NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/comp/lang/cplus/16507

Text File | 1992-11-18 | 2.6 KB | 64 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!elroy.jpl.nasa.gov!ames!haven.umd.edu!darwin.sura.net!jvnc.net!yale.edu!ira.uka.de!slsvaat!josef!kanze
From: [e-mail] (James Kanze)
Subject: Re: Two things: typeof() and exceptions
In-Reply-To: [e-mail]'s message of Mon, 16 Nov 1992 20:20:36 GMT
Message-ID: <[e-mail]>
Sender: [e-mail]
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References: <[e-mail]>
    <[e-mail]>
Date: 18 Nov 92 15:43:11
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In article <[e-mail]>, Brian McCauley
writes:

|> > Secondly, I was reading the reference manual for Dylan (Apple's new OO
|> > language) and I noticed something that was pretty hot in the exception-
|> > handling domain. Basically, in any given procedure you can define a set
|> > of instructions that are guaranteed to be executed even if the rest of
|> > the procedure is terminated early due to an exception. Currently in C++,
|> > automatic variables are destroyed as the stack is unwound. But, with
|> > a set of 'unwind' statements in a function, I could destroy all of the
|> > variables I allocated on the heap as well.

|> Yes? In C++ this is written thus:

|> void foo() {
|>   try { /* body of function */ }
|>   catch (...) { /* unwinding code */ throw; }
|> }

|> I'm sure the answer to your question can't be that simple so perhaps
|> you'd better explain again.

I don't know what Dylan does, but Modula-3 has a finally clause in
addition to a catch for a try block.

The difference is: a catch clause is only executed if an exception is
thrown.  If it is used for ressource clean-up, then the clean up code
must be duplicated outside of the catch clause.  A finally clause is
executed whether or not an exception is thrown, so it is sufficient to
put any clean-up code there.  (It also rethrows any exception
automatically, so the "throw" isn't necessary either.)

Would there be any support for adding a finally clause to C++.  It's
not likely to break any existing code, since it is a pure extension
(to exceptions, no less; how many people are actually using exceptions
at this time, as opposed to the number who wish they could?).  Except
for the new keyword, I don't see anything that it could break (just
offhand, I haven't done any real analysis).

I know that the same effect can be obtained by declaring anything
requiring clean-up as a class, and doing the clean-up in the
destructor, but I'm not sure that this is always the most appropriate
idiom.
--
James Kanze            GABI Software, Sarl.
email: [e-mail]    8 rue du Faisan
                67000 Strasbourg
                France