How to use SIGNAL and SLOT without deriving from QObject?

Question

OR other way to formulate my question (though it didn't solve my problem): 'QObject::QObject' cannot access private member declared in class 'QObject'

I need SIGNALs and SLOTS functionality in my class, but I assume it is not possible without to derive from QObject?

class MyClass
{
signals:
   importantSignal();
public slots:
   importantSlot();
};

The Problem seems to be that I need to derive from QObject to use signals and slots ... but I need the default contructor of MyClass. But I can't construct them because of the following feature of QObject: No Copy Constructor or Assignment Operator.

I tried a lot ...

So my shoul Class look like that:

#include <QObject>
class MyClass: public QObject
{
    Q_OBJECT
public:
    explicit MyClass(QObject *parent = 0); //autogenerated by qtcreator for QObject derived class
    MyClass(const MyClass * other);

signals:
    importantSignal();
public slots:
    importantSlot();
};

I need the default contructor of MyClass.

So is there any possibility do avoid the "'QObject::QObject' cannot access private member declared in class 'QObject'" error?

Or as an alternative is there any possibility to use signals and slots without QObject?

I'm glad for any advice.

Answer 1

If you want a copyable object with QObject features you need membership (by pointer) rather than inheritence.

You can derive a class Handler from QObject where Handler's slots call SomeInterface virtual functions on its parent.

struct NonQObjectHandler {
    virtual ~ NonQObjectHandler () {}
    virtual void receive (int, float) = 0;
};

class Handler : public NonQObjectHandler {
    struct Receiver;
    std :: unique_ptr <Receiver> m_receiver;
    void receive (int, float); // NonQObjectHandler
public:
    Handler ();
    Handler (const Handler &); // This is what you're really after
};

class Handler :: Receiver : public QObject {
Q_OBJECT
private:
    NonQObjectHandler * m_handler;
private slots:
    void receive (int, float); // Calls m_handler's receive
public:
    Receiver (NonQObjectHandler *);
};

Handler :: Handler ()
: m_receiver (new Receiver (this))
{
}

Handler :: Handler (const Handler & old)
: m_receiver (new Receiver (this))
{
    // Copy over any extra state variables also, but
    // Receiver is created anew.
}

Handler :: Receiver :: Receiver (NonQObjectHandler * h)
: m_handler (h)
{
    connect (foo, SIGNAL (bar (int, float)), this, SLOT (receive (int, float)));
}

void Handler :: Receiver :: receive (int i, float f)
{
    m_handler -> receive (i, f);
}

Answer 2

If you want to implement event-driven functionality using a signals/slots pattern, but do not want to work inside the confines of Qt (i.e., you want to use your class inside of STL containers, etc. that require copy-constructors), I would suggest using Boost::signal.

Otherwise, no, you can't possibly do what you're wanting without deriving from QObject since that base class is what handles the Qt runtime signals/slots functionality.