struct MyInteface {
  Q_GADGET
  QML_INTERFACE

public:
  virtual ~MyInterface();
  virtual int getAnswer() const = 0;
};

Q_DECLARE_INTERFACE(MyInterface, "com.mycompany.MyInterface")

class MyConsumer : public QObject {
  Q_OBJECT
  Q_PROPERTY(MyInterface* i READ i WRITE setI NOTIFY iChanged FINAL)

public:
  using QObject::QObject;

  MyInterface* i() const { return m_interface; }
  void setI(MyInterface* i) { 
    if (i == m_interface) return;
    m_interface = i;
    emit iChanged();
  }

signals:
  void iChanged();

private:
 MyInterface* m_interface{nullptr};
}

class MyImplementation : public QObject, public MyInterface {
  Q_OBJECT
  QML_UNCREATABLE("Created in C++")
  QML_IMPLEMENTS_INTERFACES(MyInterface)

public:
  using QObject::QObject;

  int getAnswer() const override { return 42; }
};

class MyProvider : public QObject {
  Q_OBJECT
  QML_ELEMENT
 
  Q_PROPERTY(MyImplementation* impl READ impl NOTIFY implChanged FINAL)

public:
  using QObject::QObject;

  MyImplementation* impl() const { return m_impl; }

signals:
  void implChanged();

private:
  MyImplementation* m_impl{nullptr}; // created later
};

ApplicationWindow {
  // some config
  MyProvider { id: provider }
  MyConsumer { i: provider.impl } // prints warning: Unable to assign null to MyInterface*
}