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

From cppreference.com
< 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)

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

### See also

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>`)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/scoped_allocator_adaptor/construct&oldid=109821"