Const-Correctness With Q_PROPERTY

[Sanuden]

I'm trying to develop a class that will be used in a reflection driven application and I ran into an interesting issue that I'm hoping there is a solution to. Let's say I have a class Dummy that has a few properties, one of which is a pointer to another Dummy object:

class Dummy : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int id READ id WRITE setId)
    Q_PROPERTY(QObject * child READ child)
 
    int myId;
    DummyObject *myChild;
 
public:
 
    Dummy(int id)
        : myId(id), myChild(0)
    {
        if (myId == 0)
        {
            myChild = new Dummy(myId + 1);
        }
    }
 
    ~Dummy()
    {
        delete myChild;
    }
 
    int id() const
    {
        return myId;
    }
 
    void setId(int id)
    {
        myId = id;
    }
 
    QObject * child()
    {
        return myChild;
    }
 
};

class Dummy : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int id READ id WRITE setId)
    Q_PROPERTY(QObject * child READ child)

    int myId;
    DummyObject *myChild;

public:

    Dummy(int id)
        : myId(id), myChild(0)
    {
        if (myId == 0)
        {
            myChild = new Dummy(myId + 1);
        }
    }

    ~Dummy()
    {
        delete myChild;
    }

    int id() const
    {
        return myId;
    }

    void setId(int id)
    {
        myId = id;
    }

    QObject * child()
    {
        return myChild;
    }

};

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And then lets say I create a new Dummy object with id = 0 (so it has one child with id = 1), if I pass the parent Dummy object to a function that takes a constant reference to a QObject (or a constant pointer for that matter), how can I guarantee const-correctness on the child function? I tried the obvious: Q_PROPERTY(const QObject * child READ child) and then making const QObject * child() const (which is really how I want this function to look like), but then the moc complains about: "Unable to handle unregistered datatype 'const QObject*'". I just can't figure out how to make it so that if I pass a constant reference or pointer to a Dummy object that the "child" function (through the qt property system) will expose a constant reference or pointer (to ensure const-correctness).

[high_flyer]

The property name and type and the READ function are required. The type can be any type supported by QVariant, or it can be a user-defined type.

Now look at enum QVariant::Type.
QObject * is not there.

Then:

Custom types used by properties need to be registered using the Q_DECLARE_METATYPE() macro so that their values can be stored in QVariant objects. This makes them suitable for use with both static properties declared using the Q_PROPERTY() macro in class definitions and dynamic properties created at run-time.

have a look at QVariant qVariantFromValue ( const T & value );.

One easy way would be to typedef a long as pointer type and convert it in your class back to pointer again.

[wysota]

I'd say that trying to have a static property that holds QObject pointers is a bad idea without even dwelling about whether it works or not. Qt has another facility for holding hierarchies of objects that you should be using here. If you want to hold some reference to the child object then have that property contain a unique id to the child object that can be used to retrieve the real object pointer for the id.

Something like this should work:

class MyObject : public QObject {
  Q_OBJECT
  Q_PROPERTY(int id READ id WRITE setId)
  Q_PROPERTY(int childId READ childId WRITE setChildId)
public:
  // ...
  void setId(int id) {
    if(m_id == id) return;
    if(ObjectManager::instance()->hasId(m_id)) {
        ObjectManager::instance()->unregister(m_id);
    }
    if(ObjectManager::instance()->register(id, this)){
        m_id = id;
    } else {
        qWarning("Id %d already taken. Operation failed.", id);
    }
  }
  void setChildId(int id) {
    if(id == m_childId) return;
    if(!ObjectManager::instance()->hasId(id)){
        qWarning("No object with id %d exists. Operation failed.", id);
        return;
    }
    m_childId = id;
  }
};
 
// ...
 
QObject* ObjectManager::objectById(int id) const {
  return m_objects.value(id, 0);
}

class MyObject : public QObject {
  Q_OBJECT
  Q_PROPERTY(int id READ id WRITE setId)
  Q_PROPERTY(int childId READ childId WRITE setChildId)
public:
  // ...
  void setId(int id) {
    if(m_id == id) return;
    if(ObjectManager::instance()->hasId(m_id)) {
        ObjectManager::instance()->unregister(m_id);
    }
    if(ObjectManager::instance()->register(id, this)){
        m_id = id;
    } else {
        qWarning("Id %d already taken. Operation failed.", id);
    }
  }
  void setChildId(int id) {
    if(id == m_childId) return;
    if(!ObjectManager::instance()->hasId(id)){
        qWarning("No object with id %d exists. Operation failed.", id);
        return;
    }
    m_childId = id;
  }
};

// ...

QObject* ObjectManager::objectById(int id) const {
  return m_objects.value(id, 0);
}

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provided ObjectManager is a singleton handling registration of objects in the system.