NAME¶

std::experimental::pmr::polymorphic_allocator::construct - std::experimental::pmr::polymorphic_allocator::construct

Synopsis¶

template< class U, class... Args > (1) (library fundamentals TS)
void construct( U* p, Args&&... args );
template< class T1, class T2, class... Args1, class...
Args2 >

void construct( std::pair<T1, T2>* p, (2) (library fundamentals TS)
std::piecewise_construct_t,
std::tuple<Args1...> x,

std::tuple<Args2...> y );
template< class T1, class T2 > (3) (library fundamentals TS)
void construct( std::pair<T1, T2>* p );
template< class T1, class T2, class U, class V > (4) (library fundamentals TS)
void construct( std::pair<T1, T2>* p, U&& x, V&& y );
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, const (5) (library fundamentals TS)
std::pair<U, V>& xy );
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, std::pair<U, (6) (library fundamentals TS)
V>&& xy );

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, memory_resource*>::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, memory_resource*>::value == true (the type U
uses allocators, e.g. it is a container) and std::is_constructible<U,
std::allocator_arg_t, memory_resource*, Args...>::value == true, then constructs the
object as if by ::new((void *) p) U(std::allocator_arg, this->resource(),
std::forward<Args>(args)...);.

Otherwise, if std::uses_allocator<U, memory_resource*>::value == true (the type U
uses allocators, e.g. it is a container) and std::is_constructible<U, Args...,
memory_resource*>::value == true, then constructs the object as if by ::new((void *)
p) U(std::forward<Args>(args)..., this->resource());.

Otherwise, the program is ill-formed.

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, memory_resource*>::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, memory_resource*>::value ==
true), and its constructor takes an allocator tag (std::is_constructible<T1,
std::allocator_arg_t, memory_resource*, Args1...>::value == true, then xprime is
std::tuple_cat(std::make_tuple(std::allocator_arg, this->resource()), std::move(x)).

2c) if T1 is allocator-aware (std::uses_allocator<T1, memory_resource*>::value ==
true), and its constructor takes the allocator as the last argument
(std::is_constructible<T1, Args1..., memory_resource*>::value == true), then xprime
is std::tuple_cat(std::move(x), std::make_tuple(this->resource())).

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::vector, that was given a std::polymorphic_allocator as the
allocator to use. Since memory_resource* implicitly converts to
polymorphic_allocator, the memory resource pointer will propagate to any
allocator-aware subobjects using polymorphic allocators.

See also¶

construct constructs an object in the allocated storage
[static] (function template)
construct constructs an object in allocated storage
(until C++20) (public member function of std::allocator<T>)

Category:¶

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