std::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>::construct
// 1)
template < class T, class... Args >
void construct( T* p, Args&&... args );
// 2)
template< class T1, class T2, class... Args1, class... Args2 >
void construct( std::pair<T1, T2>* p,
std::piecewise_construct_t,
std::tuple<Args1...> x,
std::tuple<Args2...> y );
// 3)
template< class T1, class T2 >
void construct( std::pair<T1, T2>* p );
// 4)
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, U&& x, V&& y );
// 5)
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy );
// 6)
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy );
// 7)
template< class T1, class T2, class NonPair >
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))>>
- 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, callsOUTERMOST_ALLOC_TRAITS(*this)::construct
.
Equivalent to
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)...
)
);
-
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 isstd::tuple<Args1&&...>(std::move(x))
. (It is also required thatstd::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 ta
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
std::allocator_traits<O>::construct( OUTERMOST,
p,
std::piecewise_construct,
std::move(xprime),
std::move(yprime) );
- 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.
- Equivalent to
construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(x)),
std::forward_as_tuple(std::forward<V>(y)))
- Equivalent to
construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first),
std::forward_as_tuple(xy.second))
- 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)))
- 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 |
---|---|---|---|
LWG 2975 | C++11 | first overload is mistakenly used for pair construction in some cases | constrained to not accept pairs |
P0475R1 | C++11 | pair piecewise construction may copy the arguments | transformed to tuples of references to avoid copy |
LWG 3525 | C++11 | no overload could handle non-pair types convertible to pair | reconstructing overload added |