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Section: 20.2.3 [pointer.traits] Status: C++23 Submitter: Glen Joseph Fernandes Opened: 2021-04-20 Last modified: 2023-11-22
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 20.2.9.2 [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.
Previous resolution [SUPERSEDED]:
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.
[2022-09-27; Jonathan provides new wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 20.2.3.1 [pointer.traits.general] as indicated:
-1- The class template pointer_traits supplies a uniform interface to certain attributes of pointer-like types.
namespace std { template<class Ptr> struct pointer_traits {using pointer = Ptr;using element_type = see below;using difference_type = see below;template<class U> using rebind = see below;static pointer pointer_to(see below r);see below; }; template<class T> struct pointer_traits<T*> { using pointer = T*; using element_type = T; using difference_type = ptrdiff_t; template<class U> using rebind = U*; static constexpr pointer pointer_to(see below r) noexcept; }; }Modify 20.2.3.2 [pointer.traits.types] as indicated:
-?- The definitions in this subclause make use of the following exposition-only class template and concept:
template<class T> struct ptr-traits-elem // exposition only { }; template<class T> requires requires { typename T::element_type; } struct ptr-traits-elem<T> { using type = typename T::element_type; }; template<template<class...> class SomePointer, class T, class... Args> requires (!requires { typename SomePointer<T, Args...>::element_type; }) struct ptr-traits-elem<SomePointer<T, Args...>> { using type = T; }; template<class Ptr> concept has-elem-type = // exposition only requires { typename ptr-traits-elem<Ptr>::type; }-?- If Ptr satisfies has-elem-type, a specialization pointer_traits<Ptr> generated from the pointer_traits primary template has the members described in 20.2.3.2 [pointer.traits.types] and 20.2.3.3 [pointer.traits.functions]; otherwise, such a specialization has no members by any of the names described in those subclauses or in 20.2.3.4 [pointer.traits.optmem].
using pointer = Ptr;using element_type =see belowtypename ptr-traits-elem<Ptr>::type;
-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 instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the specialization is ill-formed.using difference_type = see below;-2- Type: Ptr::difference_type if the qualified-id Ptr::difference_type is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, ptrdiff_t.
template<class U> using rebind = see below;-3- Alias template: Ptr::rebind<U> if the qualified-id Ptr::rebind<U> is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, SomePointer<U, Args> if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind is ill-formed.
[2022-10-11; Jonathan provides improved wording]
[2022-10-19; Reflector poll]
Set status to "Tentatively Ready" after six votes in favour in reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 20.2.3.1 [pointer.traits.general] as indicated:
-1- The class template pointer_traits supplies a uniform interface to certain attributes of pointer-like types.
namespace std { template<class Ptr> struct pointer_traits {using pointer = Ptr;using element_type = see below;using difference_type = see below;template<class U> using rebind = see below;static pointer pointer_to(see below r);see below; }; template<class T> struct pointer_traits<T*> { using pointer = T*; using element_type = T; using difference_type = ptrdiff_t; template<class U> using rebind = U*; static constexpr pointer pointer_to(see below r) noexcept; }; }
Modify 20.2.3.2 [pointer.traits.types] as indicated:
-?- The definitions in this subclause make use of the following exposition-only class template and concept:
template<class T> struct ptr-traits-elem // exposition only { }; template<class T> requires requires { typename T::element_type; } struct ptr-traits-elem<T> { using type = typename T::element_type; }; template<template<class...> class SomePointer, class T, class... Args> requires (!requires { typename SomePointer<T, Args...>::element_type; }) struct ptr-traits-elem<SomePointer<T, Args...>> { using type = T; }; template<class Ptr> concept has-elem-type = // exposition only requires { typename ptr-traits-elem<Ptr>::type; }-?- If Ptr satisfies has-elem-type, a specialization pointer_traits<Ptr> generated from the pointer_traits primary template has the following members as well as those described in 20.2.3.3 [pointer.traits.functions]; otherwise, such a specialization has no members by any of those names.
using pointer = see below;-?- Type: Ptr.
using element_type = see below;-1- Type: typename ptr-traits-elem<Ptr>::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 instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the specialization is ill-formed.using difference_type = see below;-2- Type: Ptr::difference_type if the qualified-id Ptr::difference_type is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, ptrdiff_t.
template<class U> using rebind = see below;-3- Alias template: Ptr::rebind<U> if the qualified-id Ptr::rebind<U> is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, SomePointer<U, Args> if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind is ill-formed.
Modify 20.2.3.4 [pointer.traits.optmem] as indicated:
-1- Specializations of pointer_traits may define the member declared in this subclause to customize the behavior of the standard library. A specialization generated from the pointer_traits primary template has no member by this name.
static element_type* to_address(pointer p) noexcept;-1- Returns: A pointer of type element_type* that references the same location as the argument p.