std::pmr::polymorphic_allocator<T>::construct
// 1)
template < class U, class... Args >
void construct( U* p, Args&&... args );
// 2)
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 );
// 3)
template< class T1, class T2 >
void construct( std::pair<T1, T2>* p );
// 4)
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, U&& x, V&& y );
// 5)
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy );
// 6)
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy );
// 7)
template< class T1, class T2, class NonPair >
void construct( std::pair<T1, T2>* p, NonPair&& non_pair );
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.
- 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. (do C++20)
-
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
) andstd::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 isstd::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 isstd::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));
-
-
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. -
Equivalent to
construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(x)),
std::forward_as_tuple(std::forward<V>(y)))
- Equivalent to
construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first),
std::forward_as_tuple(xy.second))
- 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)))
- This overload participates in overload resolution only if given the exposition-only function template
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));
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
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::pmr::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 |
LWG 3525 | C++17 | no overload could handle non-pair types convertible to pair | reconstructing overload added |