# std::scoped_allocator_adaptor::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) |
template< class T1, class T2 > void construct( std::pair<T1, T2>* p ); | (3) |
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, U&& x, V&& y ); | (4) |
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy ); | (5) |
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy ); | (6) |

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, determines the outermost allocator type `OUTERMOST`: it is the type that would be returned by calling this->outer_allocator(), and then calling the `outer_allocator()` member function recursively on the result of this call until reaching the type that has no such member function. That type is the outermost allocator.

Then:

1) If std::uses_allocator<T, inner_allocator_type>::value==false (the type `T` does not use allocators) and if std::is_constructible<T, Args...>::value==true, then calls

std::allocator_traits<OUTERMOST>::construct( OUTERMOST(*this),
p,
std::forward<Args>(args)... );

Otherwise, if std::uses_allocator<T, inner_allocator_type>::value==true (the type `T` uses allocators, e.g. it is a container) and if std::is_constructible<T, std::allocator_arg_t, inner_allocator_type&, Args...>::value==true, then calls

std::allocator_traits<OUTERMOST>::construct( OUTERMOST(*this),
p,
std::allocator_arg,
inner_allocator(),
std::forward<Args>(args)... );

Otherwise, std::uses_allocator<T, inner_allocator_type>::value==true (the type `T` uses allocators, e.g. it is a container) and if std::is_constructible<T, Args..., inner_allocator_type&>::value==true, then calls

std::allocator_traits<OUTERMOST>::construct( OUTERMOST(*this),
p,
std::forward<Args>(args)...,
inner_allocator());

Otherwise, compilation error is issued because although `std::uses_allocator<T>` claimed that `T` is allocator-aware, it lacks either form of allocator-accepting constructors.

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 `x`, unmodified. (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::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::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 (this also requires that all types in Args1... and Args2... are `CopyConstructible`), constructs the pair `p` in allocated storage by calling

std::allocator_traits<OUTERMOST>::construct( OUTERMOST(*this),
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)))

### 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.

### See also

construct [static] | constructs an object in the allocated storage (function template)
construct (deprecated in C++17) | constructs an object in allocated storage (public member function of `std::allocator`)

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