std::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>::construct

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)	(until C++20) (exposition only*)

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.

2) Let xprime be concat-args <T1>(std::move(x)), yprime be concat-args <T2>(std::move(y)), calls outermost-construct (p,std::piecewise_construct,std::move(xprime),std::move(yprime)).

3) Equivalent to construct(p, std::piecewise_construct, std::tuple<>(), std::tuple<>());.

4-6) Equivalent to construct(p, std::piecewise_construct,std::forward_as_tuple(xarg), std::forward_as_tuple(yarg));, where xarg and yarg are defined as follows:

Overload	`xarg`	`yarg`
(4)	`std::forward<U>(x)`	`std::forward<V>(y)`
(5)	`xy.first`	`xy.second`
(6)	`std::forward<U>(xy.first)`	`std::forward<V>(xy.second)`

7) 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:

If uses_inner is false and std::is_constructible<T, Args...>::value is true, returns std::tuple<Args&&...>(std::move(tup)).
Otherwise, if uses_inner and std::is_constructible<T, std::allocator_arg_t,inner_allocator_type&,Args...>::value are both true, returns std::tuple_cat(std::tuple<std::allocator_arg_t, inner_allocator_type&>(std::allocator_arg, inner_allocator()),std::tuple<Args&&...>(std::move(tup))).
Otherwise, if uses_inner and std::is_constructible<T, Args..., inner_allocator_type&>::value are both true, returns std::tuple_cat(std::tuple<Args&&...>(std::move(tup)),std::tuple<inner_allocator_type&>(inner_allocator()).
Otherwise, the program is ill-formed.

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

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.

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-initializing an `inner_allocator_type` object	obtained by calling `inner_allocator()`
LWG 2511 (P0475R1)	C++11	`concat-args` might copy elements of `std::tuple`s	eliminated all element copy operations
LWG 2586	C++11	only constructions from `inner_allocator_type` rvalues were checked	checks constructions from non-const `inner_allocator_type` lvalues instead
LWG 2975	C++11	overload (1) was not constrained	constrained to refuse `std::pair`

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::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>::construct

Parameters

Notes

Defect reports

See also