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From: langley@bigbird.rtp.dg.com (Mark L Langley)
Newsgroups: comp.lang.c++
Subject: default arguments
Message-ID: <820@xyzzy.UUCP>
Date: 5 Mar 90 14:23:17 GMT
Sender: usenet@xyzzy.UUCP
Reply-To: langley@bigbird.rtp.dg.com (Mark L Langley)
Organization: Data General Corporation, Research Triangle Park, NC
Lines: 46



Ron Guilmette replies to article <4800087@m.cs.uiuc.edu> nelson@m.cs.uiuc.edu

>
>               void bill (int b = printf ("default")) {};
>
>(That is to say that we can assume that if length is not supplied, then the
       >  passed data is actually a string.)
>
>But it gets upset since "data" is not yet defined/in that scope.  Is there
>  a way to get around this???

// No.  A default value expression for a formal parameter can be any kind of
// expression (including function calls) and it can be arbitrarily complex,
// but the expression has to be valid IN THE CONTEXT where it appears (just 
// like all other expressions).
	...
// Ron Guilmette (rfg@ics.uci.edu)


Ah, not so... but I concur insomuch as I think it *should* be true...

It is not *IN THE CONTEXT*, as rfg asserts, rather it must be legal outside 
of any local context.  In other words, Cfront gathers up the context at a 
global scope and uses only that global context.  Sad but true consider this...

   extern "C" { extern printf(...); };

   void bar(int x=i+j) {
   printf("%d\n", x);
   }

   main() {
   int i=20, j=30;
    bar();
    }
    "default.c", line 3: error:  i undefined
    "default.c", line 3: error:  j undefined
    "default.c", line 3: error:  + of void*
    3 errors

I intend to propose this as a change in ANSI C++, and have already
proposed it to AT&T in private correspondance.

Mark Langley
langley@dg-rtp.dg.com