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2335. array<array<int, 3>, 4> should be layout-compatible with int[4][3]

Section: 24.3.7 [array] Status: NAD Submitter: Jeffrey Yasskin Opened: 2013-10-04 Last modified: 2020-11-09

Priority: 3

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Discussion:

In order to replace some uses of C arrays with std::array, we need it to be possible to cast from a std::array<> to an equivalent C array. Core wording doesn't appear to be in quite the right state to allow casting, but if we specify that appropriate types are layout-compatible, we can at least write: 

union {
  array<array<array<int, 2>, 3>, 4> arr;
  int carr[4][3][2];
};

to view memory as the other type: C++14 CD [class.mem]p18.

I believe it's sufficient to add "array<T, N> shall be layout-compatible (6.8 [basic.types]) with T[N]." to 24.3.7.1 [array.overview], but we might also need some extension to 11.4 [class.mem] to address the possibility of layout-compatibility between struct and array types.

I checked that libc++ on MacOS already implements this, although it would be good for someone else to double-check; I haven't checked any other standard libraries.

[2020-02-14, Prague]

LWG discussions and decision for NAD.

[2020-11-09 Status changed: Tentatively NAD → NAD.]

Rationale:

The desire to use std::array like this seems like an "XY problem". The goal should be "replace C arrays" not "replace C arrays with std::array", because std::array is not suitable here. There are superior solutions being proposed, and will be available in a future version of C++ (e.g. using mdspan as a multi-dimensional view on an array).

Proposed resolution: