Re: include guards

["Steve Graegert" <graegerts@gmail.com>]

Shriramana,

Please see inline.  Thanks.

On 6/15/07, Shriramana Sharma <samjnaa@gmail.com> wrote:
> Hello.
>
> To prevent header files from being included more than once in the same
> translation unit, we use include guards like
>
> # ifndef FOO_H
> # define FOO_H
> ...
> # endif
>
> Recently I came to know that I can use simply:
>
> # pragma once
>
> instead of the above group of sentences and the desired effect is still
> accomplished.
>
> This leads me to think of two things:
>
> 1. why use the ifndef-define-endif method when the pragma once method is
> simpler and cleaner?

pragma(s) are, as most language extensions, not portable.

> 2. why should we need to use either method at all? If it is a
> universally undesirable behaviour that the same header file is included
> in a translation unit more than once, then an intelligent compiler (or
> preprocessor) itself can by default take of this, right?

Yes, if a header file is contained entirely in a `#ifndef'
conditional, then the preprocessor records that fact.  But if a
subsequent `#include' specifies the same file, and the macro in the
#ifndef is already defined, then the file is entirely skipped, without
even reading it.  How else should a preprossessor deal with this in a
portable manner?  Keep in mind that preprocessing is a distinct step
in the compilation process.

> I understand that to write portable code that compiles on
> not-so-intelligent compilers, we may need to do something manually, so
> question 2 is answered, but question 1 still stands...

Does it?  Correct, pragmas are not standard, and probably never will
be, due to the difficulty of specifying exactly what it is that
'#pragma once' is supposed to do. (Think of two identical copies of a
header file in different source directories, and a translation unit
that #includes both of them. What is the effect of '#pragma once'
here?)  Inclusion safe guards work perfectly well, are
standard-compliant and portable.

	\Steve

--

Steve Grägert <steve@graegert.com>
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