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Re: [std-proposals] In re: Issue2157

From: Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]>
Date: Tue, 5 Dec 2023 16:36:58 -0500
On Tue, Dec 5, 2023 at 4:03 PM Ryan Nicholl via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> I want to raise my personal concern around the proposed solution
> to Issue2157.
> I do not believe that std::array<T, 0> {{}}; should be required to be
> valid syntax. This prevents std::array<T, 0> from being implemented as an
> empty struct. It is also just logically inconsistent with an array that
> contains no elements. I believe this eliminates empty base class
> optimization and presents several other issues that I believe diverge from
> the 0 overhead principle. For this reason I think the solution ought to be
> reconsidered.

(You're talking about LWG 2157 <https://cplusplus.github.io/LWG/issue2157>,
"How does std::array<T,0> initialization work when T is not

There are two caveats you don't seem to be taking into account:
(1) No user-programmer ever should write `array<T,0> t = {{}};` — `array`,
like every other STL container and algebraic type, is intended to be used
with a single pair of braces `array<T,0> t = {};`. Yes, in all present-day
library implementations this relies on brace elision
<https://eel.is/c++draft/dcl.init.aggr#16.sentence-1>; that's okay.
(2) No library vendor implements `array<T,0>` as an empty type.

Asking for `array<T,0>` to be "maybe empty, maybe not" would cause (minor)
pain for user-programmers. Asking for it to be "always empty" would cause
(major ABI-breaking) pain for vendors. The proposed resolution of LWG 2157
seems to fix some infelicities in the current wording, without causing pain
for (1) any user-programmer or (2) any existing vendor. So it seems like an
improvement to me.


Received on 2023-12-05 21:37:12