QuickOPC is exposed to the C/C++ world in form of Microsoft COM objects. The State argument appears as Windows VARIANT structure (msdn.microsoft.com/en-us/library/windows/desktop/ms221627(v=vs.85).aspx ) there. That's all we care about on our side. On the consuming (C/C++) side, the VARIANT can be processed e.g.

1) "as is", with help of Win32 API functions that work with VARIANT-s (e.g. msdn.microsoft.com/en-us/library/windows/desktop/ms221673(v=vs.85).aspx ).

2) Using Microsoft C++ "Native COM support" - which is what you are using if you #import the type library. In this case, VARIANT is wrapped as _variant_t (msdn.microsoft.com/en-us/library/x295h94e.aspx ).

3) Using Microsoft ATL; in this case, one uses the CComVariant class (msdn.microsoft.com/en-us/library/ac97df2h.aspx ). Actually, in some our example, we combine ATL with the native COM support, because ATL allows to write the event hooking code better.

4) and obviously the list is open and other methods can be used as well. For example, I am aware that one of our customers have used a portable library called VOLE which has its own encapsulation for VARIANT-s.

Yes, I suggest to use integers to store the pointers. Besides traditional options such as VT_INT or VT_LONG, there are even pointer-sized VARTYPE-s (see msdn.microsoft.com/en-us/library/windows/desktop/ms221170(v=vs.85).aspx ) such VT_INTPTR, VT_UINTPTR that are properly sized to the bitness of the project. It might be even be possible to use VT_PTR, but we have not tested it.

BTW, are you aware of the SubscribeMultipleItems example project for C++ which comes with the product? It does not have the State passing part, but besides that, it is a good starting point.

Best regards

_variant_t is a C++ wrapper around Windows VARIANT structure, which can hold many different data types, including 32- and 64-bit integers which can represent pointers. QuickOPC, in fact, has no knowledge of the fact that the 'object' in the underlying .NET interface becomes a VARIANT when exported to COM, and then this further is transformed to a _variant_t when you use #import to import the COM type library of QuickOPC into C/C++ (which is just one of the options).

Passing a pointer is therefore a matter of storing it into _variant_t, and then extracting it back. Various _variant_c constructors and other members are described here: msdn.microsoft.com/en-us/library/x295h94e.aspx .

Let me know if you need further help with that.

Best regards