Difference between CComPtr and CComQIPtr?

CComQIPtr is derived from CComPtr. QI stands for Query Interface. When you construct with a CComQIPtr it can result in nullptr., which means the cast has not worked. This is useful to check the type of the pointer. For example:

   CComQIPtr<PartDocument> pPartDoc(pDoc) ;

   if (pPartDoc == nullptr) {

       // pDoc did not point to a PartDocument

       return (false) ;

   }

...and another example...

   CComQIPtr<AssemblyDocument> pAsmDoc(pDoc);

   if (pAsmDoc == nullptr) {

       // pDoc did not point to an AssemblyDocument

       return (nullptr) ;

   }

.