[boost] Re: #pragma once

From: Alberto Barbati (abarbati_at_[hidden])
Date: 2002-12-30 05:18:41

Gennaro Prota wrote:
> Hmm... frankly I haven't used it anymore. I'm under the impression
> they have fixed it now, but last time I checked it had a lot of bugs.
> It was easy, for instance, to end up including the same file twice if
> it was reached through different paths (e.g.: #include "subdir/file.h"
> in A.cpp and #include "file.h" in subdir/file2.h). Usually one doesn't
> notice the error, because he uses both the pragma and the canonical
> include guard, but that means of course that the speed gain comes to
> nothing. Probably it was VC++ 5.0 though.

You are missing the point here. "#pragma once" is not really 
"functional",  in the sense that it's not (or should not be) used 
*alone* to realize the "include me once" effect. That effect is better 
achieved by the canonical include guards that should be used anyway (and 
*will* be used anyway in portable code).

"#pragma once" is just an optimization issue. If a file is included a 
second time in the same TU, a dumb compiler will re-open it and give it 
to the preprocessor who strips its contents entirely because the include 
guard is already defined. Re-opening + preprocessing takes little but 
significant time.

With the "#pragma once" the programmer just gives an hint to the 
compiler that re-opening is unnecessary and ignores the #include 
directive immediately. You see, there *is* a speed gain even in the 
presence of canonical include guards.

Of course, a smarter compiler like g++ will recognize the canonical 
include guards and deduce that re-opening is unnecessary without any 
explicit hint given by the programmer.

> [...] In effect, the whole story about file inclusion should be
> the other way round: any source file is included at most once for each
> TU, unless the programmer requires otherwise ;-)

I agree, but we can't give the burden of avoiding multiple inclusions to 
the "user". Realistically, with large projects such intent is hardly 
achievable by anyone.

Alberto