Exception Processing Question -Reply

Question

I don't use Forte (yet--that's why I joined the mailing list), but since your
question is a generic "engineering practices" question, I'll chime in. I do
use exceptions both in C++ in Delphi, and have developed some opinions
about how to use them:
- Use exceptions for handling "behavior unexpected in the normal course
of your algorithm". In other words, instead of returning error codes
when something goes wrong, throw an exception.
One example of this is in a parser: if you get an illegal token, just throw
an exception.
- When you need to make a choice based on a return status, don't throw
an exception. In the parser example, if you get token 'A', you need to do
one thing and if you get token 'B' you need to do another.
- There's a fine distinction between "handling an error" and "making a
choice", and there are cases where you could argue one way or the
other. I'd say that an error throws an exception when it would normally
terminate the normal flow of execution. For example, "out of memory" or
"illegal token".
- Catch exceptions at "high level" process start/end points, or where it is
required to clean up resources held and rethrow.
- Don't catch exceptions in every single method--it defeats the purpose.
User Interfaces:
- If your user-interface dictates a logical place to unwind no further,
catch exceptions there. For example: In the Windows world most apps
are a window with a menu. If you pull down and pick a menu item to
execute a command, and that command throws an exception, the
exception should be caught, a modal message box indicating the error
should be presented, and then when the user clicks "ok", the program
returns to its normal waiting state with the window intact.
- If you have a modal dialog up, throwing an exception should either stop
at the dialog's event loop or take the dialog down with it.
I'd say that on-screen data validation should NOT use exceptions
because "user typing invalid data" is expected behavior for a UI.
Besides, the UI designer should be smart enough to deal with giving you
a good model for data validation. OTOH, if you take the data out of the
dialog and then pass the information as parameters to a function, and
that function can't handle one of the parameter values, it should throw an
exception.
Nevertheless, if your dialog runs out of memory for some reason, it
should throw an exception and the dialog should come down.
Hope this helps.
-- Conrad Herrmann
Jerry Fatcheric <[email protected]> 07/24/96 03:29am >>>
I have a toss up question for 50 points -
We have finished the first phase of our project and have place
exception processing in the places where we saw exceptions could be
raise. But what about those other exceptional exceptions?
Is this just good programming practice to have a high level generic
exception handler (and what should it really do??) or is it critical.
In addition, when handling data validation on specific widgets on a
screen (e.g. the account number entered must be one of the existing
10,000 valid ones), should this be handled via exception processing or
should it just be handled with code in the event loop?
Any opinions - please I don't want to hear the Forte Party line.
I would like to hear opinions based upon real experience.
Jerry Fatcheric
Relational Options, Inc.
Florham Park, New Jersey
201-301-0200
201-301-00377 (FAX) [email protected]

Answer 1

We have finished the first phase of our project and have place
exception processing in the places where we saw exceptions could
be raise. But what about those other exceptional exceptions?
Is this just good programming practice to have a high level
generic exception handler (and what should it really do??) or
is it critical.IMHO, you need to program for these ones that could occur because
if you don't the exception will still happen except without
your normal graceful (or standard) approach manner. I think it is good programming
practice to trap on the ones you think can possible occur if they require
something different in how they react. Otherwise, use of GenericException
with a proper response is called for.
>
In addition, when handling data validation on specific widgets
on a screen (e.g. the account number entered must be one of the
existing 10,000 valid ones), should this be handled via exception
processing or should it just be handled with code in the event loop?
I use event loop processing because most of the time I am also
keeping the user on the same widget after cancelling all events
posted. So, in effect they are back where they started from.
Any opinions - please I don't want to hear the Forte Party line.
I would like to hear opinions based upon real experience.
Jerry Fatcheric
[email protected]@Sagesoln.com
my 2 cents, or should I say my attempt at 50 points, Jerry.
Len Leber
ATG Partners