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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)

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 only participates in overload resolution 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)))

(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`

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

construct [static]	constructs an object in the allocated storage (function template)
construct (deprecated in C++17)(removed in 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