C / C++ / MFC

// export:
// header included in DLL files. Here's the declaration of your class
class __declspec(dllexport) CYourClass
{
public:
   int m_iSomeInt;
   void SomeMethod();
   ....
};

// cpp file included with DLL files. Here's the implementation of your class
void CYourClass::SomeMethod()
{
...
}

// put YourClass.DLL in the lib path in project settings
// include YourClass.h in your program

// YourClass.h:
class __declspec(dllimport) CYourClass
{
public:
   int m_iSomeInt;
   void SomeMethod();
   ....
};

// SomeOtherClass.h:
#include YourClass.h
class CSomeOtherClass : public CYourClass
{
.....
};