Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!cs.utexas.edu!sm.unisys.com!oberon!stromboli.usc.edu!english
From: english@stromboli.usc.edu (Joe English)
Newsgroups: comp.lang.c++
Subject: Re: Improved switch statement (was Re: goodbye cpp ???)
Summary: Not really an improvement...
Message-ID: <14104@oberon.USC.EDU>
Date: 16 Dec 88 23:01:45 GMT
References: <6590072@hplsla.HP.COM> <1106@etive.ed.ac.uk> <33528@bbn.COM> <1907@ogccse.ogc.edu> <574@redsox.UUCP>
Sender: news@oberon.USC.EDU
Reply-To: english@stromboli.usc.edu (Joe English)
Distribution: na
Organization: University of Southern California, Los Angeles, CA
Lines: 36


In article <574@redsox.UUCP> campbell@redsox.UUCP (Larry Campbell) writes:
>In article <1907@ogccse.ogc.edu> wm@ogccse.UUCP (Wm Leler) writes:
>}I don't think you could extend switch statements to work on strings,
>}since there is some ambiguity between whether you want to compare
>}the strings by comparing their contents (using strcmp) or by comparing
>}their pointers (a very reasonable thing to do).
>
>You leave that decision to the person who implemented == for the class.
>I think it would be eminently sensible for the switch statement to operate
>on any type that has an == operator defined; of course the case labels would
>have to be the correct type (or coerced thereto).
>
>A common and useful idiom I learned in BLISS which is, unfortunately, not
>possible in C or C++ today, is the following code fragment (transliterated
>from BLISS into pseudo-C), which checks a number of boolean flags:
>
[code fragment using switch(), equivalent to if (flag1)...else if (flag2)... ]

This is useful only as syntactic sugar:  the switch statement 
in C/C++ does *not* compile into a series of

if (expr == case1) ... else if (expr == case2) ...

statements, as your suggestion implies.  Instead, (in all the
implementations I've seen, anyway) it does a vectored jump, or if the
(case_i) values are not close together (e.g., case 10: ... case
100:... case 1000:...) into a table lookup with a vectored jump, both
of which are much more efficient than a series of if () tests.  The
switch statement you suggest would (of necessity) compile into a bunch
of if()s, which is not really what switch was designed to do...


      /|/| "How do you convince your dermatologist 
-----< | |                      that you're being sexually responsible?
  O   \|\| english%lipari@oberon.usc.edu