Design Principles for Collection and Enumerator Interfaces

Design Principles for Collection and Enumerator Interfaces

There are different design principles behind each type of interface:

A collection interface provides random access to a single item in the collection via the Item method, it lets clients discover how many items are in the collection via the Count property, and often allows clients to add and remove items.

An enumerator interface provides serial access to multiple items in a collection, it doesn't allow the client to discover how many items are in the collection (until the enumerator stops returning items), and it doesn't provide any way of adding or removing items.

Each type of interface plays a different role in providing access to the elements in a collection.

ATL provides the following classes to help you implement collections and enumerators.

Class	Description
ICollectionOnSTLImpl	Collection interface implementation
IEnumOnSTLImpl	Enumerator interface implementation (assumes data stored in an STL-compatible container)
CComEnumImpl	Enumerator interface implementation (assumes data stored in an array)
CComEnumOnSTL	Enumerator object implementation (uses IEnumOnSTLImpl)
CComEnum	Enumerator object implementation (uses CComEnumImpl)
_Copy	Copy policy class
_CopyInterface	Copy policy class
CAdapt	Adapter class (hides operator & allowing CComPtr, CComQIPtr, and CComBSTR to be stored in STL containers)