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From: rfg@ics.uci.edu (Ronald Guilmette)
Newsgroups: comp.lang.c++
Subject: Re: default arguments
Message-ID: <25F8CCEE.9928@paris.ics.uci.edu>
Date: 10 Mar 90 09:46:23 GMT
References: <820@xyzzy.UUCP>
Reply-To: rfg@ics.uci.edu (Ronald Guilmette)
Organization: UC Irvine Department of ICS
Lines: 59

In article <820@xyzzy.UUCP> langley@bigbird.rtp.dg.com (Mark L Langley) writes:
>
>
>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

Mark, you have obviously misconstrued what I said.  You are considering the
context in which the default value expression actually comes into play (i.e.
at a call point).  The point at which a default expression comes into play is
obviously different from the point in the text where it actually appears.

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

If you are suggesting that the default expressions be semantically
evaluated in the context(s) in which they come into play, think again.
The difficulty of implementing such a treatment far outweighs the usefulness
of having such a treatment.

// Ron Guilmette (rfg@ics.uci.edu)
// C++ Entomologist
// Motto:  If it sticks, force it.  If it breaks, it needed replacing anyway.