Basically, you should import your function declarations. However, we does not provide an importer for C++, the importer is available only for Delphi (Pascal).
So, the only way is to write Delphi (*.pas) unit with abstract base classes, and include that unit into your C++ project. Then, declare a descendant of these classes and override abstract methods in C++ code. Initial Delphi unit must be imported, using ScriptPack importer.
All this is shown in the ObjectModel C++ demo. Also, please read ScriptPack documentation to learn about importing units.
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http://stackoverflow.com/questions/7620282/call-and-execute-a-function-on-a-dll-using-vbscript