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".

class Delegator {
    string delegate() hasDelegate;
 
    string operation() {
        if (hasDelegate is null)
            return "Default implementation";
        return hasDelegate();
    }
 
    typeof(this) setDg(string delegate() dg) {
        hasDelegate = dg;
        return this;
    }
}
 
void main() {
    import std.stdio;
    auto dr = new Delegator;
    string delegate() thing = () => "Delegate implementation";
 
    writeln(dr.operation());
    writeln(dr.operation());
    writeln(dr.setDg(thing).operation());
}