Path: utzoo!attcan!utgpu!watmath!watdragon!akwright
From: akwright@watdragon.waterloo.edu (Andrew K. Wright)
Newsgroups: comp.lang.c++
Subject: Re: goodbye cpp ???
Message-ID: <10468@watdragon.waterloo.edu>
Date: 16 Dec 88 13:44:41 GMT
References: <6590072@hplsla.HP.COM> <1106@etive.ed.ac.uk> <33528@bbn.COM>
Reply-To: akwright@watdragon.waterloo.edu (Andrew K. Wright)
Organization: U. of Waterloo, Ontario
Lines: 44

In article <33528@bbn.COM> lpringle@labs-n.bbn.com (Lewis G. Pringle) writes:
	...  example omitted
>This example reminds me of a suggestion for an extention to C++.  The problem
>with the above code is that switch statements require the cases to be integer
>constants - though there is really no good reason for this.
	...
>When c++ sees a switch statement, it could generate the skip-chain
>of if-then-elseifs from that, so long as the argument to the switch
>and the cases where all =='able.  (As an optimization - of course -
>when the cases are all constant it could just output the C switch statement
>as-is.)

There is one crucial difference between an if-else chain and a C switch:
the case arms of a C switch are checked for duplication.
Allowing switch to operate on data of any type makes this duplication
check impossible at compile time.

An idea i had a few years ago, but have never had the time to test,
is this:  a switch executes *all* cases which match.  This could
be quite useful in many cases, altho code generation for it is a bit
more complex.  It is a bit difficult to fit into C, since C cases dont
really have an "endcase".  But you can imagine one, and consider:

	switch( x ) {
	case 1: case 2:
		code for x == 1 or 2;
	endcase;
	case 2:
		code for x == 2 only;
	endcase;
	case 1: case 2: case 3:
		code for x == 1, 2, or 3;
	endcase;
	}

To code this example in ordinary C you must use two switches (or some ifs).
A C switch allows the flow of control from two case arms to merge, but
not to branch.  this construct allows both merging and branching.

Wierd?  maybe.  if your suggestion was a kitchen sink, this is a
veritable jacuzzi.

Andrew K. Wright      akwright@watmath.waterloo.edu
CS Dept., University of Waterloo, Ont., Canada.