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).