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Section: 20.4 [mem.res] Status: WP Submitter: Alisdair Meredith Opened: 2020-07-27 Last modified: 2022-02-10
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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.
[2020-08-03; Billy comments]
It's worth noting that the resolution of CWG 2382 has impact on implementors for this issue.
[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Previous resolution [SUPERSEDED]: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].]
Modify 22.214.171.124 [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);
[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 6.7.2 [intro.object], 20.2.12 [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.
[2022-01-31; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
This wording is relative to N4885.
Modify 126.96.36.199 [mem.res.public] as indicated:
[[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);
Equivalent to: returndo_allocate(bytes, alignment) ;