Objective-C: Assertion vs. Exception vs. Error

Question

In Cocoa, when should I use NSAssert, NSException, NSError?

Here's what I've been thinking:

NSAssert - When creating any client program used for the programmers own benefit to double check rules, conventions, assumptions, or pre-conditions and post-conditions?

NSException - When creating a third-party library for the benefit of other programmers that use the library, so that they immediately know when an input is invalid?

NSError - When interfacing with an external system to get data like a file, database, or web service that isn't guaranteed to give me a result?

Answer 1

An NSAssert will throw an exception when it fails. So NSAssert is there to be short and easy way to write and to check any assumptions you have made in your code. It is not (in my opinion) an alternative to exceptions, just a shortcut. If an assertion fails then something has gone terribly wrong in your code and the program should not continue.

One thing to note is that NSAssert will not be compiled into your code in a release build, so this is typically used for sanity checking during development. I actually tend to use a custom assert macro that is always active.

The times you would @throw your own NSException are when you definitely want it in a release build, and in things like public libraries/interface when some arguments are invalid or you have been called incorrectly. Note that it isn't really standard practice to @catch an exception and continue running your application. If you try this with some of Apple's standard libraries (for example Core Data) bad things can happen. Similar to an assert, if an exception is thrown the app should generally terminate fairly quickly because it means there is a programming error somewhere.

NSErrors should be used in your libraries/interfaces for errors that are not programming errors, and that can be recovered from. You can provide information/error codes to the caller and they can handle the error cleanly, alert the user if appropriate, and continue execution. This would typically be for things like a File-not-found error or some other non-fatal error.