std::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>::construct
Defined in header <scoped_allocator>
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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, |
(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) |
(5) | (until C++20) | |
(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))>>
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.
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(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
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x | - | the constructor arguments to pass to the constructor of T1
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y | - | the constructor arguments to pass to the constructor of T2
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xy | - | the pair whose two members are the constructor arguments for T1 and T2
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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
[static] |
constructs an object in the allocated storage (function template) |
(deprecated in C++17)(removed in C++20) |
constructs an object in allocated storage (public member function of std::allocator<T> ) |