How to define implementation of a COM coclass?

Question

I am new to COM, and I looked around on the internet to find how to create a COM class in C++ (to be used by C#). I've seen that in the .idl file, I must put something like this:

[
object,
uuid(a93164ee-e6d4-44be-aa27-f00ce6972719),
helpstring("interface ITopologyQuery")
]

interface ITopologyQuery : IUnknown
{
  HRESULT LoadFile(BSTR completeFileName);

  HRESULT SetParameters(int above, int percentage);
}

[
  uuid(a958f2af-8b55-43c4-9fc3-c39d83fc1376)
]
library TopologyQueryLib
{
  [
    uuid(cc814992-31ec-4a1f-a41e-111ade27bdfe),
    helpstring("TopologyQuery class")
  ]
  coclass CTopologyQuery
  {
    [default] interface ITopologyQuery;
  };
}

Now my question is, where do you define the CTopologyQuery class? If I define it as a regular c++ class in another file, will the compiler link my class correctly with the interface? The c++ class code looks like this (it is implemented in a .cpp file):

class CTopologyQuery : public ITopologyQuery
{
  public:

    __stdcall CTopologyQuery();

    //IUnknown interface 
    HRESULT __stdcall QueryInterface(REFIID riid, void **ppObj);
    ULONG   __stdcall AddRef();
    ULONG   __stdcall Release();

    //ITopologyQuery interface
    HRESULT __stdcall LoadTopologyFile(BSTR completeFileName);
    HRESULT __stdcall SetParameters(int above, int percentage);

  private:
};

For now it compiles either if I put the library section in the .idl file or if I don't. I am a little lost as what is the good thing to do here? I understand that the coclass definition is supposed to give a default implementation to the interface, but for me it looks like an empty class with no methods...

Answer 1

The IDL (Interface Definition Language) part defines the binary interface in a language-independent way that just looks a lot like C++.

It's generally used to build a binary type library, which is a DLL with info about your COM class. You can use Microsoft's COM/OLE Viewer to browse existing type libraries.

For C++ consumption of your non-IDispatch COM class you don't need that info about your class, since all that the C++ compiler needs to know (as minimum) is in your C++ header file.

But the IDL description and type library generated from that helps to generate the glue code for using your COM class from C^# and other languages. As I recall it can also help to generate the Windows registry keys and values needed to use your COM class via CoCreateInstance and friends. So it's a bit of infra-structure machinery, so to speak, essentially supporting a language-independent view and usage of your COM class.

Cheers & hth.,