c# - Casting COM types in C++ -

i have following com types: project, containeritem , node. project has collection property append function accepts containeritems.

in c#, using type libraries can send node object append function , library works expected:

var prj = new project(); var node = new node(); prj.collection.append(node); 

in c++ tried direct pointer cast expecting c# doing, ends in error:

projectptr prj; prj.createinstance(__uuidof(project)); nodeptr node; node.createinstance(__uuidof(node)); prj->collection->append((containeritem**)&node.getinterfaceptr()); 

is there specific way these type of com pointer casts in c++? missing?

com casting done queryinterface() method. object queried support of interface (based on guid), , if interface supported, internal reference counter incremented (see addref()) , pointer interface returned. msdn has more detail on inner workings.

c++ not directly support code generation "com cast" c# does, implementation simple enough.

struct bad_com_cast : std::runtime_error {     bad_com_cast() : std::runtime_error("com interface not supported") {} };  template <class to, class from> to* qi_cast(from* iunknown) {     hresult hr  = s_ok;     to* ptr = null;     if (iunknown) {         hr = iunknown->queryinterface(__uuidof(to), (void**)(&ptr));         if (hr != s_ok) {             throw bad_com_cast(); // or return null         }     }     return ptr; } 

using above "cast", sample can implemented follows;

containeritem* ci = qi_cast<containeritem>(node); prj->collection->append(&ci); 

if atl library being used, can use atl::ccomqiptr<> directly obtain equivalent semantics;

auto ci = ccomqiptr<containeritem>(node); if (ci) {     // ... }