Difference between revisions of "cpp/memory/scoped allocator adaptor/construct"

Revision as of 06:37, 18 March 2018

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, determines the outermost allocator type OUTERMOST: it is the type that would be returned by calling this->outer_allocator(), and then calling the outer_allocator() member function recursively on the result of this call until reaching the type that has no such member function. That type is the outermost allocator.

Then:

1) If std::uses_allocator<T, inner_allocator_type>::value==false (the type T does not use allocators) and if std::is_constructible<T, Args...>::value==true, then calls

std::allocator_traits<OUTERMOST>::construct( OUTERMOST(*this),
                                             p,
                                             std::forward<Args>(args)... );

Otherwise, if std::uses_allocator<T, inner_allocator_type>::value==true (the type T uses allocators, e.g. it is a container) and if std::is_constructible<T, std::allocator_arg_t, inner_allocator_type&, Args...>::value==true, then calls

std::allocator_traits<OUTERMOST>::construct( OUTERMOST(*this),
                                             p,
                                             std::allocator_arg,
                                             inner_allocator(),
                                             std::forward<Args>(args)... );

Otherwise, std::uses_allocator<T, inner_allocator_type>::value==true (the type T uses allocators, e.g. it is a container) and if std::is_constructible<T, Args..., inner_allocator_type&>::value==true, then calls

std::allocator_traits<OUTERMOST>::construct( OUTERMOST(*this),
                                             p,
                                             std::forward<Args>(args)...,
                                             inner_allocator());

Otherwise, compilation error is issued because although std::uses_allocator<T> claimed that T is allocator-aware, it lacks either form of allocator-accepting constructors.

This overload participates in overload resolution only if U is not a specialization of std::pair.

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 x, unmodified. (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::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::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 (this also requires that all types in Args1... and Args2... are Template:concept), constructs the pair p in allocated storage by calling

std::allocator_traits<OUTERMOST>::construct( OUTERMOST(*this),
                                             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)))

Parameters

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.

See also