Modifying the Inheritance of RCustomRowset
To add the IRowsetLocate interface to the simple read-only provider example, modify the inheritance of CCustomRowset. Initially, CCustomRowset inherits from CRowsetImpl. You need to modify it to inherit from CRowsetBaseImpl.
To do this, create a new class, CCustomRowsetImpl, in CustomRS.h:
////////////////////////////////////////////////////////////////////////
// CustomRS.h
template <class T, class Storage, class CreatorClass, class ArrayType = CAtlArray<Storage>>
class CMyRowsetImpl:
public CRowsetImpl<T, Storage, CreatorClass, ArrayType, CSimpleRow, IRowsetLocateImpl< T, IRowsetLocate >>
{
...
};
Now, edit the COM interface map in CustomRS.h to be as follows:
BEGIN_COM_MAP(CMyRowsetImpl)
COM_INTERFACE_ENTRY(IRowsetLocate)
COM_INTERFACE_ENTRY_CHAIN(_RowsetBaseClass)
END_COM_MAP()
This code creates a COM interface map that tells CMyRowsetImpl to call QueryInterface for both the IRowset and IRowsetLocate interfaces. To get all of the implementation for the other rowset classes, the map links the CMyRowsetImpl class back to the CRowsetBaseImpl class defined by the OLE DB Templates; the map uses the COM_INTERFACE_ENTRY_CHAIN macro, which tells OLE DB templates to scan the COM map in CRowsetBaseImpl in response to a QueryInterface call.
Finally, link CCustomRowset to CMyRowsetBaseImpl by modifying CCustomRowset to inherit from CMyRowsetImpl, as follows:
class CCustomRowset : public CMyRowsetImpl<CCustomRowset, CCustomWindowsFile, CCustomCommand>
CCustomRowset can now use the IRowsetLocate interface while taking advantage of the rest of the implementation for the rowset class.
When this is done, you can dynamically determine columns returned to the consumer.