Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!agate!bionet!apple!bloom-beacon!tut.cis.ohio-state.edu!osu-cis!att!pegasus!hansen
From: hansen@pegasus.ATT.COM (Tony L. Hansen)
Newsgroups: comp.lang.c++
Subject: Re: Managing C++ Libraries: Dependencies and Headers
Summary: another method
Keywords: header files, multiple includes, nmake, cpp
Message-ID: <3222@pegasus.ATT.COM>
Date: 8 Nov 88 18:30:46 GMT
References: <5078@thorin.cs.unc.edu> <7573@nsc.nsc.com>
Reply-To: hansen@pegasus.UUCP (XT1554000-Tony L. Hansen;LZ 3B-315;3207)
Organization: AT&T Bell Laboratories, Lincroft, NJ
Lines: 31

< >7.  A program written using our management system should read only the 
< >    header files that are necessary and should read them only once. 
<
< OK.  So how about a slightly more intelligent pre-processor which would
< (a) keep a list of all files included so far, and (b) avoid including
< files which have already been included once (even if they are called for
< again via further #include's)?  This would have the same effect as the
< encapsulation of header files within #ifdef's (which many people are using
< now).
< ...
< The only problem is that this approach is *not* compatible with certain
< tricky uses of header files (i.e. those cases when a given header file
< *must* actually be included more than once for some reason).  Maybe this
< is a good thing. Such "tricky" uses of header files are probably not
< compatible with "good software engineering practices".

The 4th generation make system, nmake, comes with an enhanced version of the
C preprocessor which does what you suggest with a major modification:

A header file will not be #included twice unless it is re-#included from the
same level as previously.

In other words, if both a.c and b.h #include <stdio.h>, and a.c #includes
<stdio.h> as well, <stdio.h> will be #included only once. However, if a.c
should #include <regex.h> twice, it will actually get #included twice.

This takes care of almost all cases where you don't want files #included
more than once, plus it still allows header files which do tricky things to
be #included more than once if need be.

					Tony Hansen
				att!pegasus!hansen, attmail!tony