Question

I can understand defining the functions in the @interface of the header file, but why the instance variables? Shouldn’t the instance variables be private, only accessible through messages?

Answer 1

The reason is so it can calculate offsets of variables for subclasses.

@interface Bird : NSObject {
    int wingspan;
}
@end
@interface Penguin : Bird {
    NSPoint nestLocation;
    Penguin *mate;
}
@end

Without knowing the structure of the “Bird” class, the “Penguin” class can’t calculate the offset of its fields from the beginning of the structure. The penguin structure looks kind of like this:

struct Penguin {
    int refcount; // from NSObject
    int wingspan; // from Bird
    NSPoint nestLocation; // from Penguin
    Penguin *mate; // from Penguin
}

This has a side effect: if you change the size of a class in a library, you break all the subclasses in apps that link to that library. The new properties work around this problem.