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2310. Public exposition only member in std::array

Section: 24.3.7.1 [array.overview] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-09-30 Last modified: 2017-07-30

Priority: 4

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Discussion:

24.3.7.1 [array.overview] shows std::array with an "exposition only" data member, elems.

The wording in 16.3.3.5 [objects.within.classes] that defines how "exposition only" is used says it applies to private members, but std::array::elems (or its equivalent) must be public in order for std::array to be an aggregate.

If the intention is that std::array::elems places requirements on the implementation to provide "equivalent external behavior" to a public array member, then 16.3.3.5 [objects.within.classes] needs to cover public members too, or some other form should be used in 24.3.7.1 [array.overview].

[Urbana 2014-11-07: Move to Open]

[Kona 2015-10: Link to 2516]

[2015-11-14, Geoffrey Romer provides wording]

[2016-02-04, Tim Song improves the P/R]

Instead of the build-in address-operator, std::addressof should be used.

[2016-03 Jacksonville]

Move to Ready.

Proposed resolution:

This wording is relative to N4582.

  1. Edit 24.3.7.1 [array.overview] as indicated

    […]

    -3- An array […] 

    namespace std {
  template <class T, size_t N>
  struct array {
    […]
    T elems[N]; // exposition only
    […]
  };
}

    -4- [Note: The member variable elems is shown for exposition only, to emphasize that array is a class aggregate. The name elems is not part of array's interface. — end note]

  2. Edit [array.data] as follows:

    constexpr T* data() noexcept;
constexpr const T* data() const noexcept;

    -1- Returns: elemsA pointer such that [data(), data() + size()) is a valid range, and data() == addressof(front()).