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Showing content from https://en.cppreference.com/w/cpp/language/../memory/scoped_allocator_adaptor/construct.html below:

std::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>::construct - cppreference.com

template< class T, class... Args > void construct( T* p, Args&&... args );

(1) (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

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)

Given

std::uses_allocator<T, inner_allocator_type>::value

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.

[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-initializing
an 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 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 [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<T>) [edit]

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