Type safety in for in loop

Question

I've been experimenting with type safety in objective c for a while now. I think I got some of it, but I am wondering if the following is possible.

NSMutableArray <NSNumber *> *x = [NSMutableArray new];
[x addObject:@14];
[x addObject:@"s"]; // <--- Gives warning, good!

for (NSUInteger i = 0; i < x.count; i++) {
    NSString *s = [x objectAtIndex:i]; // <-- Gives warning, good!
}

NSString *d = x[0]; // <-- Gives warning, good!


//but
for (NSString *s in x) // <-- expected warning but didn't get it
    NSLog(@"%@", [s stringByAppendingString:@"s"]; // <-- no warning just run time error

So my question is can a for in loop give a warning when an incorrect object is used. I want to use the for in since it is fast and hides details of implementation.

Answer 1

Here's the issue.

Most of the NSArray/NSMutableArray methods such as addObject: and objectAtIndexedSubscript: (which allows for the modern [index] syntax) take or return ObjectType values. ObjectType is a special indicator that means "use the generic type" specified for the array.

The fast enumeration comes from the NSFastEnumeration protocol and its countByEnumeratingWithState:objects:count: method. Unfortunately, the objects parameter is a C-array of id. It doesn't use ObjectType. Since the objects are type id, the compiler can't do any type checking like it can for the other methods of NSArray.