My 2 cents here would be that like you mentioned before:
neither include guard nor pragma is a standard solution.
This means that we either rely on a bug prone hack or non standard solution to obtain the effect of:

“Include / import the file only once.”

Which seems like it’s a missing feature,
it’s such a common use case that it should be standardized.
Also not changing other file but stating:
#include once “header.hpp”
Is preferred in my opinion since you might not own the other file maybe it’s a part of some other repo.

P.S.
Maybe worth mentioning is the fact that I never want to include a file more than once.

P.S.S.
Should we not standardize include once since we have a non standard option available everywhere?
Shouldn’t we standardize everything that’s useful and thus lower the chance for bugs?

Cheers, Filip

> Wiadomość napisana przez Bo Persson via Std-Proposals <std-proposals_at_[hidden]> w dniu 24 mar 2025, o godz. 08:11:
>
> On 2025-03-24 at 02:51, Muhammad via Std-Proposals wrote:
>
>> Last thing about #include once, As with all preprocessing directive it get affected with the conditional #if family of preprocessing directive, for example:
>> //d.hpp
>> #ifdef D_HPP
>> # include once
>> #endif
>> The file d.hpp will get included every time it appears in the #include until the macro D_HPP gets defined.
>
> What if D_HPP gets undefined again in another header? There are tiny corner cases all around!
>
> You also have the problem of how you find the header:
>
> "A preprocessing directive of the form
>
> # include < h-char-sequence > new-line
>
> searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the < and > delimiters, and causes the replacement of that directive by the entire contents of the header. How the places are specified or the header identified is implementation-defined."
>
> (https://eel.is/c++draft/cpp.include#2)
>
> There are two "implementation-defined" right here.
>
> Later it continues:
>
> "The method by which a sequence of preprocessing tokens between a < and a > preprocessing token pair or a pair of " characters is combined into a single header name preprocessing token is implementation-defined.
>
> The implementation shall provide unique mappings for sequences consisting of one or more nondigits or digits ([lex.name]) followed by a period (.) and a single nondigit. The first character shall not be a digit. The implementation may ignore distinctions of alphabetical case."
>
> So d.HPP and D.hpp might be different files? Or not?
>
> I have worked on mainframes (IBM z/OS) where the file system doesn't have directories or extensions on file names. Another corner case (where d.hpp might be HPP(D) on disk).
>
>
>
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