Signals Slots and Class Pointers

[wysota]

We'd have to see your actual code as currently with those little pieces you posted it is hard to tell anything. In general you need your method to be declared as a slot and you need to be passing a pointer to an instance of that class to the connect() statement to be able to connect to the slot.

[ChrisW67]

Also look at the debug output of your program for warnings of the type:

QObject::connect: No such slot GLWidget::SetValue(int)

QObject::connect: No such slot GLWidget::SetValue(int)

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indicating that the SetValue() function is not declared as a slot. The glwidget.h file should look like:

...
class GLWidget: public ...
{
  Q_OBJECT
public:
  GLWidget(...);
...
public slots:  // <<<< this 
  void SetValue(int value);
...
};
...

...
class GLWidget: public ...
{
  Q_OBJECT
public:
  GLWidget(...);
...
public slots:  // <<<< this 
  void SetValue(int value);
...
};
...

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and be included in the PRO file HEADERS variable.

[Atomic_Sheep]

Looks like HEADERS in pro are fine.

As for the code...

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
#include "glwidget.h"
 
namespace Ui {
    class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
    int m_Angle;
    void AngleUpdate(int m_Angle);
private:
    Ui::MainWindow *ui;
};
 
#endif // MAINWINDOW_H

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include "glwidget.h"

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

    int m_Angle;
    void AngleUpdate(int m_Angle);
private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

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glwidget.h

#ifndef GLWIDGET_H
#define GLWIDGET_H
 
#include <QtOpenGL/QGLWidget>
#include <cmath>
#include "mainwindow.h"
#include <QMessageBox>
 
class GLWidget : public QGLWidget
{
    Q_OBJECT
public:
    explicit GLWidget(QWidget *parent = 0);
 
int m_Angle;
 
 
void initializeGL();
void paintGL();
void resizeGL(int w, int h);
 
public slots:
void SetAngle(int Angle);
 
};
 
#endif // GLWIDGET_H

#ifndef GLWIDGET_H
#define GLWIDGET_H

#include <QtOpenGL/QGLWidget>
#include <cmath>
#include "mainwindow.h"
#include <QMessageBox>

class GLWidget : public QGLWidget
{
    Q_OBJECT
public:
    explicit GLWidget(QWidget *parent = 0);

int m_Angle;


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

public slots:
void SetAngle(int Angle);

};

#endif // GLWIDGET_H

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mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(ui->horizontalSlider, SIGNAL(valueChanged(int)), ui->Widget, SLOT(SetAngle(int Angle))); //Widget is just the standard widget which I've promoted to GLWidget in form editor
    connect(ui->horizontalSlider, SIGNAL(valueChanged(int)), ui->label, SLOT(setNum(int)));
}
 
MainWindow::~MainWindow()
{
    delete ui;
}

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

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(ui->horizontalSlider, SIGNAL(valueChanged(int)), ui->Widget, SLOT(SetAngle(int Angle))); //Widget is just the standard widget which I've promoted to GLWidget in form editor
    connect(ui->horizontalSlider, SIGNAL(valueChanged(int)), ui->label, SLOT(setNum(int)));
}

MainWindow::~MainWindow()
{
    delete ui;
}

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glwidget.cpp

#include "glwidget.h"
 
GLWidget::GLWidget(QWidget *parent) :
    QGLWidget(parent)
{
 
}
 
void GLWidget::SetAngle(int Angle)
{
    if (Angle != m_Angle)
    {
        m_Angle = Angle;
        QMessageBox::information(0, "inside slot", "inside slot");
    }
}

#include "glwidget.h"

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

}

void GLWidget::SetAngle(int Angle)
{
    if (Angle != m_Angle)
    {
        m_Angle = Angle;
        QMessageBox::information(0, "inside slot", "inside slot");
    }
}

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P.S. There might be one or two mistakes here or there e.g. m_Angle used instead of Angle or something like that... I'm 99% sure I don't have that mistake in my actual program.

[ChrisW67]

In the last few posts the slot has been referred to as SetNum(), SetValue(), and now SetAngle(). I asked you to check for run time error messages like:

QObject::connect: No such slot GLWidget::SetAngle(int Angle)

QObject::connect: No such slot GLWidget::SetAngle(int Angle)

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Line 9 of mainwindow.cpp is probably generating one because the slot signature is incorrect. You do not include the argument names in the signal or slot signatures.

[Atomic_Sheep]

Forgot to say that yes I did check for those errors, I didn't see any, but I just realised that run-time errors are under "Application Output" and yes it's spitting out the "no such slot" error. i.e. :

Object::connect: No such slot GLWidget::SetAngle(int Angle) in ...\MainWindow.cpp:9
Object::connect: (sender name: 'horizontalSlider')
Object::connect: (receiver name: 'Widget')

...exited with code 0

I used different names for my slots previously because I was simply typing the code up as I was writing up the message. In the latest iteration it was copy paste with some minor editing, but the latest code that I provided should have all the naming issues sorted out.

[Atomic_Sheep]

Still unable to get my signals and slots to work .

my .cpp files are :main.cpp, button.cpp, mainWindow.cpp and mainWindowWidget.cpp.

mainWindowWidget.cpp has the following signal:

void mainWindowWidget::mousePressEvent(QMouseEvent *event)
{
    if(event->button() == Qt::LeftButton) //left mouse button - need to check this.
    {
        cMousePositionAtClick = event->pos();
        emit MouseClickLocation(cMousePositionAtClick);
        //QMessageBox::information(this, "Mouse Click Detected", "Mouse Click Detected");
    }
}

void mainWindowWidget::mousePressEvent(QMouseEvent *event)
{
    if(event->button() == Qt::LeftButton) //left mouse button - need to check this.
    {
        cMousePositionAtClick = event->pos();
        emit MouseClickLocation(cMousePositionAtClick);
        //QMessageBox::information(this, "Mouse Click Detected", "Mouse Click Detected");
    }
}

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I have a slot in my button.h and its implementation.

here's the code in main.cpp:

mainWindow w;
    Button button1(0,0,50,50);
    connect(w, SIGNAL(MouseClickLocation(QPoint)), button1, SLOT(LocationGrabber(QPoint)));
    w.show();

mainWindow w;
    Button button1(0,0,50,50);
    connect(w, SIGNAL(MouseClickLocation(QPoint)), button1, SLOT(LocationGrabber(QPoint)));
    w.show();

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However when I compile it I get the following errors:

main.cpp:11: error: cannot convert 'mainWindow' to 'SOCKET' for argument '1' to 'int connect(SOCKET, const sockaddr*, int)'

[ChrisW67]

Two basic C++, not Qt, problems:

The error message tells you that the compiler has found only function 'int connect(SOCKET, const sockaddr*, int)' when trying to compile line 11 (I assume line 3 of your posted snippet). The arguments you have given cannot be coerced into the arguments required by that function so it fails. You want QObject::connect() and you cannot access that as an unqualified name outside of the implementation of a QObject sub-class. Use the static QObject::connect() function.

QObject::connect() requires a pointer to an object, i.e. &w and &button1, not the actual objects as you are current trying to do.

[Atomic_Sheep]

Deleted... figured it out.

[Atomic_Sheep]

Thanks Chris for helping out with the previous problem.

Another question. In the Qt example with the spinning boxes:

Window::Window()
 {
     QGridLayout *mainLayout = new QGridLayout;
 
     for (int i = 0; i < NumRows; ++i) {
         for (int j = 0; j < NumColumns; ++j) {
             QColor clearColor;
             clearColor.setHsv(((i * NumColumns) + j) * 255
                               / (NumRows * NumColumns - 1),
                               255, 63);
 
             glWidgets[i][j] = new GLWidget(0, 0);
             glWidgets[i][j]->setClearColor(clearColor);
             glWidgets[i][j]->rotateBy(+42 * 16, +42 * 16, -21 * 16);
             mainLayout->addWidget(glWidgets[i][j], i, j);
 
             connect(glWidgets[i][j], SIGNAL(clicked()),
                     this, SLOT(setCurrentGlWidget()));
         }
     }

Window::Window()
 {
     QGridLayout *mainLayout = new QGridLayout;

     for (int i = 0; i < NumRows; ++i) {
         for (int j = 0; j < NumColumns; ++j) {
             QColor clearColor;
             clearColor.setHsv(((i * NumColumns) + j) * 255
                               / (NumRows * NumColumns - 1),
                               255, 63);

             glWidgets[i][j] = new GLWidget(0, 0);
             glWidgets[i][j]->setClearColor(clearColor);
             glWidgets[i][j]->rotateBy(+42 * 16, +42 * 16, -21 * 16);
             mainLayout->addWidget(glWidgets[i][j], i, j);

             connect(glWidgets[i][j], SIGNAL(clicked()),
                     this, SLOT(setCurrentGlWidget()));
         }
     }

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The code creates glWidget objects for each of the cubes to spin. I just have:

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
mainwindow::mainwindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::Jetstream41)
{
    ui->setupUi(this);
}
 
mainwindow::~mainwindow()
{
    delete ui;
}

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

mainwindow::mainwindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::Jetstream41)
{
    ui->setupUi(this);
}

mainwindow::~mainwindow()
{
    delete ui;
}

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and a .cpp file for mainwindowGLWidget.

I'm trying to connect a signal from another class to a slot of my mainwindowGLWidget class, however I don't know the name of the object of my mainwindowGLWidget class. How do I find this out?

My limited understanding is that when the compiler runs, ?moc? generates the ui.h file in the -build-desktop folder? But does this somehow solve my problem?

I know it says this in relation to my glwidget...

widget = new MyOpenGLWidget(centralWidget);

[ChrisW67]

uic generates the ui_stuff.h file from your stuff.ui file.

The name of the variable holding a pointer to your custom widget is set by you in Designer and carried over into generated UI code. In Designer:
untitled.jpg
and in the generated code is the class that the ui pointer points to an instance of:

#include <QtGui/QButtonGroup>
#include <QtGui/QFrame>
#include <QtGui/QHBoxLayout>
#include <QtGui/QHeaderView>
#include <QtGui/QWidget>
 
QT_BEGIN_NAMESPACE
 
class Ui_Form                                                                                     
{                                                                                                 
public:                                                                                           
    QHBoxLayout *horizontalLayout;                                                                
    QFrame *myCustomFrame;        // <<<<< here                                                                
 
    void setupUi(QWidget *Form)                                                                   
    {                                                                                             
 ...
        // <<<< and here
        myCustomFrame = new QFrame(Form);
        myCustomFrame->setObjectName(QString::fromUtf8("myCustomFrame"));
        myCustomFrame->setFrameShape(QFrame::Box);
...
    } // setupUi
...
};
...
#endif // UI_UNTITLED_H

#include <QtGui/QButtonGroup>
#include <QtGui/QFrame>
#include <QtGui/QHBoxLayout>
#include <QtGui/QHeaderView>
#include <QtGui/QWidget>

QT_BEGIN_NAMESPACE
                                                                                                  
class Ui_Form                                                                                     
{                                                                                                 
public:                                                                                           
    QHBoxLayout *horizontalLayout;                                                                
    QFrame *myCustomFrame;        // <<<<< here                                                                
                                                                                                  
    void setupUi(QWidget *Form)                                                                   
    {                                                                                             
 ...
        // <<<< and here
        myCustomFrame = new QFrame(Form);
        myCustomFrame->setObjectName(QString::fromUtf8("myCustomFrame"));
        myCustomFrame->setFrameShape(QFrame::Box);
...
    } // setupUi
...
};
...
#endif // UI_UNTITLED_H

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so in your code you can access it through the ui pointer thus:

ui->myCustomFrame;

ui->myCustomFrame;

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[Atomic_Sheep]

Thanks Chris... had to reread this whole thread a few times to pick out to what information I applied the principle of 'selective hearing'. Finally got everything to work as desired! Now comes the fun task of fixing the next problem caused by the solving of the old prob . Thanks again.