Direct base 'QObject' is inaccessible due to ambiguity

fem_dev

I'm developing a shared library (plugin) to my Qt C++ application.
Following many tutorials (like this) in the internet, I found that:
a) The plugin interface should inherit QObject class
b) The plugin interface MUST HAVE the Q_OBJECT macro to enable to use the same signals and slots in all developed plugins.

Here is my plugin interface: (No warnings)

#ifndef PLUGIN_API_H
#define PLUGIN_API_H

#include <QtPlugin>
#include <QString>

class Plugin_API : public QObject
{
    Q_OBJECT

public:
    virtual ~Plugin_API() = default;
    virtual void showUI(void) = 0;
    virtual void test(void) = 0;

signals:
    void sendString(QString string);
};

Q_DECLARE_INTERFACE(Plugin_API, "com.lamar.plugin")

#endif // PLUGIN_API_H

Here is my plugin class.

#ifndef MYLIB_H
#define MYLIB_H

#include <QObject>
#include <QString>

#include "myLib_global.h"
#include "plugin_api.h"

class MYLIB_EXPORT MyLib : public QObject, public Plugin_API
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "com.lamar.plugin")
    Q_INTERFACES(Plugin_API)

public:
    explicit MyLib(QObject* parent = nullptr);

    void test() override;
    void showUI() override;
};

#endif // MYLIB_H

In my plugin class myLib I got this warning message:

Direct base 'QObject' is inaccessible due to ambiguity

If I remove the QObject inheritance, I can't use the Q_OBJECT macro.
If I don't have both, I can't use MyLib(QObject* parent = nullptr);

So, what is the right way to do that?

Bonnie

You don't need to inherit QObject in MyLib because Plugin_API already has.
So MyLib already inherits QObject even without that "public QObject".
Just like when we subclass QWidget, we don't need to inherit QObject in our code, but the new class can use Q_OBJECT macro and signals/slots, because QWidget inherits QObject.