std::pmr::p...::construct(3)

💅 NAME 💅

std::pmr::polymorphic_allocator::construct — std::pmr::polymorphic_allocator::construct. Yes bestie, it said the name twice and we're keeping both 🫶. This is the std::pmr::p...::construct(3) page from the C++ Standard Libary (that spelling is literally in the header, not me 💀).

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✨ Synopsis ✨

okay so here's the whole signature lineup, every overload, no skips, no cap 👑:

template< class U, class... Args >
(1) (since C++17)
void construct( U* p, Args&&... args );
template< class T1, class T2, class... Args1, class... Args2 >

void construct( std::pair<T1, T2>* p,
(since C++17)
std::piecewise_construct_t,
(2) (until C++20)
std::tuple<Args1...> x,

std::tuple<Args2...> y );
template< class T1, class T2 >
(3) (since C++17)
void construct( std::pair<T1, T2>* p );
(until C++20)
template< class T1, class T2, class U, class V >
(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 >
(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 >
(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 >
(7) (since C++17)
void construct( std::pair<T1, T2>* p, NonPair&& non_pair );
(until C++20)

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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 down to the constructed object 💖.

1. Overload 1 — (since C++17); (until C++20). 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. Overload 2 — (since C++17); (until 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) 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, where 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. Overload 3 — (since C++17); (until C++20). 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. Overload 4 — (since C++17); (until C++20). Equivalent to construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(x)), std::forward_as_tuple(std::forward<V>(y)))
5. Overload 5 — (since C++17); (until C++20). Equivalent to construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first), std::forward_as_tuple(xy.second))
6. Overload 6 — (since C++17); (until C++20). 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. Overload 7 — (since C++17); (until C++20). 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));

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🌸 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

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🫶 Return value 🫶

(none) — literally nothing comes back, bestie. Void-coded 💅.

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🦋 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.

• LWG 2969, applied to C++17. Behavior as published: uses-allocator construction passed resource(). Correct behavior: passes *this.
• LWG 2975, applied to C++17. Behavior as published: first overload is mistakenly used for pair construction in some cases. Correct behavior: constrained to not accept pairs.
• LWG 3525, applied to C++17. Behavior as published: no overload could handle non-pair types convertible to pair. Correct behavior: reconstructing overload added.

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🌈 See also 🌈

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

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okay bestie that's the whole page 💖 source/version: http://cppreference.com, dated 2024.06.10 ✨