copy constructor of derived QT class

Question

I have a class which is publicly inherited from QWidget:

class MyWidget : public QWidget
{
    Q_OBJECT
public:
    MyWidget(const MyWidget& other)
      :
    obj1(other.obj1),
    obj2(other.obj2)

private:
    some_class obj1;
    some_class obj2;
};

When I built my project, compiler complains:

WARNING:: Base class "class QWidget" should be explicitly initialized in the copy constructor.

I checked out from other questions on stackoverflow, and got my answer. But the fact is, when I added that initialization like this:

class MyWidget : public QWidget
{
    Q_OBJECT
public:
    MyWidget(const MyWidget& other)
      :
    QWidget(other),   //I added the missing initialization of Base class
    obj1(other.obj1),
    obj2(other.obj2)

private:
    some_class obj1;
    some_class obj2;
};

I got compile error:

QWidget::QWidget(const QWidget&) is private within this context

So, please explain me what I am doing wrong.

Answer 1

QObject Class description page tells :

QObject has neither a copy constructor nor an assignment operator. This is by design. Actually, they are declared, but in a private section with the macro Q_DISABLE_COPY(). In fact, all Qt classes derived from QObject (direct or indirect) use this macro to declare their copy constructor and assignment operator to be private. The reasoning is found in the discussion on Identity vs Value on the Qt Object Model page.

That means you are not supposed to copy QT objects, since QObject is non-copyable by design.

The first warning tells you to initialize the base class (which is QWidget). If you want to do this, you are going to construct a new base object, and I doubt that is what you want to do.

The second error is telling you what I wrote above : do not copy qt objects.

Answer 2

All Qt classes are noncopyable by deriving from QObject.

It is common practice in C++ to forbid certain value-semantic operations like copying on polymorphic objects. Qt gives some examples of the problems that would arise if copy was allowed for QObject in its documentation:

A Qt Object...

• might have a unique QObject::objectName(). If we copy a Qt Object, what name should we give the copy?
• has a location in an object hierarchy. If we copy a Qt Object, where should the copy be located?
• can be connected to other Qt Objects to emit signals to them or to receive signals emitted by them. If we copy a Qt Object, how should we transfer these connections to the copy?
• can have new properties added to it at runtime that are not declared in the C++ class. If we copy a Qt Object, should the copy include the properties that were added to the original?