Question

I was debugging something in my code and realized that the following compiles

NSMutableSet *s = [[NSMutableArray alloc] init];

Card *c = [[Card alloc] init];

[s addObject:c];
[s addObject:c];
[s addObject:c];

NSLog(@"Set now contains %d cards ", [s count]); //Prints 3

But the declared type is a Set here .. why are duplicates allowed? Why does this compiles at all?

Answer 1

[[NSMutableArray alloc] init] returns type id, this is standard for Objective-C to return type id. In many cases the actual returned type depends on the class implementation, many Apple supplied classes are class clusters and return a different type. A common example of this is NSString which usually returns an instance of the class NSCFString.

It is important to be careful about matching class names.

Also see the answer by Marc Charbonneau here