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

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< cpp‎ | memory‎ | scoped allocator adaptor

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

### See also

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>`)

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