One approach to reusing code with interfaces is to have one interfaced object contain, or be contained by another. Using properties that are object types provides an approach to containment and code reuse. To support this design for interfaces, the Delphi language has a keyword implements, that makes if easy to write code to delegate all or part of the implementation of an interface to a subobject. 

Aggregation is another way of reusing code through containment and delegation. In aggregation, an outer object uses an inner object that implements interfaces which are exposed only by the outer object.