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Re: [std-proposals] Mandate layout of std::array

From: Sebastian Wittmeier <wittmeier_at_[hidden]>
Date: Sat, 1 Jul 2023 18:55:18 +0200
The stated expression should be naturally true (the sizes should be the same) and not just for the only reason that the standard says the expression evaluates to true per definitionem (although the sizes are different, special casing sizeof to return wrong values only in this case.   SCNR ;-)   -----Ursprüngliche Nachricht----- Von:Jonathan Wakely via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Sa 01.07.2023 10:12 Betreff:Re: [std-proposals] Mandate layout of std::array An:C++ Proposals <std-proposals_at_[hidden]>; CC:Jonathan Wakely <cxx_at_[hidden]>; On Sat, 1 Jul 2023, 01:34 Brian Bi via Std-Proposals, <std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]> > wrote: Would folks be interested in a proposal to require std::array<T, N> to have the exact same layout as T[N], i.e., no padding before or after the internal T[N] data member?  Yes please.    (The special case where N is zero would be excluded from the requirement, obviously).  Yes.   A lot of people are surprised when I tell them that the standard doesn't already provide this guarantee.  It's easy to check that your implementation provides it, and just assume you'll never encounter one that doesn't:  static_assert(sizeof(std::array<T,N>)==sizeof(T[N]));  But it would be nice to not need that. The spec should be as simple as saying that the equality above is true for N!=0.   I can think of some theoretical reasons why this would be useful, such as being able to serialize a T[N] and then deserialize it as std::array<T, N> using the `std::start_lifetime_as` function. However, I'd like to hear from folks here if they would actually benefit from this guarantee in a real application. If there's sufficient motivation, I'll write a paper.  I don't think implementations would actually have to do any work to conform to this requirement, so my only concern is about whether I can provide sufficient motivation to get this proposal through the committee.  Guaranteeing something that's always true in practice should be uncontroversial.     (Bonus: require std::array<T, N> to be pointer-interconvertible with its T[N] member, even when T is not standard-layout. This part of the proposal might be a bit more controversial, though; I'm not sure how hard it would be for implementors.)   -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals

Received on 2023-07-01 16:55:20