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

From cppreference.com
 
 
Dynamic memory management
Uninitialized memory algorithms
(C++17)
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(C++17)
Constrained uninitialized
memory algorithms
C Library

Allocators
Memory resources
Garbage collection support
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
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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)

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:

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:

Contents

[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

[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]