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std::scoped_allocator_adaptor::construct

From cppreference.com
 
 
Dynamic memory management
Uninitialized storage
(C++17)
Garbage collection support
Miscellaneous
(C++20)
(C++11)
(C++11)
C Library
Low level memory management
 
 
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, 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. Let O be the type of OUTERMOST.

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

Otherwise, the program is ill-formed: even though std::uses_allocator<T> claimed that T is allocator-aware, it lacks either form of allocator-accepting constructors.

This overload only participates in overload resolution if U is not a specialization of std::pair.

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)))

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

[edit] Return value

(none)

[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 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.

[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 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

[edit] See also

[static]
constructs an object in the allocated storage
(function template) [edit]
(deprecated in C++17)(removed in C++20)
constructs an object in allocated storage
(public member function of std::allocator) [edit]