Calling method on child class from parent class method (Objective-c 2.0)

Question

I have experience with object-oriented programming however this situation is unfamiliar for some reason. Consider the following Objective-c 2.0 code:

@interface A : NSObject
@end

@implementation A
- (void) f {
    [self g];
}
@end

@interface B : A
@end

@implementation B
- (void) g {
    NSLog(@"called g...");
}
@end

Is this the correct way to call a method on a child class from a method in the parent class? What happens if another child class doesn't implement method g? Perhaps there's a better way to solve this like an abstract method in the parent class A?

Answer 1

The key is to have a method in the parent class that may be overriden in the child class.

@interface Dog : NSObject
- (void) bark;
@end

@implementation Dog
- (void) bark {
    NSLog(@"Ruff!");
}
@end

@interface Chihuahua : Dog
@end

@implementation Chihuahua
- (void) bark {
    NSLog(@"Yipe! Yipe! Yipe!");
}
@end

You see, your child class will override the parent method with its own implementation. You might see it used like this:

Dog *someDog = [Chihuahua alloc] init] autorelease];
[someDog bark];

Output: Yipe! Yipe! Yipe!

Answer 2

You should implement g in the parent class, but make it do nothing. This way it can be called without error, but still can be overridden.

@interface A : NSObject
@end

@implementation A
- (void) f {
    [self g];
}
- (void) g {} // Does nothing in baseclass
@end

@interface B : A
@end

@implementation B
- (void) g {
    NSLog(@"called g...");
}
@end

Or you check for the method on the object before executing it.

if ([self respondsToSelector:@selector(g)]) {
  [self performSelector:@selector(g) withObject:nil];
}

But that can get pretty ugly.