Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!uunet!portal!cup.portal.com!John_-_Nagle
From: John_-_Nagle@cup.portal.com
Newsgroups: comp.lang.c++
Subject: Re: #pragma once (was Re: A solution to the multiple inclusion
Message-ID: <23464@cup.portal.com>
Date: 28 Oct 89 20:41:44 GMT
References: <14240@well.UUCP> <2082@plx.UUCP>
  <1989Oct27.163755.12976@paris.ics.uci.edu>
Organization: The Portal System (TM)
Lines: 34


     While there appears to be an ANSI requirement that source text bypassed
due to preprocessor commands be tokenized and error messages generated
when appropriate, there's no reason for this to happen more than once per
include file.  So objections based on that rule are spurious.

     The "#pragma once" approach is a good idea.  But a pragma represents
an explicitly nonstandard approach.  This should be standardized.

     Common LISP has "require", which has the right semantics for this
situation.  Perhaps we could use that keyword.

     It's worth considering that this new feature be added to C++ only,
not C, which is more firmly standardized.  In C++, dependencies tend to
be more complex than in C, and a larger percentage of the content of the
program tends to reside in include files in C++, since most class definitions
are in include files.  

     So how about

	#require FILENAME

as the one-time inclusion mechanism for C++. 

     The ultimate goal is that all include files contain "require" statements
for everything they need, so that order of inclusion in the ".c" files isn't
an issue and the user of a library doesn't have to worry about including
too little or too much.

     Worth noting is that the basic concept of "make" doesn't really handle
well the use of include in include files.  Something needs to be done about
this.

					John Nagle