> #once INCLUDE_GUARD_SLUG solves all the annoyance of 3-lines for one
feature and doesn't have same-file issues.

I would love this. But it was proposed before and short down. Maybe it’s
been long enough to re-propose it?

Jeremy

On Fri, Aug 30, 2024 at 05:53 Gašper Ažman via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Well:
> - 3p libraries each have their own thing, of course, but they have an
> interest in keeping it reasonable and deconflicted and I bother them if
> their include guards aren't unique
> - 1p, we have a modified bloomberg-style scheme (thanks Lakos) where we do
> INCLUDED_{company}_{full_path_to_header} where the paths are globally
> unique, and I have a linter that autofixes it so if you do #pragma once it
> just gets replaced, and if you copy-paste it somewhere else you get a fixit.
>
> This solved A LOT of build breakages, and was a prerequisite for getting
> cloud builds working at all.
>
> So, yeah, I will strongly oppose any standardization of #pragma once on a
> "same file" basis; C++ is more and more a "large systems" language, because
> small projects can choose Rust and be fine. #once INCLUDE_GUARD_SLUG solves
> all the annoyance of 3-lines for one feature and doesn't have same-file
> issues.
>
> On Fri, Aug 30, 2024 at 11:47 AM Tiago Freire <tmiguelf_at_[hidden]>
> wrote:
>
>> And what does "aware of the global namespacing" mean in practice?
>>
>> ------------------------------
>> *From:* Gašper Ažman <gasper.azman_at_[hidden]>
>> *Sent:* Friday, August 30, 2024 12:22:14 PM
>> *To:* Tiago Freire <tmiguelf_at_[hidden]>
>> *Cc:* std-proposals_at_[hidden] <std-proposals_at_[hidden]>
>> *Subject:* Re: [std-proposals] Revising #pragma once
>>
>> Because my linter is aware of the global namespacing of libraries and can
>> fix stuff *before they get into the source control structure*. My build
>> tools have to deal with build outputs, not sources. Substantially different
>> thing.
>>
>> On Fri, Aug 30, 2024 at 11:20 AM Tiago Freire <tmiguelf_at_[hidden]>
>> wrote:
>>
>>> > This is why we have linters. At least that's a problem I can fix and
>>> diagnose; I can't fix #pragma once.
>>>
>>>
>>>
>>> If your linter could figure that out, why your compiler wouldn’t?
>>>
>>>
>>>
>>> *From:* Gašper Ažman <gasper.azman_at_[hidden]>
>>> *Sent:* Friday, August 30, 2024 12:16 PM
>>> *To:* Tiago Freire <tmiguelf_at_[hidden]>
>>> *Cc:* std-proposals_at_[hidden]
>>> *Subject:* Re: [std-proposals] Revising #pragma once
>>>
>>>
>>>
>>> This is why we have linters. At least that's a problem I can fix and
>>> diagnose; I can't fix #pragma once.
>>>
>>>
>>>
>>> On Fri, Aug 30, 2024 at 11:13 AM Tiago Freire <tmiguelf_at_[hidden]>
>>> wrote:
>>>
>>> How do you ensure that your “INCLUDE_GUARD_STRING_YOU_CANNOT_OMIT” is
>>> unique across different libraries?
>>>
>>> Given standard practice, if the headers the same name, they are very
>>> likely going to have the same “INCLUDE_GUARD_STRING_YOU_CANNOT_OMIT”. And
>>> we are back to the same problem you just describe.
>>>
>>>
>>>
>>> Now multiply this by, “I renamed the file and forget to change it”, “I
>>> copy pasted a template header file and forgot to change the
>>> INCLUDE_GUARD_STRING_YOU_CANNOT_OMIT, and wasn’t caught in review because
>>> nobody looks at that”.
>>>
>>>
>>> You know what they do not have? The same filepath.
>>>
>>> If I don’t have to write it, I don’t have to worry about these problems.
>>>
>>>
>>>
>>>
>>>
>>> *From:* Gašper Ažman <gasper.azman_at_[hidden]>
>>> *Sent:* Friday, August 30, 2024 12:06 PM
>>> *To:* std-proposals_at_[hidden]
>>> *Cc:* Tiago Freire <tmiguelf_at_[hidden]>
>>> *Subject:* Re: [std-proposals] Revising #pragma once
>>>
>>>
>>>
>>> Simple as in #once INCLUDE_GUARD_STRING_YOU_CANNOT_OMIT?
>>>
>>>
>>>
>>> On Fri, Aug 30, 2024 at 11:04 AM Tiago Freire via Std-Proposals <
>>> std-proposals_at_[hidden]> wrote:
>>>
>>> I agree, bit-wise comparison is not the way to go, that’s why no
>>> compiler does this.
>>>
>>> There’s no need to over-engineer this. Keep it simple.
>>>
>>>
>>>
>>>
>>>
>>> *From:* Std-Proposals <std-proposals-bounces_at_[hidden]> *On
>>> Behalf Of *Gašper Ažman via Std-Proposals
>>> *Sent:* Friday, August 30, 2024 11:51 AM
>>> *To:* marcinjaczewski86_at_[hidden]
>>> *Cc:* Gašper Ažman <gasper.azman_at_[hidden]>;
>>> std-proposals_at_[hidden]; Tom Honermann <tom_at_[hidden]>
>>> *Subject:* Re: [std-proposals] Revising #pragma once
>>>
>>>
>>>
>>> If you knew up-front you wouldn't do it :).
>>>
>>>
>>>
>>> This happens though. There are people who generate code - whole include
>>> trees - and for plugins they end up looking very very similar or identical.
>>> And then as the build engineer my users complain that "their build doesn't
>>> work on the cloud build environment" and I have to somehow *find their bug*.
>>>
>>>
>>>
>>> Think about it. How the hell do you diagnose this silent noninclusion?
>>>
>>>
>>>
>>> On Fri, Aug 30, 2024 at 9:30 AM Marcin Jaczewski <
>>> marcinjaczewski86_at_[hidden]> wrote:
>>>
>>> pt., 30 sie 2024 o 01:20 Gašper Ažman via Std-Proposals
>>> <std-proposals_at_[hidden]> napisał(a):
>>> >
>>> > 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.
>>> >
>>>
>>> Question is do you know upfront that files are problematic?
>>> Then we could do something like this:
>>> a) we only consider normalized paths
>>> b) use have option to add path mappings that are used in path comparison
>>>
>>> Like:
>>>
>>> We have folders `libs/fooV1` and `libs/fooV2`. Both folders are symlinks
>>> but by default when compiler load `libs/fooV1/bar.h` and
>>> `libs/fooV2/bar.h`
>>> the compiler considers them as separate files even when they are the
>>> same file.
>>> Now we add compiler flag `-PI libs/fooV1:libs/fooV2` (similar to `-I`)
>>> and now
>>> when compiler load `libs/fooV1/foo.h` he consider it as if he load
>>> `libs/fooV2/bar.h` and thus next loading of `libs/fooV2/bar.h` will be
>>> blocked.
>>>
>>> And this should be ver dumb process it could be cases when
>>> `libs/fooV1/bar.h`
>>> and `libs/fooV2/bar.h` are unrelated but if you make specific maping it
>>> will
>>> override even diffrnet files. This could be useful to hacking some broken
>>> includes like:
>>>
>>> ```
>>> -PI hack/foo/bar.h:libs/fooV2/bar.h
>>> ```
>>>
>>> and we can in our files do:
>>>
>>> ```
>>> #include "hack/foo/bar.h"
>>> #include "libs/fooV2/foo.h" // it will ignore `#include "bar.h"`
>>> ```
>>>
>>> Could mechnism like this work on your build system?
>>>
>>> > 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.
>>> >> >>>
>>> >> >>> --
>>> >> >>> Std-Proposals mailing list
>>> >> >>> Std-Proposals_at_[hidden]
>>> >> >>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>> >> >>
>>> >> >>
>>> >> >> --
>>> >> >> Std-Proposals mailing list
>>> >> >> Std-Proposals_at_[hidden]
>>> >> >> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>> >> >
>>> >> > --
>>> >> > Std-Proposals mailing list
>>> >> > Std-Proposals_at_[hidden]
>>> >> > https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>> >> --
>>> >> Std-Proposals mailing list
>>> >> Std-Proposals_at_[hidden]
>>> >> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>> >
>>> > --
>>> > Std-Proposals mailing list
>>> > Std-Proposals_at_[hidden]
>>> > https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>>
>>> --
>>> Std-Proposals mailing list
>>> Std-Proposals_at_[hidden]
>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>>
>>>
>> --
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