[Development] Using '#pragma once' instead of include guards?

[Henry Skoglund]

On 2022-10-10 21:27, Thiago Macieira wrote:
> ...
> This situation is annoying either way. With include guards, you will get a
> working build, but you may spend some time trying to figure out why the changes
> you're making to the headers aren't taking effect. With the pragma, you get
> hard build errors due to redefined types, probably with both paths so you'd
> notice easily that there are two copies.
>
> The problem I have with switching to #pragma once is for whom the problem
> happens and who gets to fix it. Most people who build Qt from source are not
> developing or modifying it; they're simply building for use in their own
> content. So if the problem of incorrect include paths happens with
> preprocessor guards, most likely they will not see any ill effect because both
> copies of headers are the same. But if those headers use #pragma once, those
> people will get build errors despite the file contents being identical, because
> they are different files.

Hi, this kind of reminds me of detecting violations of ODR but instead 
of objects it's about .h files. And #pragma once can then be seen as the 
weaker sibling of the include guard mechanism, i.e. not as efficient as 
detecting multiple instances of the same .h file.
But perhaps there could be use of #pragma once anyway, as a way to 
detect those duplicate files? Say something like this at the top of an 
.h file:

#pragma once
#if defined QGLOBAL_H
#error "Multiple instances of qglobal.h detected"
#endif
#define QGLOBAL_H
...

Rgrds Henry