Question

As a C#/Java Programmer I’m having a hard time getting the following problem solved:

There is a base class “B”. In its init Method it calls a method “SetupStuff”.
For the base class this method is just empty.

Then there is a derived class “D” that inherits from “B”.
D implements the method “SetupStuff”, too (and actually does something there).

The problem is: When I create an object of D, its “SetupStuff” is never called. The init method of B is called, then the empty “SetupStuff” of is called.

What would I need to do to make the derived class’s method being called?

Answer 1

If you are trying to invoke an override from inside your initializer, it is not going to work. The reason for it is easy to understand: since the override belongs to a subclass, and because the superclass instance initialization needs to be complete before the subclass initialization can start, calling a derived method would have violated the rules that by the time a “regular” method is called the initialization of the instance has completed. Generally, calling virtuals from Java or C# constructors is not a good idea, for the same exact reason. In C++, calling virtuals from a constructor redirects to the implementation in the cosntructor’s own class (i.e. the same thing that you observe in Objective C).

Unlike C# and Java where overriding static methods is not allowed, Objective C lets you provide class-specific implementations of class methods. You can use this mechanism to achieve what you are trying to do: define a class method in the derived class, and call it from the base class, like this:

@interface TT : NSObject
-(id)init;
@end

@interface Test1 : TT
+(void)doit;
@end

@interface Test2 : TT
+(void)doit;
@end

@implementation Test1
+(void) doit {
    NSLog(@"Test1");
}
@end

@implementation Test2
+(void) doit {
    NSLog(@"Test2");
}
@end

@implementation TT
-(id) init {
    if (self=[super init]) {
        // The "magic" is in the following line:
        [self->isa doit];
    }
    return self;
}
@end

When you call

[[Test1 alloc] init];
[[Test2 alloc] init];

you see

Test1
Test2

in the log.