Plugin interface with slots and signals

[folibis]

Now I work on application with plugins.
So I have one interface and lots of plugins based on it:

class PluginWidgetInterface
{
public:
    virtual ~PluginWidgetInterface() {}
    virtual QWidget * GetWidget() = 0;
    //---------------------------- SIGNALS ---------------------------
    virtual void MessageSend() = 0;
};
 
#define PluginWidgetInterface_iid "myplugins.PluginWidgetInterface"
Q_DECLARE_INTERFACE(PluginWidgetInterface, PluginWidgetInterface_iid)

class PluginWidgetInterface
{
public:
    virtual ~PluginWidgetInterface() {}
    virtual QWidget * GetWidget() = 0;
    //---------------------------- SIGNALS ---------------------------
    virtual void MessageSend() = 0;
};

#define PluginWidgetInterface_iid "myplugins.PluginWidgetInterface"
Q_DECLARE_INTERFACE(PluginWidgetInterface, PluginWidgetInterface_iid)

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and plugin implementation:

#include "plugininterface.h"
 
class Plugin1 : public QObject,PluginWidgetInterface
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "myplugins.Plugin1")
    Q_INTERFACES(PluginWidgetInterface)
public:
    ~Plugin1() {}
    QWidget * GetWidget();
    //---------------------------- SIGNALS ---------------------------
    void MessageSend();
};

#include "plugininterface.h"

class Plugin1 : public QObject,PluginWidgetInterface
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "myplugins.Plugin1")
    Q_INTERFACES(PluginWidgetInterface)
public:
    ~Plugin1() {}
    QWidget * GetWidget();
    //---------------------------- SIGNALS ---------------------------
    void MessageSend();
};

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but at this point i have troubles with connect(). Because the Interface class not derived from QObject I cant use it with connect();
If i cast it to QObject i get 0x0.

QPluginLoader loader("plugin.dll");
PluginWidgetInterface * plugin = qobject_cast<PluginWidgetInterface *>(loader.instance());
QWidget * widget = plugin->GetWidget();
QObject * o = dynamic_cast<QObject *>(plugin); // here o = NULL
connect(o,SIGNAL(MessageSend),this,SLOT(MessageReceived));

QPluginLoader loader("plugin.dll");
PluginWidgetInterface * plugin = qobject_cast<PluginWidgetInterface *>(loader.instance());
QWidget * widget = plugin->GetWidget();
QObject * o = dynamic_cast<QObject *>(plugin); // here o = NULL
connect(o,SIGNAL(MessageSend),this,SLOT(MessageReceived));

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I would like to do it in new style

connect(o,&PluginWidgetInterface::MessageSend,this,&MainWindow::MessageReceived);

connect(o,&PluginWidgetInterface::MessageSend,this,&MainWindow::MessageReceived);

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but i got error:
no matching function for call to 'MainWindow::connect(QObject*&, void (PluginWidgetInterface::*)(), MainWindow* const, void (MainWindow::*)())


So what is right way to define interface with signals to connect inheritance to slots?