Inheriting instance variables in Objective-c

Question

In Objective-c 2.0 why do subclasses need to reference instance variables in parent classes using the self keyword?

Consider this example:

// a.h
@interface MyClass : NSObject
@property (nonatomic, retain) Object *myObject;
@end

// a.m
@implementation MyClass
@synthesize myObject;
@end


// b.h
@interface AnotherClass : MyClass
@end

// b.m
@implementation AnotherClass
- (void) someMethod {
    // error
    // Object *obj = myObject;

    // works
    // Object *obj = self.myObject;
}
@end

Answer 1

You haven't actually defined a variable, you only defined a property (which implicitly defines a variable that is private). And since property are just method, you need the dot syntax. Note that self.property is the same as [self property].

To fix this, specify a variable. I'll give you an example where the variable has a different name than the property. Most people chose the same name for both but I like to have them differ so I immediately see which one is meant.

// a.h
@interface MyClass : NSObject {
    // Instance variables are "protected" by default, except if you
    // use @private or @public.
    Object *myObjectVar;
}

@property (nonatomic, retain) Object *myObject;
@end

// a.m
@implementation MyClass
@synthesize myObject = myObjectVar;
@end


// b.h
@interface AnotherClass : MyClass
@end

// b.m
@implementation AnotherClass
- (void) someMethod {
    // works
    Object *obj = myObjectVar;

    // works
    obj = self.myObject;

    // the same as self.myObject
    obj = [self myObject];
}
@end

Note the difference when you assign: if you assign to your variable the object is not retained automatically. But it is retained if you use the property:

myObjectVar = someObject; // not retained, old object not released!
self.myObject = someObject; // old object released, new object retained
[self setMyObject:someObject]; // same as the line above

Edit: Mentioned that the synthesized instance variables are private by default, as noted by @Jason Coco. And @NSGod is right that normal instance variables are protected by default rather than public, fixed that.