> Yes, it does. It makes the behavior different from what is expected by
majority of users. And is prone to subtle errors, if different headers
happen to match in contents.

The current behavior expected by the majority of users is hard to argue
about because the semantics aren’t spelled out anywhere and every
implementation does it differently. #pragma once represents the loose
concept of single inclusion, and the semantics proposed here are not prone
to the subtle errors of mtimes happening to match, symbolic or hard links
not being handled well, certain filesystem mount behavior, or multiple
copies of the same header being unintentionally included more than once
(e.g. if a library header is copied around).

There are few to no cases where you would want to include a file with the
same exact contents with #pragma once multiple times. The only case
presented here has to do with including a header of only macros then
undefining everything then redefining. Such use probably shouldn’t be using
an include guard in the first place and it’s important to note that
traditional include guards might cause problems there too.

Jeremy

On Tue, Aug 27, 2024 at 12:28 Andrey Semashev via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On 8/27/24 20:14, Jeremy Rifkin wrote:
> > Does using contents for #pragma once break any practical or plausible
> > uses of #pragma once? Does it have any clear downsides?
>
> Yes, it does. It makes the behavior different from what is expected by
> majority of users. And is prone to subtle errors, if different headers
> happen to match in contents.
>
> > I don’t think does. It should only make things more robust and portable.
> >
> > Jeremy
> >
> > On Tue, Aug 27, 2024 at 12:07 Andrey Semashev via Std-Proposals
> > <std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]>>
> > wrote:
> >
> > On 8/27/24 17:01, Jeremy Rifkin via Std-Proposals wrote:
> > > Hi,
> > > I have drafted a proposal for standardizing #pragma once. This has
> > been
> > > previously proposed a few years ago and I recognize that on top of
> > being
> > > difficult to standardize, existing opinions on this topic may
> > render the
> > > paper dead on arrival. However, due to its widespread nature and
> > > concerns about portability contributing to it not being used more I
> > > think it's worth revisiting. I have uploaded the first draft at:
> > >
> >
> https://jeremy-rifkin.github.io/cpp-proposals/drafts/pragma-once-draft-1.html
> <
> https://jeremy-rifkin.github.io/cpp-proposals/drafts/pragma-once-draft-1.html>
> <
> https://jeremy-rifkin.github.io/cpp-proposals/drafts/pragma-once-draft-1.html
> <
> https://jeremy-rifkin.github.io/cpp-proposals/drafts/pragma-once-draft-1.html
> >>.
> >
> > I think, using file contents for `#pragma once` is wrong and should
> not
> > be standardized. The intended effect of `#pragma once` has always
> been
> > to avoid including *the same header* twice, not *the same content*.
> >
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> >
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