Re: reasons for having no inclusion guards in config.h

[Bob Friesenhahn]

On Sun, 7 Mar 2010, John Calcote wrote:
> The idea here of course is that the unwanted situation whereby the same 
> config.h file is inadvertently included in the same translation unit twice 
> could be avoided.
>
> As an aside: It's interesting to note that gcc's preprocessor has built-in 
> functionality to automatically guard against this situation.

It is also interesting to note that it is usually ok for the content 
of the header file to be parsed any number of times.  The only 
exception might be with some weird macro expansion (but that would be 
a bug).

The conditional inclusion of content is only an optimization.  The 
content of the typical config.h is quite easy to parse.  Opening the 
file to inspect it is surely much more overhead than whatever parsing 
overhead is saved by the extra conditional.  This becomes more and 
more true as time goes by.  In the early '90s the benefit was 
tangible, but probably not at all any more.

A cost/benefit analysis should show that this effort is not worth 
doing.

Bob
--
Bob Friesenhahn
address@hidden, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/