Immutable aggregation of interfaces implemented by several objects, with dynamic discovery via QueryInterface following ComponentObjectModel specification, with consistent ReferenceCounting (_AddRef, _Release).

Aggregation requires a WeakReference to the aggregated interfaces.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/com/com_9xym.asp
http://www.codeguru.com/atl/containmentAndAggregation.html

see

ComAggregation
InterfaceImplementationByDelegation