1. Exception Handling
Oo28

2. Teaching Points
Exception handling mechanisms
Try-Throw-Catch

3. Error Handling Styles
When we detect an error (exceptional condition) we can:
Terminate the program
Return a value representing the error
Return a legal value and place the program in an illegal state
Call an error function
BUT…

4. Terminate the program Too Drastic!
Many programs cannot just role over and fail
Mission critical
User can intervene to resolve error

5. Return a value representing the error
Clutter in the main line of the code. It becomes hard to see main line of the logic is.
Often there is no acceptable value which can easily be used (eg. The possible returns already cover the entire set of return values)

6. Return a legal value and place the program in an illegal state
Clutter in the main line of the code. It becomes hard to see main line of the logic is.
Probably must resort to global state variable (eg. C library errno)
Example from C

7. Call an error function
May clean up the logic a bit but we are really left with the same choices as before after the error call is made.

8. New Alternative Try-Throw-Catch
Analogy - an alternate return mechanism

9. Try-Throw-Catch try block
a program block in which an exception may be thrown
it is uncluttered with error handling support
any of the statements might throw an exception in which case the execution of the block stops with that statement

10. Try-Throw-Catch catch block
a separate block in the body of the operation
not normally executed
designed to handle a specific exception
this block is executed immediately after the specified exception is detected in the try block

11. Try-Throw-Catch throw statement
stops execution of the operation at this statement
throws an exception to the calling operation (I.e. does an alternate return with the exception value instead of the normal return value)
the thown value is an object
exceptions have their own types and are not related to the normal return type for the operation

12. Example
Example is in Java but is essentially the same mechanism as C++

13. Teaching Points
Exception handling mechanisms
Try-Throw-Catch