Proposed resolution:

This wording is relative to N4885.

1. Modify [mem.res.public] as indicated:

   [[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);
   

   -2- Effects: Equivalent to: return Allocates storage by calling do_allocate(bytes, alignment); and implicitly creates objects within the allocated region of storage.

   -?- Returns: A pointer to a suitable created object ([intro.object]) in the allocated region of storage.

   -?- Throws: What and when the call to do_allocate throws.

[ 2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP. ]

[ 2022-01-31; Reflector poll ]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[ 2021-05-20 Tim comments and updates wording ]

memory_resource::allocate is the PMR equivalent of malloc and operator new. It therefore needs the "suitable created object" wording (see [intro.object], [c.malloc]). This ensures that it creates the requisite array object automatically for all the allocation functions of polymorphic_allocator, and returns the correct pointer value in all cases.

[ 2020-08-21; Reflector prioritization ]

Set priority to 3 after reflector discussions.

Previous resolution [SUPERSEDED]:

This wording is relative to N4861.

[Drafting note: The proposed wording is inspired by the example given in the "Assertion/note" column of the expression a.allocate(n) in table [tab:cpp17.allocator].]

1. Modify [mem.res.public] as indicated:

   [[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);
   

   -2- Effects: Equivalent to: return do_allocate(bytes, alignment);

   void* p = do_allocate(bytes, alignment);
   return launder(new (p) byte[bytes]);
   

[ 2020-08-03; Billy comments ]

It's worth noting that the resolution of CWG 2382 has impact on implementors for this issue.

With the adoption of P0593R6 in Prague, std::ptr::polymorphic_allocator no longer satisfies the of Cpp17Allocator requirements. Specifically, all calls to allocate(n) need to create an object for an array of n Ts (but not initialize any of those elements). std::pmr::polymorphic_allocator calls its underlying memory resource to allocate sufficient bytes of storage, but it does not create (and start the lifetime of) the array object within that storage.