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

    From cppreference.com
     
    C++
     
    Memory management library
    (exposition only*)
    Allocators
    Uninitialized memory algorithms
    Constrained uninitialized memory algorithms
    Memory resources
    Uninitialized storage (until C++20)
    (until C++20*)
    (until C++20*)
    (until C++20*)

    Garbage collector support (until C++23)
    (C++11)(until C++23)
    (C++11)(until C++23)
    (C++11)(until C++23)
    (C++11)(until C++23)
    (C++11)(until C++23)
    (C++11)(until C++23)
     
    std::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)
    Helper function templates
    template < class T, class... Args > std::tuple</* see below */> /*concat-args*/( std::tuple<Args...>&& tup );
    		 (7) (exposition only*)
    (until C++20)
    See More

    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 by 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)
    2-6) Constructs a std::pair object by uses-allocator construction at the uninitialized memory location indicated by p using the outermost allocator.
    2) Let xprime be concat-args <T1>(std::move(x)), yprime be concat-args <T2>(std::move(y)), calls outermost-construct (p, std::piecewise_construct, std::move(xprime), std::move(yprime)).
    3) Equivalent to construct(p, std::piecewise_construct, std::tuple<>(), std::tuple<>());.
    4-6) Equivalent to construct(p, std::piecewise_construct,
    std::forward_as_tuple(xarg), std::forward_as_tuple(yarg));    , where xarg and yarg are defined as follows:
     Overload  xarg yarg
    (4) std::forward<U>(x) std::forward<V>(y)
    (5) xy.first xy.second
    (6) std::forward<U>(xy.first) std::forward<V>(xy.second)
    7) Merges the arguments contained in tup and additional arguments required by uses-allocator construction of an object of type T.
    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, 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.

    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

    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.

    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

    See also

    [static]
    		 constructs an object in the allocated storage
    (function template) [edit]
    (until C++20)
    		 constructs an object in allocated storage
    (public member function of std::allocator<T>) [edit]