What is the main use of NSAssert Vs. NSException

Question

What is the main use of NSAssert Vs. NSException. What is more recommended and when?

Answer 1

Assertions are generally used during development only and are compiled-out of the app when in release mode (this is controlled by NS_BLOCK_ASSERTIONS). Exceptions, on the other hand, can be used at all times.

When an exception is thrown, it travels back up the call chain, until it is either caught (and reported, ignored, or another exception is thrown) or it reaches the top, in which case it will cause the app to crash. It can be considered part of the contract of a class method and needs to be documented so the caller can handle this correctly.

Assertions are really a runtime developer check that ensure that something (generally a instance variable) is in a certain state and if it's not then abort() in order to bring the issue to the developers attention. It's a developer sanity check to check that something is in the state the developer expects it to be.

Answer 2

Assertions are used to find things that should never happen under any circumstances if your code is working the way you think it should be. If they are happening, there is a bug in your code and you want to know about it, at least if it happens during testing. (Most people turn off assertions in released code.)

In contrast, exceptions are used to find things that have gone wrong over which you have no control. For example, if your application is dependent on a database server and that database server is unavailable, that might raise an exception in your code. (Do not make the mistake of using exceptions for things like user input validation. If it's regular program flow--the user forgot to enter a field or whatever--that's not an exception. Exceptions should be exceptional.)