std::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>::construct
| 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, |
(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) | |
| (5) | ||
| (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 participates in overload resolution only 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 |
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.
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) |
| (until C++20) |
constructs an object in allocated storage (public member function of std::allocator<T>)
|
