std::experi...::construct(3)

👑 NAME 👑

std::experimental::pmr::polymorphic_allocator::construct — literally std::experimental::pmr::polymorphic_allocator::construct. Bestie, the NAME line repeats it and we are keeping that energy 💅. This page lives in the C++ Standard Libary manual, and its title-line vibe is std::experi...::construct(3).

✨ 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 );

Okay so the whole point is: it constructs an object in allocated, but not initialized storage pointed to by p, using the provided constructor arguments. If the object is of a type that itself uses allocators, or if it is std::pair, it 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 it 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 it 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 it 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, it 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, it 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, it 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) — bestie, it's giving “void energy” ✨.

🦋 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 💜

• Noindexed pages

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💖 source/version: http://cppreference.com · date: 2024.06.10 — that's the whole page, bestie, no cap 🎀✨