What are the most common/uncommon error messages one encounters in Objective-C and what causes them?

Question

I've been trying to maintain a list of all the error messages I ever encounter while developing and their common causes and fixes. What are the common (and the not so common) error messages you encountered and how did you fix them?

Answer 1

EXC_BAD_ACCESS when referencing a pointer that points to an object that has been dealloc'ed

Answer 2

In general I find objective-c exception and error messages tO be self-explanatory. I think you would be better served by learning to use the debugger with breakpoints on objective-c exceptions to locate the specific line causing a crash than by trying to compile a list of recipies for resolving common errors.

That said the one non-obvious error I see as a frequent point of confusion is "unrecognized selector foo: sent to BarClass". While the reason why that is an error should be clear answering how that happened can be harder and I see two common causes:

1. An app is attempting to call a subclass' method on an instance of a super class. Often a result of developers failing to change the class of objects in IB.
2. An object has been over-released or otherwise referenced after being deallocated. If a new instance of some other class now occupies that memory address the app with hit an "unrecognized selector" exception rather than a "BAD_ACCESS".