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3382. NTTP for pair and array

Section: 20.4.2 [pairs.pair], 22.3.7 [array] Status: WP Submitter: Barry Revzin Opened: 2020-01-27 Last modified: 2020-02-24

Priority: 2

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Discussion:

We had this NB ballot issue, to ensure that std::array could be a NTTP. But after P1907, we still need some kind of wording to ensure that std::array (and also std::pair) have no extra private members or base classes.

This is similar to LWG 3373 — maybe we just need to add:

The class template pair/array has the data members specified above. It has no base classes or data members other than those specified.

[2020-02 Prioritized as P2 Monday morning in Prague]

Previous resolution [SUPERSEDED]:

This wording is relative to N4849.

  1. Modify 20.4.2 [pairs.pair] as indicated:

    -1- Constructors and member functions of pair do not throw exceptions unless one of the element-wise operations specified to be called for that operation throws an exception.

    -2- The defaulted move and copy constructor, respectively, of pair is a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function.

    -3- If (is_trivially_destructible_v<T1> && is_trivially_destructible_v<T2>) is true, then the destructor of pair is trivial.

    -?- The class template pair has the data members specified above. It has no base classes or data members other than those specified.

  2. Modify 22.3.7.1 [array.overview] as indicated:

    -1- The header <array> defines a class template for storing fixed-size sequences of objects. An array is a contiguous container (22.2.1 [container.requirements.general]). An instance of array<T, N> stores N elements of type T, so that size() == N is an invariant.

    -2- An array is an aggregate (9.4.1 [dcl.init.aggr]) that can be list-initialized with up to N elements whose types are convertible to T.

    -3- An array meets all of the requirements of a container and of a reversible container (22.2 [container.requirements]), except that a default constructed array object is not empty and that swap does not have constant complexity. An array meets some of the requirements of a sequence container (22.2.3 [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.

    -?- The class template array has the data members specified in subclauses 22.3.7.1 [array.overview] and 22.3.7.5 [array.zero]. It has no base classes or data members other than those specified.

    -4- […]

[2020-02-13, Prague]

Tim Song and Tomasz were trying to come up with general wording that could be reused for both pair and array (and other types). They suggest that if it should be in scope for C++20, it would be better to provide non-general wording for pair and array (that is easier to get right).

For completeness (and future wording) the generalized wording is included. The definition of structurally compatible with:

The type T is structurally compatible with subs, if for the values t1 and t2 of type T:

Then changes for array/pair would then look like:

pair<T, U> is structurally compatible (<some-reference>) with first and second.

array<T, N> is structurally compatible with its elements (if any).

[2020-02 Status to Immediate on Friday morning in Prague.]

Proposed resolution:

This wording is relative to N4849.

  1. Modify 20.4.2 [pairs.pair] as indicated:

    -1- Constructors and member functions of pair do not throw exceptions unless one of the element-wise operations specified to be called for that operation throws an exception.

    -2- The defaulted move and copy constructor, respectively, of pair is a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function.

    -3- If (is_trivially_destructible_v<T1> && is_trivially_destructible_v<T2>) is true, then the destructor of pair is trivial.

    -?- pair<T, U> is a structural type (13.2 [temp.param]) if T and U are both structural types. Two values p1 and p2 of type pair<T, U> are template-argument-equivalent (13.6 [temp.type]) if and only if p1.first and p2.first are template-argument-equivalent and p1.second and p2.second are template-argument-equivalent.

  2. Modify 22.3.7.1 [array.overview] as indicated:

    -1- The header <array> defines a class template for storing fixed-size sequences of objects. An array is a contiguous container (22.2.1 [container.requirements.general]). An instance of array<T, N> stores N elements of type T, so that size() == N is an invariant.

    -2- An array is an aggregate (9.4.1 [dcl.init.aggr]) that can be list-initialized with up to N elements whose types are convertible to T.

    -3- An array meets all of the requirements of a container and of a reversible container (22.2 [container.requirements]), except that a default constructed array object is not empty and that swap does not have constant complexity. An array meets some of the requirements of a sequence container (22.2.3 [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.

    -?- array<T, N> is a structural type (13.2 [temp.param]) if T is a structural type. Two values a1 and a2 of type array<T, N> are template-argument-equivalent (13.6 [temp.type]) if and only if each pair of corresponding elements in a1 and a2 are template-argument-equivalent.

    -4- […]