NSAssertionHandler objects are used to raise exceptions on behalf of macros implementing assertions.
Each thread has its own assertion handler instance.

The macros work together with the assertion handler object to produce meaningful exception messages containing the name of the source file, the position within that file, and the name of the ObjC method or C function in which the assertion failed.

An NSAssertionHandler instance is created on demand for each thread and is stored in the thread's dictionary under the key NSAssertionHandler. A custom NSAssertionHandler can be used by adding it to the thread dictionary under this key.

The assertion macros are: NSAssert() , NSCAssert() , NSAssert1() , NSCAssert1() , NSAssert2() , NSCAssert2() , NSAssert3() , NSCAssert3() , NSAssert4() , NSCAssert4() , NSAssert5() , NSCAssert5() , NSParameterAssert() , NSCParameterAssert()

The NSException class helps manage errors in a program. It provides a mechanism for lower-level methods to provide information about problems to higher-level methods, which more often than not, have a better ability to decide what to do about the problems.

Exceptions are typically handled by enclosing a sensitive section of code inside the macros NS_DURING and NS_HANDLER, and then handling any problems after this, up to the NS_ENDHANDLER macro:

   NS_DURING
    code that might cause an exception
   NS_HANDLER
    code that deals with the exception. If this code cannot deal with
    it, you can re-raise the exception like this
    [localException raise]
    so the next higher level of code can handle it
   NS_ENDHANDLER
   

The local variable localException is the name of the exception object you can use in the NS_HANDLER section. The easiest way to cause an exception is using the +raise:format:,... method.

If there is no NS_HANDLER... NS_ENDHANDLER block enclosing (directly or indirectly) code where an exception is raised, then control passes to the uncaught exception handler function and the program is then terminated.
The uncaught exception handler is set using NSSetUncaughtExceptionHandler() and if not set, defaults to a function which will simply print an error message before the program terminates.