C / C++ / MFC

// Correct...
afx_msg LRESULT OnMyMessage(WPARAM wParam, LPARAM lParam);

class CBaseClass : public CObject
{
   DECLARE_DYNAMIC(CBaseClass)
};
// In the source file somewhere:
IMPLEMENT_DYNAMIC(CBaseClass,CObject)
 
class CDerivedClass : public CBaseClass
{
   DECLARE_DYNAMIC(CDerivedClass)
};
// In the source file somewhere:
IMPLEMENT_DYNAMIC(CDerivedClass,CBaseClass)
 
// Somewhere else:
CBaseClass * pBaseClass = new CDerivedClass;
 
// Now, you can pass that pointer to a function, and the function can do this:
if( pBaseClass->IsKindOf( RUNTIME_CLASS( CDerivedClass ) ) )
{
   // Do something, because we know it's a "CDerivedClass" object.
   CDerivedClass * pDerivedClass = static_cast<CDerivedClass*>(pBaseClass);
}
else
{
   // Do something else, with the CBaseClass object.
}