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std::pmr::polymorphic_allocator<T>::construct

From cppreference.com
 
 
Dynamic memory management
Smart pointers
(C++11)
(C++11)
(C++11)
(until C++17)
(C++11)
Allocators
Memory resources
Uninitialized storage
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Garbage collection support
Miscellaneous
(C++20)
(C++11)
(C++11)
C Library
Low level memory management
 
 
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,
                std::piecewise_construct_t,
                std::tuple<Args1...> x,

                std::tuple<Args2...> y );
(2) (since C++17)
(until C++20)
template< class T1, class T2 >
void construct( std::pair<T1, T2>* p );
(3) (since C++17)
(until C++20)
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, U&& x, V&& y );
(4) (since C++17)
(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) (since C++17)
(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) (since C++17)
(until C++20)

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) Creates an object of the given type U by means of uses-allocator construction at the uninitialized memory location indicated by p, using *this as the allocator. This overload participates in overload resolution only if U is not a specialization of std::pair. (until C++20)
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.
5) Equivalent to
6) Equivalent to
(until C++20)

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::pmr::vector (or another std::vector that was given a std::pmr::polymorphic_allocator as the allocator to use).

[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 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

[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<T>) [edit]