Movatterモバイル変換

[0]ホーム
URL:

This page is a snapshot from the LWG issues list, see theLibrary Active Issues List for more information and the meaning ofC++20 status.

2139. What is auser-defined type?

Section: 16.4.5.2.1[namespace.std], 19.5[syserr], 20.2.8.1[allocator.uses.trait], 22.10.15.2[func.bind.isbind], 22.10.15.3[func.bind.isplace], 22.10.19[unord.hash], 21.3.9.7[meta.trans.other], 28.3.3.1[locale], 28.3.4.2.5[locale.codecvt], 28.6.11.1.5[re.regiter.incr]Status:C++20Submitter: Loïc JolyOpened: 2012-03-08Last modified: 2021-02-25

Priority:4

View otheractive issues in [namespace.std].

View all otherissues in [namespace.std].

View all issues withC++20 status.

Discussion:

The expression "user-defined type" is used in several places in the standard, but I'm not sure what it means. More specifically, is a type defined in the standard library a user-defined type?

From my understanding of English, it is not. From most of the uses of this term in the standard, it seem to be considered as user defined. In some places, I'm hesitant, e.g. 16.4.5.2.1[namespace.std] p1:

A program may add a template specialization for any standard library template to namespacestd only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

Does it mean we are allowed to add in the namespacestd a specialization forstd::vector<std::pair<T, U>>, for instance?

Additional remarks from the reflector discussion: The traditional meaning of user-defined types refersto class types and enum types, but the library actually means here user-defined types that are not(purely) library-provided. Presumably a new term - likeuser-provided type - should be introducedand properly defined.

[2012-10 Portland: Move to Deferred ]

The issue is real, in that we never define this term and rely on a "know it when I see it"intuition. However, there is a fear that any attempt to pin down a definition is morelikely to introduce bugs than solve them - getting the wording for this precisely correctis likely far more work than we are able to give it.

There is unease at simple closing as NAD, but not real enthusiasm to provide wording either.Move to Deferred as we are not opposed to some motivated individual coming back with fullwording to review, but do not want to go out of our way to encourage someone to work on thisin preference to other issues.

[2014-02-20 Re-open Deferred issues as Priority 4]

[2015-03-05 Jonathan suggests wording]

I dislike the suggestion to change to "user-provided" type because I already find the difference between user-declared / user-provided confusing for special member functions, so I think it would be better to use a completely different term. The core languageuses "user-defined conversion sequence" and "user-defined literal" andsimilar terms for things which the library provides, so I think weshould not refer to "user" at all to distinguish entities definedoutside the implementation from things provided by the implementation.

I propose "program-defined type" (and "program-defined specialization"), defined below. The P/R below demonstrates the scope of the changes required, even if this name isn't adopted. I haven't proposed a change for "User-defined facets" in [locale].

[Lenexa 2015-05-06]

RS, HT: The core language uses "user-defined" in a specific way, including library things but excluding core language things, let's use a different term.

MC: Agree.

RS: "which" should be "that", x2

RS: Is std::vector<MyType> a "program-defined type"?

MC: I think it should be.

TK: std::vector<int> seems to take the same path.

JW: std::vector<MyType> isn't program-defined, we don't need it to be, anything that depends on that also depends on =MyType.

TK: The type defined by an "explicit template specialization" should be a program-defined type.

RS: An implicit instantiation of a "program-defined partial specialization" should also be a program-defined type.

JY: This definition formatting is horrible and ugly, can we do better?

RS: Checking ISO directives.

RS: Define "program-defined type" and "program-defined specialization" instead, to get rid of the angle brackets.

JW redrafting.

[2017-09-12]

Jonathan revises wording as per Lenexa discussion

Previous resolution [SUPERSEDED]:

This wording is relative to N4296.

  1. Add a new sub-clause to [definitions]:

    17.3.? [defns.program.defined]

    program-defined

    <type> a class type or enumeration type which is not part of the C++standard library and not defined by the implementation. [Note: Typesdefined by the implementation include extensions (4.1[intro.compliance])and internal types used by the library. —end note]

    program-defined

    <specialization> an explicit template specialization or partialspecialization which is not part of the C++ standard library and notdefined by the implementation.

  2. Change 16.4.5.2.1[namespace.std] paragraph 1+2:

    -1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespacestd or to anamespace within namespacestd unless otherwise specified. A program may add a template specializationfor any standard library template to namespacestd only if the declaration depends on auserprogram-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

    -2- The behavior of a C++ program is undefined if it declares

    […]

    A program may explicitly instantiate a template defined in the standard library only if the declarationdepends on the name of auserprogram-defined type and the instantiation meets the standard library requirements for the original template.

  3. Change 19.5[syserr] paragraph 4:

    -4- Theis_error_code_enum andis_error_condition_enum may be specialized foruserprogram-defined types to indicate that such types are eligible for classerror_code and classerror_condition automatic conversions, respectively.

  4. Change 20.2.8.1[allocator.uses.trait] paragraph 1:

    -1-Remarks: automatically detects […]. A program may specialize this template to derive fromtrue_type for auserprogram-defined typeT that does not have a nestedallocator_type but nonetheless can be constructed with an allocator where either: […]

  5. Change 22.10.15.2[func.bind.isbind] paragraph 2:

    -2- Instantiations of theis_bind_expression template […]. A program may specializethis template for auserprogram-defined typeT to have aBaseCharacteristic oftrue_type to indicate thatT should be treated as a subexpression in abind call.

  6. Change 22.10.15.3[func.bind.isplace] paragraph 2:

    -2- Instantiations of theis_placeholder template […]. A program may specialize this template for auserprogram-defined typeT to have aBaseCharacteristic ofintegral_constant<int,N> withN > 0 to indicate thatT should be treated as a placeholder type.

  7. Change 22.10.19[unord.hash] paragraph 1:

    The unordered associative containers defined in 23.5 use specializations of the class templatehash […], the instantiationhash<Key> shall:

    • […]

    • […]

    • […]

    • […]

    • satisfy the requirement that the expressionh(k), whereh is an object of typehash<Key> andk is an object of typeKey, shall not throw an exception unlesshash<Key> is auserprogram-defined specialization that depends on at least oneuserprogram-defined type.

  8. Change 21.3.9.6[meta.trans.ptr] Table 57 (common_type row):

    Table 57 — Other transformations
    TemplateConditionComments
    …
    template <class... T>
    struct common_type;    		 The member typedeftype shall be
    defined or omitted as specified below.
    […]. A program may
    specialize this trait if at least one
    template parameter in the
    specialization is auserprogram-defined type.
    […]
    …

  9. Change 28.3.4.2.5[locale.codecvt] paragraph 3:

    -3- The specializations required in Table 81 (22.3.1.1.1) […]. Other encodings can be converted by specializing on auserprogram-definedstateT type.[…]

  10. Change 28.6.11.1.5[re.regiter.incr] paragraph 8:

    -8- [Note: This means that a compiler may call an implementation-specific search function, in which caseauserprogram-defined specialization ofregex_search will not be called. —end note]

[2018-3-14 Wednesday evening issues processing; move to Ready]

After this lands, we need to audit Annex C to find "user-defined type" Example: [diff.cpp03.containers]/3

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative toN4687.

  1. Add a new sub-clause to [definitions]:

    20.3.? [defns.program.defined.spec]

    program-defined specialization

    explicit template specialization or partialspecialization that is not part of the C++ standard library and notdefined by the implementation

    20.3.? [defns.program.defined.type]

    program-defined type

    class type or enumeration type that is not part of the C++standard library and not defined by the implementation,or an instantiation of a program-defined specialization

    [Drafting note: ISO directives say the following Note should be labelled as a "Note to entry"but the C++ WP doesn't follow that rule (yet). —end drafting note]

    [Note: Typesdefined by the implementation include extensions (4.1[intro.compliance])and internal types used by the library. —end note]

  2. Change 16.4.5.2.1[namespace.std] paragraph 1+2:

    -1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespacestd or to anamespace within namespacestd unless otherwise specified. A program may add a template specializationfor any standard library template to namespacestd only if the declaration depends on auserprogram-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

    -2- The behavior of a C++ program is undefined if it declares

    […]

    A program may explicitly instantiate a template defined in the standard library only if the declarationdepends on the name of auserprogram-defined type and the instantiation meets the standard library requirements for the original template.

  3. Change 19.5[syserr] paragraph 4:

    -4- Theis_error_code_enum andis_error_condition_enum may be specialized foruserprogram-defined types to indicate that such types are eligible for classerror_code and classerror_condition automatic conversions, respectively.

  4. Change 20.2.8.1[allocator.uses.trait] paragraph 1:

    -1-Remarks: automatically detects […]. A program may specialize this template to derive fromtrue_type for auserprogram-defined typeT that does not have a nestedallocator_type but nonetheless can be constructed with an allocator where either: […]

  5. Change 22.10.15.2[func.bind.isbind] paragraph 2:

    -2- Instantiations of theis_bind_expression template […]. A program may specializethis template for auserprogram-defined typeT to have aBaseCharacteristic oftrue_type to indicate thatT should be treated as a subexpression in abind call.

  6. Change 22.10.15.3[func.bind.isplace] paragraph 2:

    -2- Instantiations of theis_placeholder template […]. A program may specialize this template for auserprogram-defined typeT to have aBaseCharacteristic ofintegral_constant<int,N> withN > 0 to indicate thatT should be treated as a placeholder type.

  7. Change 22.10.19[unord.hash] paragraph 1:

    The unordered associative containers defined in 23.5 use specializations of the class templatehash […], the instantiationhash<Key> shall:

  8. Change 21.3.9.6[meta.trans.ptr] Table 57 (common_type row):

    Table 57 — Other transformations
    TemplateConditionComments
    …
    template <class... T>
    struct common_type;    		 The member typedeftype shall be
    defined or omitted as specified below.
    […]. A program may
    specialize this trait if at least one
    template parameter in the
    specialization is auserprogram-defined type.
    […]
    …

  9. Change 28.3.4.2.5[locale.codecvt] paragraph 3:

    -3- The specializations required in Table 81 (22.3.1.1.1) […]. Other encodings can be converted by specializing on auserprogram-definedstateT type.[…]

  10. Change 28.6.11.1.5[re.regiter.incr] paragraph 8:

    -8- [Note: This means that a compiler may call an implementation-specific search function, in which caseauserprogram-defined specialization ofregex_search will not be called. —end note]

[8]ページ先頭
©2009-2026 Movatter.jp