VST SDK 3.0: HOWTO derive a class from an interface

class CMyClass: public FUnknown
{
public:
        CMyClass ();
        virtual ~CMyClass ();

        DECLARE_FUNKNOWN_METHODS  // declares queryInterface, addRef and release
};


CMyClass::CMyClass ()
{
        FUNKNOWN_CTOR // init reference counter, increment global object counter
}

CMyClass::~CMyClass ()
{
        FUNKNOWN_DTOR // decrement global object counter
}

IMPLEMENT_REFCOUNT (CMyClass) // implements reference counting

tresult CMyClass::queryInterface (const char* iid, void** obj)
{
        QUERY_INTERFACE (iid, obj, ::FUnknown::iid, CMyClass)
        return kNoInterface;
}

Developing a class with more than one interface is done by multiple inheritance. Additionally you have to provide an appropriate cast for each interface in the queryInterface method.

class CMyMultiClass:    public IPluginBase,
                        public IPlugController,
                        public IEditorFactory
{
public:
        DECLARE_FUNKNOWN_METHODS

        // declare the methods of all inherited interfaces here...
};


tresult CMyMultiClass::queryInterface (const char* iid, void** obj)
{
        QUERY_INTERFACE (iid, obj, ::FUnknown::iid, IPluginBase)
        QUERY_INTERFACE (iid, obj, IPluginBase::iid, IPluginBase)
        QUERY_INTERFACE (iid, obj, IPlugController::iid, IPlugController)
        QUERY_INTERFACE (iid, obj, IEditorFactory::iid, IEditorFactory)
        *obj = 0;
        return kNoInterface;
}