std::pmr::polymorphic_allocator::construct
| template < class U, class... Args > void construct( U* p, Args&&... args ); |
(1) | (since C++17) |
| template< class T1, class T2, class... Args1, class... Args2 > void construct( std::pair<T1, T2>* p, |
(2) | (since C++17) |
| template< class T1, class T2 > void construct( std::pair<T1, T2>* p ); |
(3) | (since C++17) |
| template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, U&& x, V&& y ); |
(4) | (since C++17) |
| (5) | (since C++17) | |
| (6) | (since C++17) | |
Constructs an object in allocated, but not initialized storage pointed to by p the provided constructor arguments. If the object is of type that itself uses allocators, or if it is std::pair, passes this->resource() down to the constructed object.
1) If std::uses_allocator<U, polymorphic_allocator>::value==false (the type U does not use allocators) and std::is_constructible<U, Args...>::value==true, then constructs the object as if by ::new((void *) p) U(std::forward<Args>(args)... );.
Otherwise, if std::uses_allocator<U, polymorphic_allocator>::value==true (the type U uses allocators, e.g. it is a container) and std::is_constructible<U, std::allocator_arg_t, polymorphic_allocator, Args...>::value==true, then constructs the object as if by ::new((void *) p) U(std::allocator_arg, *this, std::forward<Args>(args)... );.
Otherwise, if std::uses_allocator<U, polymorphic_allocator>::value==true (the type U uses allocators, e.g. it is a container) and std::is_constructible<U, Args..., polymorphic_allocator>::value==true, then constructs the object as if by ::new((void *) p) U(std::forward<Args>(args)..., *this);.
Otherwise, the program is ill-formed.
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 this->resource(), 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, polymorphic_allocator>::value==false) and std::is_constructible<T1, Args1...>::value==true, then xprime is x, unmodified.
2b) if T1 is allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==true), and its constructor takes an allocator tag (std::is_constructible<T1, std::allocator_arg_t, polymorphic_allocator, Args1...>::value==true, then xprime is
std::tuple_cat(std::make_tuple(std::allocator_arg, *this), std::move(x))
2c) if T1 is allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==true), and its constructor takes the allocator as the last argument (std::is_constructible<T1, Args1..., polymorphic_allocator>::value==true), then xprime is std::tuple_cat(std::move(x), std::make_tuple(*this)).
2d) Otherwise, the program is ill-formed.
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 as if by ::new((void *) p) pair<T1, T2>(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 memory resource 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)))
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::pmr::vector (or another std::vector that was given a std::std::polymorphic_allocator as the allocator to use).
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 2969 | C++17 | uses-allocator construction passed resource()
|
passes *this
|
| LWG 2975 | C++17 | first overload is mistakenly used for pair construction in some cases | constrained to not accept pairs |
See also
| [static] |
constructs an object in the allocated storage (function template) |
| (deprecated in C++17) |
constructs an object in allocated storage (public member function of std::allocator) |