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Topic created on: February 1, 2010 23:28 CST by ronnie291983 .

Hi,

Had a question regarding COM,

When we create an instance of interface using CoCreateInstance then the pointer returned by this API is suppose "x".

CoCreateInstance(target, NULL, flags, IID_IDispatch, tmpObj)

where "x" is "tmpobj"

Does X point to virtual function table?

does *(x) point to QueryInterface ; and
*(x+4) point to AddRef;
etc......

Is this assumption correct?

Can't find any resources for this. Appreciate any kind of help.

detlef		February 2, 2010 02:51.23 CST
Hi, tmpObj is a pointer. Hence, *(PVOID)x points to the vtable. Based upon the assumptions that x would be the vtable itself the other assumptions were right.

lallous		February 2, 2010 05:18.50 CST
This article can help you: http://www.codeproject.com/KB/COM/com_in_c1.aspx

ronnie291983		February 4, 2010 03:21.15 CST
thanks guys for ur inputs

eeewhatup		February 11, 2010 10:46.16 CST
Yes - the ptr returned points directly to the VTable. One thing that is particularly useful is that the VTable will be in the exact order as the methods are displayed on MSDN. So for example if you wanted to know about the IPersistStream interface (http://msdn.microsoft.com/en-us/library/ms690091%28VS.85%29.aspx), you can see that it has the methods isDirty(), Load(), Save(), and GetSizeMax(). To figure out their offset from the base, you need to track back through all of the classes that IPersistStream inherits. So in this case, IPersist, and IUnknown. From that information, you can construct the VTable: QueryInterface() -- IUnknown AddRef() -- IUnknown Release() -- IUnknown GetClassID() -- IPersist isDirty() -- IPersistStream Load() -- IPersistStream Save() -- IPersistStream GetSizeMax() -- IPersistStream Hope that all made sense.

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