NAME¶

std::scoped_allocator_adaptor::construct - std::scoped_allocator_adaptor::construct

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 >

void construct( std::pair<T1, T2>* p,
std::piecewise_construct_t, (2) (until C++20)
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 );

Constructs an object in allocated, but not initialized storage pointed to by p using
OuterAllocator and the provided constructor arguments. If the object is of type that
itself uses allocators, or if it is std::pair, passes InnerAllocator down to the
constructed object.

First, retrieve 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.

Define 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 uses-allocator convention expected by T's constructor, calls
OUTERMOST_ALLOC_TRAITS(*this)::construct.

This overload participates in overload resolution only if U is not a (until C++20)
specialization of std::pair.
Equivalent to
std::apply(

[p, this](auto&&... newargs)
{
OUTERMOST_ALLOC_TRAITS(*this)::construct(
OUTERMOST(*this), p, (since C++20)
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, modifies the tuples x and y
to include the appropriate inner allocator, 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, 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

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

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, constructs the pair p in allocated
storage by calling (until
std::allocator_traits<O>::construct(OUTERMOST, C++20)
p,
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 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.

Defect reports

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

DR Applied to Behavior as published Correct behavior
first overload is mistakenly used constrained to not accept
LWG 2975 C++11 for pair construction in some pairs
cases
P0475R1 C++11 pair piecewise construction may transformed to tuples of
copy the arguments references to avoid copy
LWG 3525 C++11 no overload could handle non-pair reconstructing overload added
types convertible to pair

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>)