Xref: utzoo comp.lang.c++:5203 comp.std.c:1946
Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!gem.mps.ohio-state.edu!lavaca.uh.edu!uhnix1!sugar!ficc!peter
From: peter@ficc.uu.net (Peter da Silva)
Newsgroups: comp.lang.c++,comp.std.c
Subject: Re: Conditional inclusion
Message-ID: <6662@ficc.uu.net>
Date: 24 Oct 89 17:27:17 GMT
References: <2181@dataio.Data-IO.COM>
Reply-To: peter@ficc.uu.net (Peter da Silva)
Organization: Xenix Support, FICC
Lines: 33

In article <14240@well.UUCP> nagle@well.UUCP (John Nagle) writes:
<	#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.
<With widespread use of this technique within library files, some files may
<be read a large number of times, mostly to be ignored.  This slows compilation.

In practice, you can combine this with a second technique:

	#ifndef FROB_H
	#define FROB_H

	#ifndef SYS_FOO_H
	#include <sys/foo.h>
	#endif

	#ifndef SYS_BAR_H
	#include <sys/bar.h>
	#endif

	...

	#endif

This completely resolves the efficiency question.
-- 
Peter da Silva, *NIX support guy @ Ferranti International Controls Corporation.
Biz: peter@ficc.uu.net, +1 713 274 5180. Fun: peter@sugar.hackercorp.com. `-_-'
"That particular mistake will not be repeated.  There are plenty of        'U`
 mistakes left that have not yet been used." -- Andy Tanenbaum (ast@cs.vu.nl)