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Section: 20.2.3 [pointer.traits] Status: Open Submitter: Glen Joseph Fernandes Opened: 2021-04-20 Last modified: 2022-02-26
Priority: 2
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Discussion:
P1474R1 chose to use std::to_address (a mechanism of converting pointer-like types to raw pointers) for contiguous iterators. std::to_address provides an optional customization point via an optional member in std::pointer_traits. However all iterators are not pointers, and the primary template of std::pointer_traits<Ptr> requires that either Ptr::element_type is valid or Ptr is of the form template<T, Args...> or the pointer_traits specialization is ill-formed. This requires specializing pointer_traits for those contiguous iterator types which is inconvenient for users. P1474 should have also made pointer_traits SFINAE friendly.
[2021-05-10; Reflector poll]
Priority set to 2. Send to LEWG.
Daniel: "there is no similar treatment for the rebind member
template and I think it should be clarified whether pointer_to's
signature should exist and in which form in the offending case."
[2022-01-29; Daniel comments]
This issue has some overlap with LWG 3665 in regard to the question how we should handle the rebind_alloc member template of the allocator_traits template as specified by 20.2.8.2 [allocator.traits.types]/11. It would seem preferable to decide for the same approach in both cases.
[2022-02-22 LEWG telecon; Status changed: LEWG → Open]
No objection to unanimous consent for Jonathan's suggestion to make
pointer_traits an empty class when there is no
element_type. Jonathan to provide a paper.
Proposed resolution:
This wording is relative to N4885.
Modify 20.2.3.2 [pointer.traits.types] as indicated:
As additional drive-by fix the improper usage of the term "instantiation" has been corrected.
using element_type = see below;-1- Type: Ptr::element_type if the qualified-id Ptr::element_type is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, T if Ptr is a class template
instantiationspecialization of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise,the specialization is ill-formedpointer_traits has no member element_type.