NSException raise:format: as the last statement in a method

Question

I have this method:

+ (MHTwitterParser*)createParser:(NSString*)format {
    if ([format compare:@"json"] == NSOrderedSame) {
        return [[MHJsonTwitterParser alloc] init];
    }

    [NSException raise:@"Unknown format" format:@"Unknown format of parser"];
}

Compiler complains that:

Control may reach end of non-void function

It is just a warning, but it does not matter.

Obvious fix for that is to add for example return nil; after the [NSException raise: ....

However, I think it is not needed (and is even misleading for readers), because the exception is thrown, so it is not true that "Control may reach end of non-void function". Or am I missing something ...? Is it only compiler imperfection or there is some considerable reason for this?

The compiler is Apple LLVM compiler 3.1

Answer 1

The reason is simple.

For the compiler, the method [NSException raise: ...] is a black box method. It doesn't know that the method will actually raise an exception.

If you compare it with Java or C++, their throw statements are a language feature and the compiler knows exactly what will happen when it finds it. In Obj-C it's different and sometimes it depends on runtime conditions. Consider the following.

NSException* exception = nil;

if (someCondition) {

   exception = [NSException exceptionWithName:...];
}

[exception raise];

The compiler won't know if the exception is really raised or not.

Answer 2

Replacing [exception raise]; with @throw exception; is functionally the same thing and will prevent the warning (see: Throwing Exceptions).

Answer 3

The warning is there simply because not every path through the code ends with a return statement which the compiler recognizes as potentially problematic. That said, you probably shouldn't be throwing an exception here and should instead be generating an NSError and returning nil. The differences between exceptions and errors in objective-c are explained here and here.