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3471. polymorphic_allocator::allocate does not satisfy Cpp17Allocator requirements

Section: 20.4 [mem.res] Status: WP Submitter: Alisdair Meredith Opened: 2020-07-27 Last modified: 2022-02-10

Priority: 3

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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Previous resolution [SUPERSEDED]: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].]

  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]);

[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], 20.2.11 [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.]

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 (6.7.2 [intro.object]) in the allocated region of storage.

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