CODEWITHSUNDEEP
objective-c
Andrew
Andrew

Reputation: 24846

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 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 ?

Upvotes: 2

Views: 266

Answers (1)

zoul
zoul

Reputation: 104065

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.

Upvotes: 2

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