class IPluginlInterface
{
public:
    virtual ~IPluginInterface() {};
    virtual void Init() = 0;
    virtual char* GetInterfaceName() const = 0;
    virtual char* GetInterfaceDescription() const = 0;
    virtual char* GetInterfaceVersion() const = 0;
    virtual char* GetInterfaceDateTime() const = 0;
    virtual int  GetInterfaceCategory() const = 0; 
    virtual int  GetInterfaceFlags() const = 0;
    virtual void SetCallbackNotifyer(INotifyer1* pNotyfier) = 0;
    virtual void SetIXMLConfigPointer(IXMLConfig* pXMLConfig) = 0;
    virtual void SetIDALPointer(IDAL* pIDAL) = 0;
    virtual void SetDataItem1(DataItem1* pItem) = 0; 
    virtual void SetDataItem2(DataItem2* pItem) = 0;
    virtual void SetBackupOptions(int iOptions) = 0;
    virtual INotifyer1* GetCallbackNotifyer() const = 0;
    virtual DataItem1* GetDataItem1() const = 0; 
    virtual DataItem2* GetDataItem2() const = 0;
    virtual long PerformCalculations() = 0;
    virtual void DeInit(int& aiDeInitCounter) = 0;
};

class IPluginlInterface
{
public:
    virtual ~IPluginInterface() {};
    virtual void Init() = 0;
    virtual char* GetInterfaceName() const = 0;
    virtual char* GetInterfaceDescription() const = 0;
    virtual char* GetInterfaceVersion() const = 0;
    virtual char* GetInterfaceDateTime() const = 0;
    virtual int  GetInterfaceCategory() const = 0; 
    virtual int  GetInterfaceFlags() const = 0;
    virtual void SetCallbackNotifyer(INotifyer1* pNotyfier) = 0;
    virtual void SetIXMLConfigPointer(IXMLConfig* pXMLConfig) = 0;
    virtual void SetIDALPointer(IDAL* pIDAL) = 0;
    virtual void SetDataItem1(DataItem1* pItem) = 0; 
    virtual void SetDataItem2(DataItem2* pItem) = 0;
    virtual void SetBackupOptions(int iOptions) = 0;
    virtual INotifyer1* GetCallbackNotifyer() const = 0;
    virtual DataItem1* GetDataItem1() const = 0; 
    virtual DataItem2* GetDataItem2() const = 0;
    virtual long SuspendCalculations() = 0;  // Добавлен этот новый вирт-метод!!!
    virtual long PerformCalculations() = 0;
    virtual void DeInit(int& aiDeInitCounter) = 0;
};

class IPluginlInterface2 : public IPluginlInterface
{
    virtual long SuspendCalculations() = 0;  // Добавлен этот новый вирт-метод!!!
}

class IPluginlInterfaceWrapper : public IPluginlInterface2
{
public:
    IPluginlInterfaceWrapper(IPluginlInterface * old_plugin);

    virtual void Init() { old_plugin->Init(); } 
    virtual char* GetInterfaceDescription() const { return old_plugin->GetInterfaceDescription(); }
    ...
    virtual long SuspendCalculations() { /* Как быть? */}
}

app_runtime rt[1];

int init(queryfn fn) {
  if (fn.query(rt,APP_RUNTIME_VERSION_2_1) return -1;
  rt->set_plugin_info(info);
  rt->create_thread...
  rt->add_menu_item...
  rt->set_event_listner...
  rt->add_resouce...
  ...
  return 0;
}
void done() {
  // release resources used
}

class IPluginlInterface
{
public:
     static int iid() { return 1; }

     virtual IPluginlInterface* GetInterface(int id) const = 0;
 
     template<class IF>  
       IF* Interface() const { return static_cast<IF*>(GetInterface(IF::iid())); }
      

};

class IPluginlInterface2 : IPluginlInterface
{
public:
     static int iid() { return 2; }

     virtual IPluginlInterface* GetInterface(int id) const 
     {
        if( id == iid() ) return this;
        return IPluginlInterface::GetInterface(id);
     }
};

IPluginlInterface* pp = ...;
IPluginlInterface2* pp2 = pp->Interface<IPluginlInterface2>();

CS>class IPluginlInterface
CS>{
CS>public:
CS>     static int iid() { return 1; }

CS>     virtual IPluginlInterface* GetInterface(int id) const = 0;
 
CS>     template<class IF>  
CS>       IF* Interface() const { return static_cast<IF*>(GetInterface(IF::iid())); }
      

CS>};

CS>class IPluginlInterface2 : IPluginlInterface
CS>{
CS>public:
CS>     static int iid() { return 2; }

CS>     virtual IPluginlInterface* GetInterface(int id) const 
CS>     {
CS>        if( id == iid() ) return this;
CS>        return IPluginlInterface::GetInterface(id);
CS>     }
CS>};
CS>

CS>IPluginlInterface* pp = ...;
CS>IPluginlInterface2* pp2 = pp->Interface<IPluginlInterface2>();
CS>

   template<class IF>  
      IF* Interface() const { return static_cast<IF*>(GetInterface(IF::iid())); }

IPluginlInterface* pp = ...;
IPluginlInterface2* pp2 = dynamic_cast<IPluginlInterface2*>(pp);

   Q_PLUGIN_METADATA(IID ...)
   Q_INTERFACES(...)

In Visual C++ ... COM interfaces are guaranteed not to break from version to version.

It is no coincidence that the Win32 COM object layout matches closely the C++ object layout.

A COM interface is nothing more than a named table of function pointers (methods), each of which has documented behavior.

By design, ... a COM interface is binary-compatible with a C++ abstract class.