std::pmr::polymorphic_allocator<T>::construct
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std::pmr::polymorphic_allocator
Member functions
polymorphic_allocator::construct
(C++20/26*)
Non-member functions
(until C++20)
template< class U, class... Args >
void construct( U* p, Args&&... args );
(1)
(since C++17)
void construct( U* p, Args&&... args );
template< class T1, class T2, class... Args1, class... Args2 >
(2)
(since C++17) void construct( std::pair <T1, T2>* p,
std::piecewise_construct_t,
std::tuple <Args1...> x,
(until C++20)
template< class T1, class T2 >
void construct( std::pair <T1, T2>* p );
(3)
(since C++17) void construct( std::pair <T1, T2>* p );
(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) void construct( std::pair <T1, T2>* p, U&& x, V&& y );
(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) void construct( std::pair <T1, T2>* p, const std::pair <U, V>& xy );
(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) void construct( std::pair <T1, T2>* p, std::pair <U, V>&& xy );
(until C++20)
template< class T1, class T2, class NonPair >
void construct( std::pair <T1, T2>* p, NonPair&& non_pair );
(7)
(since C++17) void construct( std::pair <T1, T2>* p, NonPair&& non_pair );
(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 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.
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)))
7) This overload participates in overload resolution only if given the exposition-only function template
(until C++20)template< class A, class B > void /*deduce-as-pair*/( const std::pair <A, B>& );
, /*deduce-as-pair*/(non_pair) is ill-formed when considered as an unevaluated operand. Equivalent to
construct<T1, T2, T1, T2>(p, std::forward <NonPair>(non_pair));
[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
non_pair
-
non-
pair argument to convert to pair for further construction
[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 |
| LWG 3525 | C++17 | no overload could handle non-pair types convertible to pair
|
reconstructing overload added |