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From: ok@goanna.cs.rmit.edu.au (Richard A. O'Keefe)
Subject: Re: Improving Ada Exceptions
Date: 1997/11/03
Message-ID: <63jqd0$21p$1@goanna.cs.rmit.edu.au>#1/1
X-Deja-AN: 287547859
References: <63072n$q6r$1@berlin.infomatch.com>
Organization: Comp Sci, RMIT University, Melbourne, Australia.
Keywords: exceptions
NNTP-Posting-User: ok
Newsgroups: comp.lang.ada
Date: 1997-11-03T00:00:00+00:00
List-Id: <comp.lang.ada>


blaak@infomatch.com (Ray Blaak) writes:

>Hello all. I am an Ada fanatic who has just started his first C++ project.
>Being thoroughly ingrained against the evils of C++ I was quite surprised to
>find it not as bad as I expected. There are many horrible aspects about it, to
>be sure, but one thing I find very cool is C++'s exception mechanism.

Does anyone know whether C++ copied the idea of throwing an object
from CLOS?  The 'condition system' in Common Lisp goes back a long way.
Maybe the idea of throwing objects is just obvious.

Of course, there _is_ the wee problem of running a constructor so you
can throw a 'storage exhausted' exception, but a few prebuilt exceptions
will fix that.

>It is a very powerful thing to be able to throw and catch arbitrary objects,

It is also rather 'heavyweight' for a lot of situations.

There's another problem.  What if an exception object belongs to a class
that was declared inside a subprogram P, and no handler inside P catches
it.  Now an object has escaped outside the region of the program where
it makes sense.  Imagine, for example

    void foo(void) {
	char buff[4];
	class my_exception : std :: bad_alloc {
	    private: char *msg;
	    public : myexception(char *m) msg(m) {}
	};
	strcpy(buff, "fu!");
	my_exception e(buff);
	throw buff;
    }

This is not supposed to be correct C++, by the way, just to illustrate
the possibility.  C++ puts severe restrictions on classes declared
inside functions (which is a rather gross violation of one Stroustrup's
design rules for C++: allow nesting whenever it makes sense).  Ada,
however, doesn't have those silly restrictions, which means that
throwing an exception object out of the scope of its class would be a
possibility that would have to be blocked some way.

Lisp, of course, makes returning objects outside their lexical scope
_work_, so the problem doesn't apply in quite these terms to CLOS.

-- 
John �neas Byron O'Keefe; 1921/02/04-1997/09/27; TLG,TLTA,BBTNOTL.
Richard A. O'Keefe; RMIT Comp.Sci; http://www.cs.rmit.edu.au/%7Eok