std::experimental::pmr::polymorphic_allocator::construct - cppreference.com (original) (raw)

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.

1. 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 isstd::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));.

1. 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.

2. Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(x)),
std::forward_as_tuple(std::forward<V>(y)))

1. Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first),
std::forward_as_tuple(xy.second))

1. 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)))

[edit] Parameters

[edit] Return value

(none)

[edit] 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.

[edit] See also

| | constructs an object in the allocated storage (function template) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | | constructs an object in allocated storage (public member function of std::allocator) [edit] |