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3219. std::array overview container requirements are incorrect

Section: 24.3.7.1 [array.overview], 24.2.2.1 [container.requirements.general] Status: New Submitter: Nevin Liber & Christian Trott Opened: 2019-06-13 Last modified: 2022-04-24

Priority: 3

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Discussion:

The requirements specified in 24.3.7.1 [array.overview] p3 are incorrect; namely:

[2019-07 Issue Prioritization]

Priority to 3 after discussion on the reflector.

Previous resolution [SUPERSEDED]:

This wording is relative to N4810.

  1. Modify 24.2.2.1 [container.requirements.general], Table 62 — "Container requirements", as indicated (This table can be identified by the "section" identifier [tab:container.req] in the next working draft):

    Table 62 — Container requirements
    Expression Return type Operational
    semantics    		 Assertion/note
    pre/post-condition    		 Complexity
    […]
    X u; Ensures: !u.empty() for array<T, N> where 0 < N, and
    Ensures: u.empty() for all other standard containers.     		constant(Note A)
    X() Ensures: !X().empty() for array<T, N> where 0 < N, and
    Ensures: X().empty() for all other standard containers.     		constant(Note A)
    […]

    Those entries marked "(Note A)" or "(Note B)" have linear complexity for array<T, N> where 0 < N and have constant complexity for all other standard containers.

  2. Modify 24.3.7.1 [array.overview] as indicated:

    -2- An array is an aggregate (9.4.2 [dcl.init.aggr]) that can be list-initialized with up to N elements whose types are convertible to T.

    -3- An array<T, 0> satisfies all of the requirements of a container and of a reversible container (24.2 [container.requirements]). An array<T, N> where 0 < N satisfies all of the requirements of a container and of a reversible container (24.2 [container.requirements]), except that a default constructed array<T, N> object is not empty and thatboth default construction and swap does not have constantlinear complexity. An array satisfies some of the requirements of a sequence container (24.2.4 [sequence.reqmts]). Descriptions are provided here only for operations on array that are not described in one of these tables and for operations where there is additional semantic information.

[2022-04-24; Daniel rebases wording on N4910]

Proposed resolution:

This wording is relative to N4910.

  1. Modify 24.2.2.2 [container.reqmts] as indicated:

    X u;
X u = X();

    -10- Postconditions: !u.empty() for array<T, N> where 0 < N, and u.empty() for all other standard containers.

    -11- Complexity: ConstantLinear for array<T, N> where 0 < N and constant for all other standard containers.

    […] 
    X u(rv);
X u = rv;

    -15- Postconditions: u is equal to the value that rv had before this construction.

    -11- Complexity: Linear for array<T, N> where 0 < N and constant for all other standard containers.

    […] 
    a.swap(b)

    -45- Result: void

    -46- Effects: Exchanges the contents of a and b.

    -47- Complexity: Linear for array<T, N> where 0 < N and constant for all other standard containers.

  2. Modify 24.3.7.1 [array.overview] as indicated:

    -2- An array is an aggregate (9.4.2 [dcl.init.aggr]) that can be list-initialized with up to N elements whose types are convertible to T.

    -3- An array<T, 0> meets all of the requirements of a container (24.2.2.2 [container.reqmts]) and of a reversible container (24.2.2.3 [container.rev.reqmts]). An array<T, N> where 0 < N meets all of the requirements of a container (24.2.2.2 [container.reqmts]) and of a reversible container (24.2.2.3 [container.rev.reqmts]), except that a default constructed array<T, N> object is not empty if N > 0 and default construction, move construction, and swap have linear complexity if N > 0. An array meets some of the requirements of a sequence container (24.2.4 [sequence.reqmts]). Descriptions are provided here only for operations on array that are not described in one of these tables and for operations where there is additional semantic information.