You don't specify a compiler or operating system, but if this is Windows and Microsoft C++, then you can use __declspec(dllexport) on a class to export it from a DLL. You then use __declspec(dllimport) on the same class when you include the header to be consumed elsewhere.
However, I tend to recommend against exporting classes from DLLs. It's an ok plan if all the DLLs always ship together and are built together. [Where the DLL is just used to delay loading of code]. If you are using DLLs to provide actual components that ship separately, you should use an actual versionable abstraction like COM.
Martyn
