W3cubDocs

/C++

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)
template< class T1, class T2, class NonPair > void construct( std::pair<T1, T2>* p, NonPair&& non_pair );	(7)	(until C++20)

Constructs an object in allocated, but not initialized storage pointed to by p using OuterAllocator and the provided constructor arguments. If the object is of type that itself uses allocators, or if it is std::pair, passes InnerAllocator down to the constructed object.

First, retrieve the outermost allocator OUTERMOST by calling this->outer_allocator(), and then calling the outer_allocator() member function recursively on the result of this call until reaching an allocator that has no such member function.

Define OUTERMOST_ALLOC_TRAITS(x) as std::allocator_traits<std::remove_reference_t<decltype(OUTERMOST(x))>>

1) Creates an object of the given type T by means of uses-allocator construction at the uninitialized memory location indicated by p, using OUTERMOST as the allocator. After adjustment for uses-allocator convention expected by T's constructor, calls OUTERMOST_ALLOC_TRAITS(*this)::construct.

This overload participates in overload resolution only if `U` is not a specialization of `std::pair`.	(until C++20)
Equivalent to `std::apply( [p, this](auto&&... newargs) { OUTERMOST_ALLOC_TRAITS(this)::construct( OUTERMOST(this), p, std::forward<decltype(newargs)>(newargs)...); }, std::uses_allocator_construction_args( inner_allocator(), std::forward<Args>(args)... ) );`	(since C++20)

2) First, if either T1 or T2 is allocator-aware, modifies the tuples x and y to include the appropriate inner allocator, 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, inner_allocator_type>::value == false), then xprime is std::tuple<Args1&&...>(std::move(x)). (It is also required that std::is_constructible<T1, Args1...>::value == true). 2b) if T1 is allocator-aware (std::uses_allocator<T1, inner_allocator_type>::value == true), and its constructor takes an allocator tag

std::is_constructible<T1, std::allocator_arg_t, inner_allocator_type&, Args1...>::value == true,
then xprime is
std::tuple_cat(std::tuple<std::allocator_arg_t, inner_allocator_type&>( std::allocator_arg, inner_allocator() ), std::tuple<Args1&&...>(std::move(x))).

2c) if T1 is allocator-aware (std::uses_allocator<T1, inner_allocator_type>::value == true), and its constructor takes the allocator as the last argument

std::is_constructible<T1, Args1..., inner_allocator_type&>::value == true,
then xprime is
std::tuple_cat(std::tuple<Args1&&...>(std::move(x)), std::tuple<inner_allocator_type&>(inner_allocator())).

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 by calling

std::allocator_traits<O>::construct(OUTERMOST,
 p,
 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 inner allocator 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)

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

Return value

(none).

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 is itself an instance 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 2975	C++11	first overload is mistakenly used for pair construction in some cases	constrained to not accept pairs
P0475R1	C++11	pair piecewise construction may copy the arguments	transformed to tuples of references to avoid copy
LWG 3525	C++11	no overload could handle non-`pair` types convertible to `pair`	reconstructing overload added

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

Return value

Notes

Defect reports

See also