A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

 Keep an optional delegate instance.
 Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:

 Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

a = new()
a.f = method(){
        .x.print()
}
 
c = new()
c.g = method(){
        (.x + 1).print()
}
 
# array of delegates
b = new()
b.delegate = new()
b.delegate[0] = a
b.delegate[1] = c
b.x = 3
b.f()
b.g()
 
# single delegate
d = new()
d.delegate = a
d.x = 7
d.f()