Loading an interface fails - c++

Question

I'd like to use a COM function : CreateInstance
http://msdn.microsoft.com/en-us/library/k2cy7zfz%28v=vs.80%29.aspx
like this
IPointer p=NULL;
HRESULT hr=p.CreateInstance(xxx);
However I don't have CLSID of xxx I only know its interface name ISubPointer
I can see its interface description inside the tlb file when I view the file with oleview. What should I do to use that CreateInstance ?

Answer 1

There are two ways to do this:
1st: a ClassFactory ,and
2nd: a helper function to create a pointer.
I found this:
int main()
{
IMath* pIMath;
HRESULT hr;
// 1. Initialize COM Library
CoInitialize(NULL);
// 2. Call CoCreateInstance to get the IMath interface pointer
hr = CoCreateInstance ( __uuidof(CMathComp), NULL, CLSCTX_INPROC_SERVER,
__uuidof(IMath), (void**) &pIMath );
if ( FAILED(hr) )
{
return 0;
}
// 3. Call the interface functions
int sum = pIMath->Add(1, 3);
printf("Sum = %d \n", sum);
int sub = pIMath->Sub(4, 3);
printf("Sub = %d \n", sub);
// 4. Release the interface pointer if you are done
pIMath->Release();
// 5. Un-Initialize COM Library
CoUninitialize();
return 0;
}
Also see MSDN:
HRESULT CoCreateInstance(
_In_ REFCLSID rclsid,
_In_ LPUNKNOWN pUnkOuter,
_In_ DWORD dwClsContext,
_In_ REFIID riid,
_Out_ LPVOID *ppv
);
If you can gather the CLSID from OLEVIEW use it, otherwise there must be documentation about this. You can't deliver a component without exposing ist CLSID.

Answer 2

You have a couple of options for obtaining the class ID of the object you want to create. You can use the OLE Viewer to generate the header files or you can directly import the type library into your source file using the #import directive. The CreateInstance function that you refer to is a non-static member of _com_ptr_t and requires you to use an instance of it.
The following example should get you on your way.
#include <comip.h> // _com_ptr_t
#import "tlbname.tlb" // Change to the name of your type library
int main()
{
CoInitialize(NULL);
::_com_ptr_t<ISubPointer> ptr;
// CoISubPointer is the class ID specified in the type library
// you will need to change the name accordingly.
ptr.CreateInstance(__uuid(CoISubPointer), NULL, CLSCTX_INPROC_SERVER);
CoUninitialize();
return 0;
}
When main() has finished ptr will automatically release it's reference to the ISubPointer object it holds.

Answer 3

You can't create COM object without knowing class ID. I suggest to read the basic of COM in this article http://www.codeproject.com/Articles/633/Introduction-to-COM-What-It-Is-and-How-to-Use-It