stanton@fortune.UUCP (W. Dean Stanton) (08/04/84)
Someone suggested "marking" a header file with a
#define HeaderLoaded 1
so some file that doesn't want to load it twice can check.
I have two further suggestions.
1. Either the file about to include can check:
#ifndef HeaderLoaded
# include "header.h"
#endif HeaderLoaded
or a careful include file can check:
#ifndef HeaderLoaded
# define HeaderLoaded
# . . . /* stuff to only include once */
#endif HeaderLoaded
(I feel it is probably the include file which should know if
what it does can be done more than once.) It is interesting
to note that some CPPs don't care if the #endif is left off
a "successful" #if or #ifdef, so be careful to include it.
2. As long as you are defining something, I make it a general rule
to prefix my include files with BOTH documentation-type comments,
including who changed what when, and ALSO a "flag" which gives
CPP the ability to check the versions if it cares:
# define HeaderVersion 840803
This could be a string, of course, but the "integer" (assuming such
a number fits in your CPP's "integer") allows checks. For example,
version numbers in two files damn well better match. Or the code
better be newer than the header file, or bomb (which I will take
as an indication that I should check the changes in the header file
for compatibility).
Of course, this last trick is most useful if your CPP allows you
to print error messages at CPP time if some unfriendly set of
values is discovered. Does anyone's CPP support this?
- W. Dean Stanton, Graphics Software
UUCP: {ihnp4,ucbvax!amd,hpda,sri-unix,harpo}!fortune!stanton
USPS: Fortune Systems Corp, 101 Twin Dolphin Drive, Redwood City, CA 94065
Phone: (415) 594-2835