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".

enum OTHER, OPERATION
 
function operation(object o)
    integer rid = o[OPERATION]
    if rid!=NULL then
        return call_func(rid,{})
    end if
    return "no implementation"
end function
 
function xthing()
    return "default implementation"
end function
 
function newX()
    return {1,routine_id("xthing"),2}
end function
 
function newY()
    object res = newX()
    res[OTHER] = "something else"
    -- remove delegate:
    res[OPERATION] = NULL
    return res
end function
 
function zthing()
    return "delegate implementation"
end function
 
function newZ()
    object res = newX()
    -- replace delegate:
    res[OPERATION] = routine_id("zthing")
    return res
end function
 
object x = newX(),
       y = newY(),
       z = newZ()
 
?operation(x)
?operation(y)
?operation(z)