6.9 Tricks of the Trade: Testing with Exception Handling

Ada's exception handling provides a powerful tool for designing programs whose behavior is predictable even if its inputs are badly formed or out of range. If exception handling were not available, it would be the programmer's responsibility to validate all incoming data--for example, by checking its range with an IF statement. Indeed, Ada programs can certainly be written this way--with no use of exception handling--but the result would not take advantage of this built-in power.

Even if exception handling is used to advantage, however, the programmer still has several important responsibilities in this area:

Analyze your program so that you know the places where exceptions may be raised, and be sure to place exception handlers in appropriate blocks in your program. This will ensure that exceptions are not unexpectedly passed back to the calling program or to the run-time system.
When you test your program, be sure to test it with badly formed or-out-of range data so that your exception-handling flow is tested. When you are finished testing, you should be confident that you know exactly what your program will do under each set of input conditions. The test data supplied to Program 6.12 shows an example of how this is done.

In summary, Ada's exception handling provides a useful way to take account of unusual circumstances in your program but does not relieve you of the responsibility to design and test carefully so that your program's behavior will always be predictable.