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[[Precondition]]s, [[postcondition]]s and [[invariant]]s validation are also part of defensive programming. This may involve checking arguments to a function or method for validity before execution of the body of the function. After the body of a function, doing a check of object state (in [[OO]] languages) or other held data and the return value before exits (break/return/throw/error code) is also wise.
Within functions, you may want to double check that you are not referencing something that is not valid (i.e., null) and that array lengths are valid before referencing elements with indexes on all temporary/local instantiations. A good heuristic is to not trust the libraries you did not write either. So any time you call them, check what you get back from them. It often helps to create a small library of "asserting" and "checking" functions to do this along with a logger so you can trace your path and reduce the need for extensive [[debugging]] cycles in the first place. With the advent of logging libraries and [[aspect oriented programming]], many of the tedious aspects
Generally speaking then, it is preferable to throw intelligible exception messages that enforce part of your [[application programming interface|API]] contract and guide the client [[programmer]] instead of returning values that a client programmer is likely to be unprepared for and hence minimize their complaints and increase robustness and security of your software.
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