std::scoped...::construct(3)

🌸 NAME 🌸

okay so bestie, this is the manual page for std::scoped_allocator_adaptor::construct — filed under std::scoped...::construct(3) — from the C++ Standard Library. The header spells it “C++ Standard Libary” and honestly? iconic. The NAME section is basically the same name twice with a dash, so: std::scoped_allocator_adaptor::construct — that’s the construct member of std::scoped_allocator_adaptor. periodt. 💅

✨ Synopsis ✨

Defined in header <scoped_allocator>

template< class T, class... Args >
(1)
void construct( T* p, Args&&... args );

template< class T1, class T2, class... Args1, class... Args2 >
(2) (until C++20)
void construct( std::pair<T1, T2>* p,
                std::piecewise_construct_t,
                std::tuple<Args1...> x,
                std::tuple<Args2...> y );

template< class T1, class T2 >
(3) (until C++20)
void construct( std::pair<T1, T2>* p );

template< class T1, class T2, class U, class V >
(4) (until C++20)
void construct( std::pair<T1, T2>* p, U&& x, V&& y );

template< class T1, class T2, class U, class V >
(5) (until C++20)
void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy );

template< class T1, class T2, class U, class V >
(6) (until C++20)
void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy );

template< class T1, class T2, class NonPair >
(7) (until C++20)
void construct( std::pair<T1, T2>* p, NonPair&& non_pair );

💖 Description 💖

The big picture: this thing constructs an object in allocated, but not initialized storage pointed to by p, using OuterAllocator and the constructor arguments you pass in. If the object is of a type that itself uses allocators, or if it is a std::pair, it passes InnerAllocator down to the constructed object. allocator nepotism, but make it standards-compliant. 🫶

First, it retrieves the outermost allocator OUTERMOST by calling this->outer_allocator(), and then calling the outer_allocator() member function recursively on the result of this call until reaching an allocator that has no such member function.

Then it defines OUTERMOST_ALLOC_TRAITS(x) as std::allocator_traits<std::remove_reference_t<decltype(OUTERMOST(x))>>.

---

1) Creates an object of the given type T by means of uses-allocator construction at the uninitialized memory location indicated by p, using OUTERMOST as the allocator. After adjustment for the uses-allocator convention expected by T’s constructor, it calls OUTERMOST_ALLOC_TRAITS(*this)::construct.

Until C++20, this overload participates in overload resolution only if U is not a specialization of std::pair.

Equivalent to (since C++20):

std::apply(
    [p, this](auto&&... newargs)
    {
        OUTERMOST_ALLOC_TRAITS(*this)::construct(
            OUTERMOST(*this), p,
            std::forward<decltype(newargs)>(newargs)...);
    },
    std::uses_allocator_construction_args(
        inner_allocator(),
        std::forward<Args>(args)...
    )
);

2) First, if either T1 or T2 is allocator-aware, it modifies the tuples x and y to include the appropriate inner allocator, resulting in the two new tuples xprime and yprime, according to these three rules:

• 2a) if T1 is not allocator-aware (std::uses_allocator<T1, inner_allocator_type>::value == false), then xprime is std::tuple<Args1&&...>(std::move(x)). (It is also required that std::is_constructible<T1, Args1...>::value == true).
• 2b) if T1 is allocator-aware (std::uses_allocator<T1, inner_allocator_type>::value == true), and its constructor takes an allocator tag where std::is_constructible<T1, std::allocator_arg_t, inner_allocator_type&, Args1...>::value == true, then xprime is:

std::tuple_cat(std::tuple<std::allocator_arg_t,
                          inner_allocator_type&>(
                              std::allocator_arg, inner_allocator()
                          ),
               std::tuple<Args1&&...>(std::move(x)))

• 2c) if T1 is allocator-aware (std::uses_allocator<T1, inner_allocator_type>::value == true), and its constructor takes the allocator as the last argument where std::is_constructible<T1, Args1..., inner_allocator_type&>::value == true, then xprime is:

std::tuple_cat(std::tuple<Args1&&...>(std::move(x)),
               std::tuple<inner_allocator_type&>(inner_allocator()))

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 by calling (until C++20):

std::allocator_traits<O>::construct(OUTERMOST,
                                    p,
                                    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 inner allocator 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)))

7) 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::vector, that was given a std::scoped_allocator_adaptor as the allocator to use. Since inner_allocator is itself an instance of std::scoped_allocator_adaptor, this function will also be called when the allocator-aware objects constructed through this function start constructing their own members. it’s recursive bestie behavior, but make it allocator lore. 💜

💅 Defect reports 💅

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• LWG 2975 · applied to C++11 — as published: first overload is mistakenly used for pair construction in some cases; correct behavior: constrained to not accept pairs.
• P0475R1 · applied to C++11 — as published: pair piecewise construction may copy the arguments; correct behavior: transformed to tuples of references to avoid copy.
• LWG 3525 · applied to C++11 — as published: no overload could handle non-pair types convertible to pair; correct behavior: reconstructing overload added.

🔥 See also 🔥

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

---

okay bestie, that’s the whole page 💖 source/version/date check: http://cppreference.com, 2024.06.10, page std::scoped...::construct(3). periodt. 🫶