Decorator

Decorator: wrap an object to add behavior

Problem: you want to add responsibilities to a single object without subclassing or editing its class.

PawWalk scenario: a base walk price, then optional GPS and insurance surcharges. Wrap the price object; the wrapper's total adds its surcharge to the wrapped total.

Ruby's SimpleDelegator forwards everything to the wrapped object so you override only the method you're changing — but a plain wrapper class works just as well.