[Development] Using #pragma once

[Thiago Macieira]

On Monday, 8 October 2018 01:50:04 PDT Edward Welbourne wrote:
> On Sunday, 7 October 2018 01:56:47 PDT Lars Knoll wrote:
> >> Just a quick question: Does anybody have any good arguments against
> >> us starting to use #pragma once instead of header guards throughout
> >> our code base?
> 
> Thiago Macieira (7 October 2018 20:39) wrote:
> > For example, I have ~/src as a bind-mount to ~/dev/src. That means
> > 
> >         ~/src/qt/qt5/qtbase/src/corelib/global/qglobal.h
> >         ~/dev/src/qt/qt5/qtbase/src/corelib/global/qglobal.h
> > 
> > are actually the same file, but they aren't for the effects of #pragma
> > once because the paths differ.
> 
> How could the compiler end up trying to #include both of those files ?

THAT is the million dollar question.

Of course it shouldn't, but it could happen in an improperly-configured build, 
with stale files. And now instead of no error due to same include guard, you 
get C++ error which make no sense.

> Wherever they appear in its include-path for a given compilation unit,
> one of them must appear before the other and thus be used consistently.
> I suppose you could put one directory in the sys-include path for
> #include <qglobal.h> and the other in the source-include path for
> #include "qglobal.h" ... but this seems like a strangely contrived thing
> to do and I'd be inclined to say "well don't do that, then" to anyone
> who has problems with it.  What am I missing ?

The ability to debug and who has to do it.

If this problem happens, it happens for someone *using* Qt, not those of us 
developing it. And figuring out what went wrong is extremely difficult.

> The other reasons make more sense to me (and I'm "not a fan" of #pragma
> in general),

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center