Objective-c class create methods

Let's say we have a class Base.

@inerface Base : NSObject
{
}
+(id) instance;
@end

@implementation Base
+(id) instance
{
    return [[[self alloc] init] autorelease];
}
-(id) init
{
...
}
@end

And we have a derived class Derived.

@interface Derived : Base
{
}
@end

Which reimplements the init method.

Now we want to create an instance of Derived class using class method +(id) instance.

id foo = [Derived instance];

And now foo is actually a Base class.

How to achieve foo to be a Derived cla开发者_高级运维ss in this case?

Should I reimplement all the class method for derived classes ? (actually immplementation will be totally the same).

Is there a more elegant way ?

When you create an instance using [Derived instance], that instance will be of class Derived. Try it. The trick is messaging self in the instance method:

+(id) instance
{
    return [[[self alloc] init] autorelease];
}

When you send the instance message to Base, self is Base. When you send the same message to Derived, self is Derived and therefore the whole thing works as desirable.