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From: bs@research.att.com (Bjarne Stroustrup <9758-26353> 0112760)
Subject: Re: What Should An Exception Handling Do? -- Clarification of rules
Message-ID: <Doq3sv.MzA@research.att.com>
Organization: Info. Sci. Div., AT&T Bell Laboratories, Murray Hill, NJ
References: <4irn11$7ln@mimas.brunel.ac.uk> <Pine.Sola.3.91.960322041345.17711C-100000@ux5.cso.uiuc.edu> <4j03p4$fbt@hoho.quake.net>
Date: Sat, 23 Mar 1996 13:50:55 GMT
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billf@jovial.com (Bill Foote) writes


 > > quote explaining why C++ (and Java) uses the termination model
 > > of exception handling
 > 
 > A cynic might conclude something more along these lines:  "It would be hard
 > to implement resumable exceptions in C++, so they decided to punt."

and that "cynic" would be wrong, guilty of not having done his homework,
and guilty of making unkind conjectures without basis in facts.
 
 > It's not too hard to come up with examples where it would be nice to have
 > resumable exceptions.  One example is resource acquisition.  Upon failure
 > to acquire a resource (a file descriptor, some memory, etc.) it might be
 > nice if a top-level exception handler could try to free up some resources,
 > and if it can, try to resume from the exception.

This point, and many others were seriously studied by the C++ standards
committee before it approved my design of exception handling for C++.
The arguments for and against resumption can be found in chapter 16 of

	Bjarne Stroustrup:
	The Design and Evolution of C++
	Addison-Wesley ISBN 0-201-54330-3

together with a description of the complete language features and examples
of their use.

Here, I'd like to quote a key section:

	Then, at the Palo Alto meeting in November 1991, we heard a
	brilliant summary of the arguments for termination semantics
	backed with both personal experience and data from Jim Mitchell
	(from Sun, formerly from Xerox PARC). Jim had used exception
	handling in half a dozen languages over a period of 20 years
	and was an early proponent of resumption semantics as one of
	the main designers and implementers of Xerox's Cedar/Mesa system.
	His message was

		``termination is preferred over resumption; this is
		not a matter of opinion but a matter of years of
		experience. Resumption is seductive, but not valid.''

	He backed this statement with experience from several operating
	systems. The key example was Cedar/Mesa: It was written by people
	who liked and used resumption, but after ten years of use, there
	was only one use of resumption left in the half million line
	system -- and that was a context inquiry. Because resumption
	wasn't actually necessary for such a context inquiry, they removed
	it and found a significant speed increase in that part of the
	system. In each and every case where resumption had been used
	it had -- over the ten years -- become a problem and a more
	appropriate design had replaced it. Basically, every use of
	resumption had represented a failure to keep separate levels
	of abstraction disjoint

	Mary Fontana presented similar data from the TI Explorer system
	where resumption was found to be used for debugging only, Aron
	 Insinga presented evidence of the very limited and nonessential
	use of resumption in DEC's VMS, and Kim Knuttilla related exactly
	the same story as Jim Mitchell for two large and long-lived
	projects inside IBM. To this we added a strong opinion in favor
	 of termination based on experience at L.M.Ericsson relayed to
	us by Dag Bruck.

	Thus, the C++ committee endorsed termination semantics.

 > Oh well.

Indeed

	- Bjarne