C++ /Qt Proper way to access ui from another class in qt //Edited

Question

I'm trying to access the ui member which is private in the MainWindow class.

I would like to update a lineEdit (Xvaldisp) when I release the mousebutton (with the amount the mouse moved)from a gl widget.

After searching a bit around I found that I need to create a function/Method in mainwindow then access it through a pointer to Mainwindow from my GLWidget

The problem:

The lineEdit remains blank, The method( displaymessage() ) that should update it seems to get called.

To check that I've created a string(str) to see if displaymessage was getting called, this string gets updated with a new value when displaymessage() gets called.

The on_Button_clicked() method below displaymessage() also updates the same lineEdit when a pushbutton is clicked and works just fine it displays the content of str

Here's my code:

glwidget.h

#ifndef GLWIDGET_H
#define GLWIDGET_H

#include <QGLWidget>
#include <QTimer>
#include <QMouseEvent>
#include "mainwindow.h"
#include <QObject>
#include <QLineEdit>



class GLWidget : public QGLWidget
{
Q_OBJECT


public:

    explicit GLWidget(QWidget *parent = 0);


void mousePressEvent(QMouseEvent *e);
void mouseReleaseEvent(QMouseEvent *e);

void initializeGL();
void paintGL();
void resizeGL(int w, int h);

private:

    QTimer timer;

QPoint pointMpressed;
QPoint diff;


protected:


signals:
    void valueCh();     


    };

    #endif // GLWIDGET_H

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QLineEdit>
#include <QObject>

#include "glwidget.h"

namespace Ui {
    class MainWindow;
}


class MainWindow : public QMainWindow {
    Q_OBJECT


public:
    MainWindow(QWidget *parent = 0);
   ~MainWindow();




public slots:


   void on_Button_clicked();
   void displayMessage();


protected:
    void changeEvent(QEvent *e);


private:
    Ui::MainWindow *ui;

};




#endif // MAINWINDOW_H

glwidget.cpp.

#include "glwidget.h"
#include <GL/glut.h>



GLWidget::GLWidget(QWidget *parent) :
    QGLWidget(parent)
{

    connect (&timer,SIGNAL(timeout()),this,SLOT(updateGL()));
    timer.start(16);


}

void GLWidget::mousePressEvent(QMouseEvent *e){

    pointMpressed=e->pos();


}

void GLWidget::mouseReleaseEvent(QMouseEvent *e){

    diff=(e->pos())- pointMpressed ; //calculate position difference between click  and release


    MainWindow *mwd;

        mwd=  new MainWindow;


    //  mwd->displayMessage();   //tried this first then with signals and slots below same result

        QObject::connect(this, SIGNAL(valueCh()), mwd ,SLOT(displayMessage()) );

    emit valueCh();


   delete mwd;


    }


void GLWidget::initializeGL(){


}



void GLWidget::resizeGL(int w, int h){


}

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "glwidget.h"



QString str="none activated";  //I used this string to check if the methods were getting called




MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

}



MainWindow::~MainWindow()
{

    delete ui;

}




void MainWindow::changeEvent(QEvent *e)
{
    QMainWindow::changeEvent(e);
    switch (e->type()) {
    case QEvent::LanguageChange:
        ui->retranslateUi(this);
        break;
    default:
        break;
    }
}





 void MainWindow::displayMessage(){   //this method should  update the lineEdit (Xvaldisp)  the lineEdit however remains blank  but str gets updated


     ui->Xvaldisp->setText(str);

     str ="displayMessage hs rn";   //displaymessage has run

 }





    void MainWindow::on_Button_clicked(){ // this is a pushbutton(Button) that once pushed  also updates the same lineEdit(Xvaldisp)  this works just fine  If I clicked and released the mouse before on the GLWidget it would show the by displaymessage() updated string else it would be the orginal value   

        ui->Xvaldisp->setText(str);



    }

Answer 1

The root of your problem is that you seem to be misunderstanding the concept of pointers. Specifically, calling new is not the only way to obtain a pointer - a pointer is just a variable that holds the address of some object (or function). The new operator returns a pointer to a dynamically-allocated object, but there are other ways too, at least three of which are relevant to you:
1) Have someone else give you a pointer, for example as a function parameter;
2) Use & to take the address of an object.
3) Use this to get a pointer to the object you are currently working with.

Now that we have that out of the way, take a look at your code. MainWindow has two slots:

class MainWindow : public QMainWindow {
    Q_OBJECT
    ...
public slots:
   void on_Button_clicked();
   void displayMessage();

Slots are member functions - they are called on an object.

When you create an object of type MainWindow, the on_Button_clicked slot of your MainWindow object is automatically connected to the clicked signal of Button (via the meta-object compiler, a Qt-specific thing, because of the particular naming convention that you used).

What happens to the other one? Here's your code:

void GLWidget::mouseReleaseEvent(QMouseEvent *e){  
     MainWindow *mwd;  
     mwd=  new MainWindow;  
     QObject::connect(this, SIGNAL(valueCh()), mwd ,SLOT(displayMessage()) );  
     emit valueCh();  
     delete mwd;  
}

Rather than connecting to the slot of the original MainWindow object, you are creating a new object, calling its function, and promptly destroying it. So, while the slot does get called, it doesn't get called on the object that is actually your gui.

This is happening because you figured you needed a pointer to a MainWindow object, and used new to get one. The problem is, that pointer didn't point to the object you actually care about (i.e. your actual gui).

One (inelegant) solution is to not create a new MainWindow; instead, pass the address of the original MainWindow to the widget, and use that address (pointer) in the connect statement.

The much better solution is to skip all that stuff and use signals and slots the way they were intended, so that individual widgets don't have to know anything about the objects they are being connected to.

Change your MainWindow constructor to the following:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(ui->glWidget, SIGNAL(mouseReleaseEvent()),this,SLOT(displayMessage()));

}

and get rid of everything but the emit statement inside GLWidget::displayMessage.