Question

I have two classes under QT, one to make a form, the other to send the collected data on the serial port. There is a button to submit and send the information on the serial port. The first class “myform” contains the file descriptor variable and the slot function for the submit button.

    class myform: public QWidget
    {
      Q_OBJECT
      private slots:
      void submitclicked(void);
    public:
        myform(QWidget *parent = 0);

        // some vars...  
        QPushButton *submit;
        int serialfd;
    };

The second class “serialcom” inherits the first class, since I want to implement the slot function in that “serialcom” class.

    class serialcom : public myform
    {
        Q_OBJECT
    public:
        int serialdev_init(void);
        serialcom(myform *parent=0);

    private:
        // some vars...
    };

The serialfd file descriptor is getting initialized in the constructor for serialcom through serialdev_init(). I have checked, it is initialized properly. The problem is that when the SLOT for submit button is called, serialfd has garbage value (I mean inside the submitclicked() slot ), not the initialized value.

Isn’t it supposed to preserve the value, or am I wrong to assume that ? I am pretty new to this QT or even C++ business, so please mind my stupidities if any…

Here is the main function…

    int main(int argc,char **argv)
    {
        QApplication app(argc,argv);

        myform *trial = new myform;

        serialcom *serial = new serialcom(trial);


        trial->show();

        return app.exec();
   }

Any other suggestions are also welcome.

Answer 1

Derived class construction involves base class sub-object construction first followed by derived class sub-object. So, derived class object has two sub-objects.

 myform *trial = new myform;

trail has a sub-objects of type QWidget, myform. Now this myform sub-object has it’s own member variable(s) serialfd which is uninitialized. Now, trial cannot access serialcom members because a derived class can access base class members but the otherwise is not true.

 serialcom *serial = new serialcom(trial);

The same happens with this statement too except that serial has it’s own sub-objects QWidget, myform, serial.

The serialfd file descriptor is getting initialized in the constructor for serialcom through serialdev_init().

Now the two myform sub-objects has no relation. You are setting serialfd of this sub-object but seeing the serialfd of trail sub-object.

 trial->show();

Hope you understood what you are doing wrong.