include guards

include guards

[Shriramana Sharma]

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?

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?

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...

Shriramana Sharma.

Re: include guards

[wwp]

[-- Attachment #1: Type: text/plain, Size: 1240 bytes --]

Hello Shriramana,


On Fri, 15 Jun 2007 09:13:59 +0530 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?
> 
> 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?
> 
> 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...

#pragma is simply not supported by all pre-processors, is that
directive present in any standard at least?


Regards,

-- 
wwp

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 189 bytes --]

Re: include guards

[Steve Graegert]

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>
-
To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html