Question

B is a subclass of class A.

A * a = [[B alloc] init];
B * b = [[A alloc] init];

Which of these is invalid and why?

When I typed this in I got a warning for the second thing, but I couldn’t understand what it meant. It showed “Incompatible pointer types initializing ‘B * __strong’ with an expression of type ‘A *'”. Also may some one tell me if the second expression can be made valid or not and how to do the same.

Answer 1

If B is a subclass of A, its instances may be used everywhere where instances of A could be used. The inverse is not true.

Here is a real-life example using Apple’s classes: consider NSArray and its subclass NSMutableArray. Since NSMutableArray is an NSArray, the following assignment is valid:

NSArray *myArray = [[NSMutableArray alloc] init];

However, since NSArray is not necessarily an NSMutableArray, the following assignment is invalid:

NSMutableArray *myArray = [[NSArray alloc] init];

EDIT From the language point of view, both assignments are valid: the code is going to compile, and may even run if you steer clear of B‘s methods not also supported by A, thanks to the dynamic method dispatch mechanism of Objective C. But the compiler can no longer validate the code that involves the variable, and tell you of other potential problems.