Aggregation in ATL

Can anyone explain, how aggregation is achieved using ATL. I tried to create an aggregated component using ATL. But since the implementation of Query Interface is hidden, i am not sure whether it should be manually overridden or is it done automatically. Also during the initialization of Aggregating object, when i tried the CoCreateInstance of the aggregated object, it gave a compilation error saying
" ambiguous conversions from 'class CWallLibrary *const ' to 'struct IUnknown *'"
Can anyone help me out ??