This class implements IUnknown for an aggregated or nonaggregated object.
template< class contained > class CComPolyObject : public IUnknown, public CComObjectRootEx < contained::_ThreadModel::ThreadModelNoCS >
- Your class, derived from CComObjectRoot or CComObjectRootEx, as well as from any other interfaces you want to support on the object.
CComPolyObject implements IUnknown for an aggregated or nonaggregated object.
When an instance of CComPolyObject is created, the value of the outer unknown is checked. If it is NULL, IUnknown is implemented for a nonaggregated object. If the outer unknown is not NULL, IUnknown is implemented for an aggregated object.
The advantage of using CComPolyObject is that you avoid having both CComAggObject and CComObject in your module to handle the aggregated and nonaggregated cases. A single CComPolyObject object handles both cases. This means only one copy of the vtable and one copy of the functions exist in your module. If your vtable is large, this can substantially decrease your module size. However, if your vtable is small, using CComPolyObject can result in a slightly larger module size because it is not optimized for an aggregated or nonaggregated object, as are CComAggObject and CComObject.
If the DECLARE_POLY_AGGREGATABLE macro is specified in your object's class definition, CComPolyObject will be used to create your object. DECLARE_POLY_AGGREGATABLE will automatically be declared if you use the ATL Project Wizard to create a full control or Internet Explorer control.
If aggregated, the CComPolyObject object has its own IUnknown, separate from the outer object's IUnknown, and maintains its own reference count. CComPolyObject uses CComContainedObject to delegate to the outer unknown.
For more information about aggregation, see the article Fundamentals of ATL COM Objects.