Question

In Objective-C, why can’t we use

@interface Animal : NSObject {
    int state = AnimalStateAlive;   // a constant which is 1 to indicate alive
    int x = 0;
    int y = 0;
}

@properties int energyLevel = 100;

and the compiler can fill in those values right after the alloc has happened?

Answer 1

Because there’s no such thing as a constructor in objective-C. +alloc is a method implemented by NSObject specifically, and the compiler has no idea when/how it is called. To provide initialization, the runtime (class_createInstance(), to be precise) zeroes out the whole instance upon creation, and in fact, this is what is implemented in +alloc.