How to derive a class from an interface

In the first example we derive a class directly from FUnknown, using some of the helper macros provided by the SDK.

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 Steinberg::IPluginBase,
            public Steinberg::IPlugController,
            public Steinberg::IEditorFactory
{
public:
    DECLARE_FUNKNOWN_METHODS

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

IMPLEMENT_REFCOUNT (CMyMultiClass) // implements reference counting

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