialized memory location indicated by p using the outermost allocator.
Given std::uses_allocator<T, inner_allocator_type>::value as uses_inner: If uses_inner is false and std::is_constructible<T, Args...>::value is true, calls outermost-construct (p, std::forward<Args>(args)...). Otherwise, if uses_inner and std::is_constructible<T, std::allocator_arg_t, inner_allocator_type&, Args...>::value are both true, calls outermost-construct (p, std::allocator_arg, inner_allocator(), std::forward<Args>(args)...). Otherwise, if uses_inner and std::is_constructible<T, Args..., inner_allocator_type&>::value are both true, calls outermost-construct (p, std::forward<Args>(args)..., inner_allocator()). Otherwise, the program is ill-formed. This overload participates in overload resolution only if T is not a specialization of std::pair. (until C++20)
Equivalent to std::apply ( [p, this](auto&&... newargs) { outermost-construct (p, std::forward<decltype(newargs)>(newargs)...); }, std::uses_allocator_construction_args (inner_allocator(), std::forward<Args>(args)...) ); . (since C++20)

2-6) Constructs a std::pair object by uses-allocator construction at the uninitialized memory location indicated by p using the outermost allocator.

  1. Let xprime be _concat-args_ <T1>(std::move(x)), yprime be _concat-args_ <T2>(std::move(y)), calls _[outermost-construct](helpers.html#outermost-construct "cpp/memory/scoped allocator adaptor/helpers")_ (p, std::piecewise_construct, std::move(xprime), std::move(yprime)).

  2. Merges the arguments contained in tup and additional arguments required by uses-allocator construction of an object of type T.

Given std::uses_allocator<T, inner_allocator_type>::value as uses_inner:

[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
tup - the arguments to be merged

[edit] Notes

This function is called (through std::allocator_traits) by any allocator-aware object, such as std::vector, that was given a std::scoped_allocator_adaptor as the allocator to use. Since inner_allocator_type is itself a specialization of std::scoped_allocator_adaptor, this function will also be called when the allocator-aware objects constructed through this function start constructing their own members.

[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 2203 C++11 inner allocators were obtained by value-initializingan inner_allocator_type object obtained by calling inner_allocator()
LWG 2511(P0475R1) C++11 concat-args might copy elements of std::tuples eliminated all element copy operations
LWG 2586 C++11 only constructions frominner_allocator_type rvalues were checked checks constructions from non-constinner_allocator_type lvalues instead
LWG 2975 C++11 overload (1) was not constrained constrained to refuse std::pair

[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] |