On 30 August 2024 06:07:42 BST, Tiago Freire via Std-Proposals <std-proposals_at_[hidden]> wrote:
>I'm going to one up you on this.
>How do you prevent people from using the same macro for that include guard?

That's fair, however global name collision is a concern that libraries already need to deal with outside of include guards too. The pragma once collision problem is entirely novel compared to this.

In theory include guards collision can be eliminated as a concern, as you don't need good names for them. For some of my personal projects I use something like UUID_<uuid generator output>, and have a macro in my editor that puts the guard pattern in place. No chance that this collides with anything, and I don't need to deal with renaming them when I rename/move the header.

>
>Which is a case I've run into when using include guards, and there's no build sanitation that you can do in that case other than changing the source code.
>
>That's why I don't use include guards, they are even more broken, takes longer to write, easy to get it wrong, and you need to get creative.
>
>
>________________________________
>From: Std-Proposals <std-proposals-bounces_at_[hidden]> on behalf of Jeremy Rifkin via Std-Proposals <std-proposals_at_[hidden]>
>Sent: Friday, August 30, 2024 1:29:31 AM
>To: Gašper Ažman <gasper.azman_at_[hidden]>
>Cc: Jeremy Rifkin <rifkin.jer_at_[hidden]>; std-proposals_at_[hidden] <std-proposals_at_[hidden]>; Tom Honermann <tom_at_[hidden]>
>Subject: Re: [std-proposals] Revising #pragma once
>
>> #include "library2/public.hpp" # boom, library2/utils.hpp does not get included
>
>This has already been presented. While I have shared I think this is
>rare and such a "main header" should not use include guards in the
>first place, it is compelling.
>
>In case it's not already abundantly clear: A content-based definition
>is the only way to make #pragma once work for a setup like yours with
>multiple mounts. But perhaps such a feature shouldn't bend over
>backwards to support such a case.
>
>Jeremy
>
>On Thu, Aug 29, 2024 at 6:20 PM Gašper Ažman <gasper.azman_at_[hidden]> wrote:
>>
>> Darlings,
>>
>> byte-identical is just plain incorrect. Consider.
>>
>> [library1/public.hpp]
>> #pragma once
>> #include "utils.hpp"
>>
>> [library1/utils.hpp]
>> class lib1 {};
>>
>> library2/public.hpp
>> #pragma once
>> #include "utils.hpp"
>>
>> [library2/utils.hpp]
>> class lib2 {};
>>
>> [main.cpp]
>> #include "library1/public.hpp"
>> #include "library2/public.hpp" # boom, library2/utils.hpp does not get included
>>
>> same-contents also means same-relative-include-trees. Congratulations.
>>
>> On Fri, Aug 30, 2024 at 12:15 AM Jeremy Rifkin via Std-Proposals <std-proposals_at_[hidden]> wrote:
>>>
>>> > In this very thread there are examples showing why taking only the content into account doesn't work but it was brushed off as "that can be fixed".
>>>
>>> I'm sorry you feel I have brushed any presented examples off. I have
>>> found them all immensely helpful for consideration. It's hard for me
>>> to imagine times when you'd want the same include-guarded content
>>> included twice, however I found the example of a "main header"
>>> compelling. The example of a header that only defines macros you undef
>>> is also not impractical.
>>>
>>> However, there are also compelling examples for filesystem identity.
>>> Mainly the multiple mount point issue.
>>>
>>> I think both can be reasonable, however, I have been trying to
>>> understand the most probable failure modes. While I originally
>>> proposed a content-based definition, I do think a filesystem-based
>>> definition is closer to current semantics and expectations.
>>>
>>> Jeremy
>>>
>>> On Thu, Aug 29, 2024 at 5:24 PM Breno Guimarães via Std-Proposals
>>> <std-proposals_at_[hidden]> wrote:
>>> >
>>> > To add to that, the whole idea is to standardize standard practice. If the first thing you do is to change spec to something else, then you're not standardizing standard practice, you are adding a new feature that inconveniently clashes with an existing one.
>>> >
>>> > In this very thread there are examples showing why taking only the content into account doesn't work but it was brushed off as "that can be fixed".
>>> >
>>> > None of this make sense to me.
>>> >
>>> >
>>> > Em qui., 29 de ago. de 2024 18:59, Tiago Freire via Std-Proposals <std-proposals_at_[hidden]> escreveu:
>>> >>
>>> >> Again, hashing content... totally unnecessary.
>>> >>
>>> >> There's no need to identify "same content" which as far as I can see can be defeated by modifications that don't change the interpretation, like spaces, which although not technically a violation of "same content" it clearly defeats the intent.
>>> >>
>>> >> An include summons a resource, a pragma once bars that resources from bey re-summonable. That's it. File paths should be more than enough.
>>> >>
>>> >> I'm unconvinced that the "bad cases" are not just a product of bad build architecture, if done properly a compiler should never be presented with multiple alternatives of the same file. And putting such requirements on compilers puts an unnecessary burden on developers to support a scenario that it is that is arguably bad practice.
>>> >>
>>> >> The argument is "prgma once" is supported everywhere it is good, we should make it official in the standard, effectively no change to a compiler should occur as a consequence.
>>> >> If a change needs to occur, then in fact "your version" of what you mean by "pragma once" is actually "not supported" by all the major compilers.
>>> >>
>>> >> Current compiler support of "pragma once" and it's usage on cross platform projects have a particular way of dealing with dependencies in mind. That workflow works. It's pointless to have this discussion if you don't understand that flow, and you shouldn't tailor the tool to a workflow that doesn't exist to the detriment of all.
>>> >>
>>> >>
>>> >> ________________________________
>>> >> From: Std-Proposals <std-proposals-bounces_at_[hidden]> on behalf of Jeremy Rifkin via Std-Proposals <std-proposals_at_[hidden]>
>>> >> Sent: Thursday, August 29, 2024 9:56:18 PM
>>> >> To: Tom Honermann <tom_at_[hidden]>
>>> >> Cc: Jeremy Rifkin <rifkin.jer_at_[hidden]>; std-proposals_at_[hidden] <std-proposals_at_[hidden]>
>>> >> Subject: Re: [std-proposals] Revising #pragma once
>>> >>
>>> >> Performance should be fine if using a content definition. An implementation can do inode/path checks against files it already knows of, as a fast path. The first time a file is #included it’s just a hash+table lookup to decide whether to continue.
>>> >>
>>> >> Regarding the filesystem definition vs content definition question, while I think a content-based definition is robust I can see there is FUD about it and also an argument about current practice being a filesystem-based definition. It may just be best to approach this as filesystem uniqueness to the implementation’s ability, with a requirement that symbolic links/hard links are handled. This doesn’t cover the case of multiple mount points, but we’ve discussed that that’s impossible with #pragma once without using contents instead.
>>> >>
>>> >> Jeremy
>>> >>
>>> >> On Thu, Aug 29, 2024 at 13:06 Tom Honermann <tom_at_[hidden]> wrote:
>>> >>>
>>> >>> On 8/28/24 12:32 AM, Jeremy Rifkin via Std-Proposals wrote:
>>> >>>
>>> >>> Another question is whether the comparison should be post translation
>>> >>> phase 1.
>>> >>>
>>> >>> I gave this some thought while drafting the proposal. I think it comes
>>> >>> down to whether the intent is single inclusion of files or single
>>> >>> inclusion of contents.
>>> >>>
>>> >>> Indeed. The proposal currently favors the "same contents" approach and offers the following wording.
>>> >>>
>>> >>> A preprocessing directive of the form
>>> >>> # pragma once new-line
>>> >>> shall cause no subsequent #include directives to perform replacement for a file with text contents identical to this file.
>>> >>>
>>> >>> The wording will have to define what it means for contents to be identical. Options include:
>>> >>>
>>> >>> The files must be byte-for-byte identical. This makes source file encoding observable (which I would be strongly against).
>>> >>> The files must encode the same character sequence post translation phase 1. This makes comparisons potentially expensive.
>>> >>>
>>> >>> Note that the "same contents" approach obligates an implementation to consider every previously encountered file for every #include directive. An inode based optimization can help to determine if a file was previously encountered based on identity, but it doesn't help to reduce the costs when a file that was not previously seen is encountered.
>>> >>>
>>> >>>
>>> >>> Tom.
>>> >>>
>>> >>> Jeremy
>>> >>>
>>> >>> On Tue, Aug 27, 2024 at 3:39 PM Tom Honermann via Std-Proposals
>>> >>> <std-proposals_at_[hidden]> wrote:
>>> >>>
>>> >>> On 8/27/24 4:10 PM, Thiago Macieira via Std-Proposals wrote:
>>> >>>
>>> >>> On Tuesday 27 August 2024 12:35:17 GMT-7 Andrey Semashev via Std-Proposals
>>> >>> wrote:
>>> >>>
>>> >>> The fact that gcc took the approach to compare file contents I consider
>>> >>> a poor choice, and not an argument to standardize this implementation.
>>> >>>
>>> >>> Another question is whether a byte comparison of two files of the same size is
>>> >>> expensive for compilers.
>>> >>>
>>> >>> #once ID doesn't need to compare the entire file.
>>> >>>
>>> >>> Another question is whether the comparison should be post translation
>>> >>> phase 1. In other words, whether differently encoded source files that
>>> >>> decode to the same sequence of code points are considered the same file
>>> >>> (e.g., a Windows-1252 version and a UTF-8 version). Standard C++ does
>>> >>> not currently allow source file encoding to be observable but a #pragma
>>> >>> once implementation that only compares bytes would make such differences
>>> >>> observable.
>>> >>>
>>> >>> Tom.
>>> >>>
>>> >>> --
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>>> >>
>>> >>
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>>> >
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