Query: Use of std::vector<const something> in Chromium?

[Harald Alvestrand]

Hi,

I recently got pinged on https://crbug.com/webrtc/15829 - an external user of WebRTC had his compile break on this line:

std::vector<const int> foo = {1, 2, 3};

It seems that const inside a std::vector has been disallowed in the C++ spec for a while. But the chromium compile didn't catch it, and some grepping found quite a bit of usage inside Chrome.

Is this something that someone's working on fixing?

Harald

[Peter Kasting]

I'm not aware of any effort to find or fix this.

I would file a bug, link to the relevant c++ spec bits, and if you have time send some patches to improve things.

PK

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[Hans Wennborg]

Looks like Nico just filed crbug.com/323708866

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[Adam Rice]

Very odd that you can just wrap your const value in a struct and the problem goes away:

https://godbolt.org/z/z4f6qdr6h

The restriction seems very artificial.

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[Tomasz Sniatowski]

I recall this popping up from time to time, breaking on libstdc++, and getting fixed as part of the libstdc++ community effort in https://issues.chromium.org/issues/41455655 (formerly known as crbug.com/957519). So there shouldn't be very many "old" instances, at least in //content and below.

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Tomasz Śniatowski

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From: 'Peter Kasting' via cxx <c...@chromium.org>
Sent: Monday, February 5, 2024 20:16
To: Harald Alvestrand <h...@google.com>
Cc: cxx <c...@chromium.org>
Subject: Re: Query: Use of std::vector<const something> in Chromium?

 

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[Dana Jansens]

On Tue, Feb 6, 2024 at 1:10 AM Adam Rice <ri...@chromium.org> wrote:

Very odd that you can just wrap your const value in a struct and the problem goes away:

https://godbolt.org/z/z4f6qdr6h

The restriction seems very artificial.

Yeah I think what vector really wants is move-constructible and move-assignable, which would fail with a const member in a struct. But it may go out of its way and do non-optimal things if a type is not move-assignable.

 

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[Harald Alvestrand]

the patch to llvm to allow us to not fix this was kind of illuminating.

https://github.com/llvm/llvm-project/pull/80711/files

It just does const_cast to strip away the constness, and then uses memcpy on the result.

In other words - the const keyword is being ignored by the std::vector implementation in order to make std::vector work.

Not requiring this seems like a Good Thing.

[David Benjamin]

Huh. How does that interact with Adam's example in https://godbolt.org/z/z4f6qdr6h? Has that been ignoring the const all this time too?

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[Harald Alvestrand]

My impression is that memcpy() ignores most things about the inner workings of the object it's copying... so the outer const is the only part that needs to be ignored in order to make memcpy() eat the object.

[David Benjamin]

Right, but std::vector isn't defined to call memcpy. That's an optimization libc++ does when a type is trivially movable. If we have a vector of a type that isn't movable at all, to say nothing of trivially so, it's surprising that libc++ is calling memcpy. I.e. my question isn't why the memcpy compiles. It's whether there's a deeper problem in memcpy getting chosen at all.