When the polymorphic allocator infrastructure was moved from the Library Fundamentals TS to the C++17 working draft, pmr::resource_adaptor was left behind. It is not clear whether this was an omission or a deliberately conservative move, but the absence of resource_adaptor in the standard is a hole that must be plugged for a smooth transition to the ubiquitous use of polymorphic_allocator, as proposed in P0339 and P0987. This paper proposes that pmr::resource_adaptor be moved from the LFTS and added to the C++20 working draft.
Added a note for LWG to consider clarifying the alignment requirements for resource_adaptor<A>::do_allocate(). Changed rebind type from char to byte. Rebased to July 2018 draft of the C++ WP.
It is expected that more and more classes, especially those that would not otherwise be templates, will use pmr::polymorphic_allocator<byte> to allocate memory. In order to pass an allocator to one of these classes, the allocator must either already be a polymorphic allocator, or must be adapted from a non-polymorphic allocator. The process of adaptation is facilitated by pmr::resource_adaptor, which is a simple class template, has been in the LFTS for a long time, and has been fully implemented. It is therefore a low-risk, high-benefit component to add to the C++ WP.
pmr::resource_adaptor is a pure library extension requiring no changes to the core language nor to any existing classes in the standard library.
This proposal is based on the Library Fundamentals TS v2, N4617 and the July 2018 draft of the C++ WP, N4762.
In section 19.12.1 [mem.res.syn] of the C++ WD, add the following declaration immediately after the declaration of operator!=(const polymorphic_allocator...):
// 19.12.x resource adaptor
// The name resource_adaptor_imp is for exposition only.
template <class Allocator> class resource_adaptor_imp;
template <class Allocator>
using resource_adaptor = resource_adaptor_imp<
typename allocator_traits<Allocator>::template rebind_alloc<byte>>;Insert between sections 19.12.3 [mem.poly.allocator.class] and 19.12.4 [mem.res.global] of the C++ WP, the whole of section 8.7 [memory.resource.adaptor] from the LFTS v2, including all of its subsections, renumbered appropriately.
LWG NOTE: There was some concern that alignment support was unclear for resource_adaptor. We might consider replacing the Returns clause of do_allocate with something like the following (note the use of normative encouragement in the last sentence:
Returns: a pointer to allocated storage obtained by calling the allocate member function on a suitably rebound copy of m_alloc such that the size and alignment of the allocated memory meet the requirements for a class derived from memory_resource. If the rebound Allocator supports over-aligned memory, then resource_adaptor<Allocator> should also support over-aligned memory.
N4762: Working Draft, Standard for Programming Language C++, Richard Smith, editor, 2018-07-07.
N4617: Programming Languages - C++ Extensions for Library Fundamentals, Version 2, 2016-11-28.
P0339: polymorphic_allocator<> as a vocabulary type, Pablo Halpern, 2018-04-02.
P0987: polymorphic_allocator