qt signal in a Plugin base class

[trallallero]

I have created a GUI with a plugin system.
There's a PluginInterface abstract class:

class PluginInterface
{
   virtual methods ...
}

class PluginInterface
{
   virtual methods ...
}

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A plugin has a class like this:

class TextValidatorPlugin : public QObject, public PluginInterface
{
   Q_OBJECT
   Q_INTERFACES(DB_PluginSystem::PluginInterface)
   ...
 
   signals:
      void PluginSaveSignal(const QString& pluginName, const QString& config);
}

class TextValidatorPlugin : public QObject, public PluginInterface
{
   Q_OBJECT
   Q_INTERFACES(DB_PluginSystem::PluginInterface)
   ...

   signals:
      void PluginSaveSignal(const QString& pluginName, const QString& config);
}

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Now the problem is in the loader:

QPluginLoader loader(m_PluginsDir.absoluteFilePath(fileName));
 
         QObject *plugin = loader.instance();
         if (plugin)
         {
            PluginInterface* pi = qobject_cast<PluginInterface*>(plugin);
            if (pi)
            {
               pi->SetParent(m_Parent);
               pi->SetMenu  (pluginMenu);
               ...
               connect(pi  , SIGNAL(PluginSaveSignal(const QString&, const QString&)),
                       this, SLOT  (PluginSaveSlot  (const QString&, const QString&)));

QPluginLoader loader(m_PluginsDir.absoluteFilePath(fileName));

         QObject *plugin = loader.instance();
         if (plugin)
         {
            PluginInterface* pi = qobject_cast<PluginInterface*>(plugin);
            if (pi)
            {
               pi->SetParent(m_Parent);
               pi->SetMenu  (pluginMenu);
               ...
               connect(pi  , SIGNAL(PluginSaveSignal(const QString&, const QString&)),
                       this, SLOT  (PluginSaveSlot  (const QString&, const QString&)));

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The connect of course fails because the plugin loader knows only the base class (the interface) and not the class of the plugin.
But I cannot put the signal in the base class as it's not inherited from QObject.
And I cannot derive the plugin base class from QObject because the plugin classes (those derived from the plugin base class) already inherit from QObject...

is there as solution for this ?

---

Added after 11 minutes:

Ok, solved by my self.
Stupid me, I don't have to use PluginInterface* pi pointer but the QObject* plugin one in the connect.

---

Added after 1 39 minutes:

But there's still a problem:

The plugin loader assumes that all the plugins have a signal

void PluginSaveSignal(const QString&, const QString&)

void PluginSaveSignal(const QString&, const QString&)

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but I cannot force who writes the plugins to implement it.

I know it sounds weird, but is it possible somehow to create a signal pure virtual ?
Like a:

void PluginSaveSignal(const QString&, const QString&) = 0;

void PluginSaveSignal(const QString&, const QString&) = 0;

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