Overriding closeEvent without child class

qmad

I saw an application with following override

void QDockWidget::closeEvent(QCloseEvent* e)
{
  //handling close event here
}

How it is not a redefinition? Qt must be defining the QDockWidget::closeEvent() function by itself. How can we redefine that function? I know C++ override, but I thought it is possible only by creating a child class. But there is no child class in this this case. By the way the above code is working perfectly fine !! How?

raven-worx

@qmad said in Overriding closeEvent without child class:

But there is no child class in this this case.

i am not quite sure what your question is, but i try it anyway.
The QWidget base class implements closeEvent(). Additionally it's declared as virtual

ambershark

QDockWidget's parent is QWidget. So it is a child to QWidget. And QWidget::closeEvent() is a protected virtual so it can be overridden.

Likewise if you wanted to override closeEvent in a class you derive from QDockWidget you would be able to do that as well.

So the override is legit since QDockWidget is a child of QWidget. ;)

Edit: I really should have read @raven-worx 's answer before I wrote that since he said exactly what I said.

qmad

I can understand if there is a child class of QDocWidget and closeEvent is overriding in it. In this case simply there is a QDockWidget::closeEvent(QCloseEvent* e){----} code. I was assuming that this definition will be already there in Qt libraries. Do you mean, in QtDocWidget, it is only a declaration and they left definition of us ?

jsulm

@qmad Where is this code from?

void QDockWidget::closeEvent(QCloseEvent* e)
{
  //handling close event here
}

It is just a plain old C++ method definition, which in this case happens to be an inherited virtual method, hence it is overridden in QDockWidget.

ambershark

@qmad I don't think you get it. The QDockWidget IS the child class where it is overridden. You can of course derive from QDockWidget and override it again yourself.

qmad

@jsulm

Let us say class A is defined in a library as follows

class A
{
  public:
   virtual int fun();
};

int A::fun()
{
   cout<<"Called from library";
}

And in my main.cpp can I have below code?

main()
{
  A a;
  a.fun();
}

int A::fun()
{
  cout<<"called from Main";
}

ambershark

OH! I think I see what you are talking about. The QDockWidget::closeEvent override was in some random cpp file. Not in the Qt files. I thought you were confused on inheritance and virtual functions, but really you're saying someone just randomly redefined a virtual from a class it wasn't part of?

qmad

@ambershark
Yes
In my opinion, the only way it can work is Qt left the closeEvent() function undefined inside QDocWidget class.
Or is there any C++ or Qt magic I am not aware of?

ambershark

@qmad Now that I get what you're asking that is indeed weird. The one thing I can think of as well is that it is not defined in Qt itself, so there is no naming conflict when redefining it.

Seems like a dangerous game that author was playing regardless though. I'd like to write some test code and play with that a bit more. It doesn't seem valid. :)