Objective-c class create methods

Question

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 class 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 ?

zoul · Accepted Answer · 2011-02-18 09:29:39Z

2

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.

edited Feb 18, 2011 at 9:29

answered Feb 18, 2011 at 9:23

zoul

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3 Comments

Andrew Over a year ago

Damn, i've written return [[[Base alloc] init] autorelease]; in instance method in my code. Thanks a lot!

Andrew Over a year ago

Can you explain how can i use self word in a class method. What does it mean?

zoul Over a year ago

“Within the [method] implementation, both self and super refer to the receiving object,” see Language Summary in the Objective-C book by Apple.

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