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Next: What exception to throw Up: Note 190: Exceptions Changes for AIPS++ Previous: Exception mechanism

When to throw an exception

Exceptions are intended to be used in exceptional circumstances -- those that the application programmer cannot reasonably be expected to test for -- not as an alternative return mechansim. If nothing else, throwing an exception is expensive: Meyers²notes that throwing an exception might be three orders or magnitude more expensive than a normal function return.

As a rule of thumb then, a library routine should rarely have a catch clause. Anticipated problems should be handled with normal programming constructs (error returns and the like). The AIPS++ library already follows this rule.

Looging through the aips package, an exception is thrown³ in about 1800 places. Examining about half of these reveals the following reasons that exceptions are thrown:

Argument checking 45%

Internal invariant/state verification 40%

Memory allocation 10%

I/O errors 2.5%

Misc. 2.5%

It seems to me that we are generally throwing exceptions appropriately.

The only errors that a running program might plausibly be able to handle (other than via a graceful exit or a next iteration of a high level loop) are the memory allocation and I/O errors.

Next: What exception to throw Up: Note 190: Exceptions Changes for AIPS++ Previous: Exception mechanism

Argument checking	45%
Internal invariant/state verification	40%
Memory allocation	10%
I/O errors	2.5%
Misc.	2.5%