Friday, February 12, 2010

Asserts vs Exceptions

I still see people with the doubt: "shall I use an assert or throw an exception?".
Let me whine a bit about the usage of asserts first. Overall people code badly and I mean it, hence assertions are not used enough. Having say that, "assert" and "exception" have different
meanings and behaves.

A failure assert just terminates the normal program execution (reporting source file name and line position where the error occurred), an exception thrown has a chance to be caught. Asserts disappear/vanish when NDEBUG is defined (usually in production code); this fact is a newbie oversight and what they claim about the massive usage of assert is the overhead introduced by asserts. If I wasn't clear: "Asserts disappear in production code". What didn't you get from: "Asserts disappear in production code"?

Those two facts already give an hint on when to use an assert and when an exception. If the error can be managed (by user or code itself) then an exception must be used, if the error has no chance to be managed by anyone then assert is your friend. This is not the only rule to follow indeed as said: "Asserts disappear in production code" and then in production code that error will not be "detected", errors that you expect to happen in production code can not be spotted by an assert.
Asserts are meant to detect coding error and there is no reason to happen in production code. When writing a method there are some assumption about the internal class status (pre-condition), better to check those first in order to not do something bad, at the end of the same method better check that the internal status of class is in a consistent status (post-condition).
Unfortunately, there are a great deal of people that are using exceptions for case that really ought to be assertions. Throwing exceptions instead of wrapping simple pre/post-conditions into a simple assertion macro is an hint of the fact that you're coding badly. Asserts are used to protect by coding error: using a null pointer for example; exception are to protect by normal program life: server not available, file not writable, etc.

It should be always possible to write a unit test that is able to make an exception to be thrown, if you are not able to then it means that that piece of code is a dead code; to the other side it should be impossible do the same with assert, if you are able to write an unit test that is able to make an assert fail then most likely that assert should be an exception, or of course the code is wrong.

A nice rule that drives the lazy bone programmers to use more assert is: who ever write a code that provides a set of data that asserted then the same person is responsible to fix it. This rule drives library writer to protect them self by an incorrect usage of their library at the cost to fix even the code using the library.

If you have missed it: "Asserts disappear in production code".

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