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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: