Dealing with IDispatch parameter and COM

Question

I have a function that return a pointer to an interface pointer via a paramater (project) :

CreateProject(std::string str, IDispatch** project);

Given two other Interface that implement IDispatch : A and B, is the following code legit given that the real type of project is A. ( I am trying to work with COM VCProjectEngine.CreateProject)

A** a;
B** b;

CreateProject("test.vcxproj", a); //should work but I don't know why
CreateProject("test.vcxproj", b); //should not work but I don't know why

Can someone explain me how this kind of thing is suppose to work ? I am sorry I am a little bit new with COM objects.

Roman R. Roman R. · Accepted Answer · 2014-03-09T14:13:19

IDispatch** project argument typically assumes that you pass a pointer to IDispatch* variable, which is to be filled with actual interface pointer:

IDispatch* pDispatch;
pDispatch = NULL; // Sanity, optional
CreateProject("test.vcxproj", &pDispatch);
assert(pDispatch != NULL); // Filled by call above
// ...
pDispatch->Release();

Since dealing COM interface pointers make you care about proper reference counting, you typically want to use wrapper classes, instead of raw pointers:

CComPtr<IDispatch> pDispatch;
CreateProject("test.vcxproj", &pDispatch);
ATLASSERT(pDispatch != NULL);

Read up on CComPtr on MSDN.

Dealing with IDispatch parameter and COM

1 Answers