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3411. [fund.ts.v3] Contradictory namespace rules in the Library Fundamentals TS

Section: 8.3 [fund.ts.v3::memory.resource.syn] Status: C++23 Submitter: Thomas Köppe Opened: 2020-02-28 Last modified: 2023-11-22

Priority: 3

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Discussion:

Addresses: fund.ts.v3

The Library Fundamentals TS, N4840, contains a rule about the use of namespaces (paragraph 1), with the consequence:

"This TS does not define std::experimental::fundamentals_v3::pmr"

However, the TS then goes on to define exactly that namespace.

At the time when the subclause memory.resource.syn was added, the IS didn't use to contain a namespace pmr. When the IS adopted that namespace and the TS was rebased, the namespace rule started conflicting with the material in the TS.

I do not have a workable proposed resolution at this point.

[2020-04-07 Issue Prioritization]

Priority to 3 after reflector discussion.

[2021-11-17; Jiang An comments and provides wording]

Given namespaces std::chrono::experimental::fundamentals_v2 and std::experimental::fundamentals_v2::pmr are used in LFTS v2, I think that the intent is that

If we follow the convention, perhaps we should relocate resource_adaptor from std::experimental::fundamentals_v3::pmr to std::pmr::experimental::fundamentals_v3 in LFTS v3. If it's decided that resource_adaptor shouldn't be relocated, I suppose that LWG 3411 can be by striking the wrong wording in 4.1 [fund.ts.v3::general.namespaces] and using qualified std::pmr::memory_resource when needed.

Previous resolution [SUPERSEDED]:

This wording is relative to N4853.

[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

Option A:

  1. Modify 4.1 [fund.ts.v3::general.namespaces] as indicated:

    -2- Each header described in this technical specification shall import the contents of outer-namespacestd::experimental::fundamentals_v3 into outer-namespacestd::experimental as if by 

    namespace outer-namespacestd::experimental::inline fundamentals_v3 {}

    where outer-namespace is a namespace defined in the C++ Standard Library.

  2. Modify [fund.ts.v3::memory.type.erased.allocator], Table 5, as indicated:

    Table 5 — Computed memory_resource for type-erased allocator
    If the type of alloc is then the value of rptr is
    […]
    any other type meeting the requirements (C++20 ¶16.5.3.5) a pointer to a value of type pmr::experimental::resource_adaptor<A> where A is the type of alloc. rptr remains valid only for the lifetime of X.
    […]

  3. Modify 8.3 [fund.ts.v3::memory.resource.syn], header <experimental/memory_resource> synopsis, as indicated:

    namespace std::pmr::experimental::inline fundamentals_v3::pmr {
[…]
} // namespace std::pmr::experimental::inline fundamentals_v3::pmr

Option B:

  1. Modify 4.1 [fund.ts.v3::general.namespaces] as indicated:

    -1- Since the extensions described in this technical specification are experimental and not part of the C++ standard library, they should not be declared directly within namespace std. Unless otherwise specified, all components described in this technical specification either:

    1. — modify an existing interface in the C++ Standard Library in-place,

    2. — are declared in a namespace whose name appends ::experimental::fundamentals_v3 to a namespace defined in the C++ Standard Library, such as std or std::chrono, or

    3. — are declared in a subnamespace of a namespace described in the previous bullet, whose name is not the same as an existing subnamespace of namespace std.

    [Example: This TS does not define std::experimental::fundamentals_v3::pmr because the C++ Standard Library defines std::pmr. — end example]

  2. Modify 7.2 [fund.ts.v3::func.wrap.func], class template function synopsis, as indicated:

    namespace std {
  namespace experimental::inline fundamentals_v3 {

    template<class> class function; // undefined

    template<class R, class... ArgTypes>
    class function<R(ArgTypes...)> {
    public:
      […]
      std::pmr::memory_resource* get_memory_resource() const noexcept;
    };
[…]
} // namespace experimental::inline fundamentals_v3
[…]
} // namespace std

  3. Modify [fund.ts.v3::memory.type.erased.allocator], Table 5, as indicated:

    Table 5 — Computed memory_resource for type-erased allocator
    If the type of alloc is then the value of rptr is
    […]
    any other type meeting the requirements (C++20 ¶16.5.3.5) a pointer to a value of type experimental::pmr::resource_adaptor<A> where A is the type of alloc. rptr remains valid only for the lifetime of X.
    […]

  4. Modify 8.4.1 [fund.ts.v3::memory.resource.adaptor.overview] as indicated:

    -1- An instance of resource_adaptor<Allocator> is an adaptor that wraps a std::pmr::memory_resource interface around Allocator. […] 

    // The name resource_adaptor_imp is for exposition only.
template<class Allocator>
class resource_adaptor_imp : public std::pmr::memory_resource {
public:
  […]
  
  virtual bool do_is_equal(const std::pmr::memory_resource& other) const noexcept;
};

  5. Modify 8.4.3 [fund.ts.v3::memory.resource.adaptor.mem] as indicated:

    -6- bool do_is_equal(const std::pmr::memory_resource& other) const noexcept;

    […]

  6. Modify 32.10.6 [futures.promise], class template promise synopsis, as indicated:

    namespace std {
  namespace experimental::inline fundamentals_v3 {

    template<class R>
    class promise {
    public:
      […]
      std::pmr::memory_resource* get_memory_resource() const noexcept;
    };
[…]
} // namespace experimental::inline fundamentals_v3
[…]
} // namespace std

  7. Modify 32.10.10 [futures.task], class template packaged_task synopsis, as indicated:

    namespace std {
  namespace experimental::inline fundamentals_v3 {

    template<class R, class... ArgTypes>
    class packaged_task<R(ArgTypes...)> {
    public:
      […]
      std::pmr::memory_resource* get_memory_resource() const noexcept;
    };
[…]
} // namespace experimental::inline fundamentals_v3
[…]
} // namespace std

[2022-10-12; Jonathan provides updated wording]

The LWG telecon decided on a simpler form of Option A. The changes to 4.1 [fund.ts.v3::general.namespaces] generated some questions and disagreement, but it was decided that they are not needed anyway. The normative synopses already depict the use of inline namespaces with the stated effects. That paragraph seems more informative than normative, and there were suggestions to strike it entirely. It was decided to keep it but without making the edits. As such, it remains correct for the contents of std::experimental::fundamentals_v3. It doesn't apply to pmr::resource_adaptor, but is not incorrect for that either. The rest of the proposed resolution fully specifies the pmr parts.

[2022-10-19; Reflector poll]

Set status to "Tentatively Ready" after eight 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 N4920.

  1. Modify [fund.ts.v3::memory.type.erased.allocator], Table 5, as indicated:

    Table 5 — Computed memory_resource for type-erased allocator
    If the type of alloc is then the value of rptr is
    […]
    any other type meeting the requirements (C++20 ¶16.5.3.5) a pointer to a value of type pmr::experimental::resource_adaptor<A> where A is the type of alloc. rptr remains valid only for the lifetime of X.
    […]

  2. Modify 8.3 [fund.ts.v3::memory.resource.syn], header <experimental/memory_resource> synopsis, as indicated:

    namespace std::pmr::experimental::inline fundamentals_v3::pmr {
[…]
} // namespace std::pmr::experimental::inline fundamentals_v3::pmr