From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID
	autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: fac41,9a0ff0bffdf63657
X-Google-Attributes: gidfac41,public
X-Google-Thread: 103376,4b06f8f15f01a568
X-Google-Attributes: gid103376,public
X-Google-Thread: 1108a1,9a0ff0bffdf63657
X-Google-Attributes: gid1108a1,public
X-Google-Thread: f43e6,9a0ff0bffdf63657
X-Google-Attributes: gidf43e6,public
From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Software landmines (loops)
Date: 1998/09/01
Message-ID: <m3vhn8q3kh.fsf@mheaney.ni.net>#1/1
X-Deja-AN: 386663466
Sender: matt@mheaney.ni.net
References: <slrn6sopmg.4fg.jdege@jdege.visi.com>
 <rfzpddz3s2.fsf@bnp-eng.remcomp.com>
 <902934874.2099.0.nnrp-10.c246a717@news.demon.co.uk> <dewar.903074236@merv>
 <6r1glm$bvh$1@nnrp1.dejanews.com> <dewar.903281994@merv>
 <6r9f8h$jtm$1@nnrp1.dejanews.com> <6renh8$ga7$1@nnrp1.dejanews.com>
 <6rf59b$2ud$1@nnrp1.dejanews.com> <l5HC1.6840$wN.1856764@news.giganews.com>
 <6rfra4$rul$1@nnrp1.dejanews.com> <35DBDD24.D003404D@calfp.co.uk>
 <m3ogt3qgca.fsf@mheaney.ni.net> <6sbuod$fra$1@hirame.wwa.com>
 <35f51e53.48044143@ <m3af4mq7f4.fsf@mheaney.ni.net>
 <904556531.666222@miso.it.uq.edu.au> <m31zpxqutn.fsf@mheaney.ni.net>
 <6sf87j$47n$1@hirame.wwa.com>
NNTP-Posting-Date: Mon, 31 Aug 1998 20:44:24 PDT
Newsgroups: comp.lang.eiffel,comp.object,comp.software-eng,comp.lang.ada
Date: 1998-09-01T00:00:00+00:00
List-Id: <comp.lang.ada>

"Robert Martin" <rmartin@oma.com> writes:

> >Would the implementation be better by not using multiple returns?
> 
> Yes.  Imagine that you had to change the function to make it thread safe;
> and that the way to do that was to sieze and release a mutex while the
> function was executing.  As written you would have to add the release in
> three separate places.  But if you had avoided the multiple returns, you
> would have had a single release.

Well...  The proper way to use a mutex is to wrap it in a controlled
type, so that release is called automatically as a result of subprogram
exit, no matter what the reason.  (Controlled types in Ada have
operations that are roughly analagous to constructors and deconstructors
in C++.)

Something like:

function "=" (L, R : Stack_Type) return Boolean is

   Lock : Mutex_Lock (Mutex'Access);
begin
   <same body as before>
end "=";

When the Lock object initializes (automatically), it calls the seize
operation of the mutex.

When the Lock object finalizes (automatically, as a result of subprogram
termination - for whatever reason), it calls the release operation of
the mutex.

A subprogram could terminate because there was an unhandled exception,
which is a form of return.  In that case, release wouldn't get called.
(Unless you remembered to include a catch-all handler, to release the
mutex.  But you probably won't remember.)

So I don't buy your thread-safety argument, because the issue you raise
is a problem for any algorithm that requires a mutex, not just an
algorithm implemented using multiple returns.

> bool operator==(Stack& l, Stack& r)
> {
>   bool equal = true;
>   for (int index = 1; index < l.top() && equal == true; index++)
>   {
>     if (l[index] != r[index])
>       equal = false;
>   }
>   return equal;
> }
> 
> If this is more complex (something that is arguable) it is not *much* more
> complex.  

I find that this is indeed more complex.  A decision table for the
predicate has 4 rules instead of just 2.

> On the other hand, it is easier to maintain.  The thread-safety
> issue I talked about above would be easier to add to this function than to
> the one with multiple returns.

I agree with you, but as I pointed out, there are problems with that
implementation too.