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From: matthew@sunpix.UUCP ( Sun Visualization Products)
Newsgroups: comp.lang.c
Subject: Re: A solution to the multiple inclusion problem
Keywords: #include c c++
Message-ID: <946@friar-taac.UUCP>
Date: 24 Oct 89 13:46:46 GMT
References: <14240@well.UUCP> <15316@bloom-beacon.MIT.EDU>
Reply-To: matthew@friar-taac.UUCP (Matthew Stier - Sun Visualization Products)
Distribution: comp
Organization: Sun Microsystems, RTP, NC
Lines: 70

In article <15316@bloom-beacon.MIT.EDU> tada@athena.mit.edu (Michael J Zehr) writes:
|In article <14240@well.UUCP> nagle@well.UUCP (John Nagle) writes:
|>[problem of multiple exclusion, generally solved by:]
|>	#ifndef XXX
|>	#define XXX
|>	...content...
|>	#endif
|>This works, but on the second inclusion, the file still has to be read and
|>parsed, at least by the level of processing that reads "#" statements.
|>[slowing compilation, particularly in C++ programs]
|
|>[proposed compiler optimization binding the 'XXX' defined above to the
|>included file and not including it a second time.]
|
|there's another solution you can manage without having to get your
|compiler vendor to make such an extension:
|
|<foo.h:>
|#ifndef FOO
|#define FOO
|#include "foo_real.h"
|#endif
|
|admittedly there's an extra file open for the first time reading
|through, and you still have to open files when #including the second
|time, but it's not as bad as having to read the entire file in
|(particularly if you have some really *large* files).  library files
|could be changed to this with relatively little effort and no compiler
|changes, and users' header files for large systems could be changed to
|work faster *today* without having to wait for the vendors to decide to
|do this.
|
|(unless i'm missing something obvious and being really stupid this
|works....)
|
|-michael j zehr

or if your sure all your header files use the:

#ifndef XXX
#define XXX
...content...
#endif

constructs, you could just alter you source code a bit, and do an '#ifndef', '#endif' around
each '#include'.  This would require adding two lines for each included file, and no extra files
open.


/*
 * Start of source code
 */

#ifndef STDIO.H
#include <stdio.h>
#endif
#ifndef CTYPE.H
#include <ctype.h>
#endif


     Now, if only library writers used the '#ifndef', '#endif' construct consistantly.



-- 
Matthew Lee Stier                            |
Sun Microsystems ---  RTP, NC  27709-3447    |     "Wisconsin   Escapee"
uucp:  sun!mstier or mcnc!rti!sunpix!matthew |
phone: (919) 469-8300 fax: (919) 460-8355    |