Question

Problem is that I keep getting the ‘No Such Slot’ runtime error in Qt Creator every time I launch a ‘settings’ window from my main window. I’ve found Qt to be quite counter-intuitive so far and this slots ‘n signals concept seems a bit of a stretch from simply passing vars or function calls. Basically, I have menu with a settings option, that when clicked, opens a settings window which needs to grab a double from the user and update a var in the main window.

SettingsWindow.h

class SettingsWindow : public QWidget
{
      Q_OBJECT
  public:
      SettingsWindow(QWidget *parent = 0);
  signals:
      void ValChanged(double newVal);
  public slots:
      void Accept();
  private:
      QLineEdit *le1;
};

The settings window has an accept button which calls Accept() which emits the ValChanged signal with newVal set as the user input in le1 as a double.

SettingsWindow.cpp

void SettingsWindow::Accept(){
    emit ValChanged(le1->text().toDouble());
    this->close();
}

This settings window is called by the application’s main window: MainWindow

MainWindow.cpp

class MainWindow : public QMainWindow
{
      Q_OBJECT  
  public:
      MainWindow(QWidget *parent = 0);
  public slots:
      void SetVal(double x);
  private slots:
      void NewWindow();
  private:
      double theVal;
};

This main window has a menu which one would select settings from. This creates a new window with a field for one to enter a number.

MainWindow.cpp

void MainWindow::NewWindow()
{
    SettingsWindow *MySettings=new SettingsWindow(this);
    QObject::connect(MySettings, SIGNAL(ValChanged(double)), this, SLOT(SetVal(double)));
    MySettings->show();
    MySettings->raise();
}

void MainWindow::SetVal(double x){
    theVal = x;
}

My hope is that when the settings window is opened, the user can enter a val into the field which then emits the ValChanged Signal which sets theVal to the value specified by the user. Most of the time I saw an issue with people not including Q_OBJECT macro, but I’ve included it both times. Any suggestions on why this doesn’t work?

Answer 1

The issue you are having is almost certainly due to a moc file not being recreated, a typo in your call to connect or a typo in the declaration of the slot in question.

You may want to consider that this is a lot more work than necessary for getting input from a dialog. A simpler method would be to connect the “Accept” button clicked signal to a slot in main window and then fetch the value you want directly from the instance of the settings window through a getXXX() method.

If you eventually have a settings dialog with a lot of values, instead of fetching each value through getters, have the “Accept” button signal return a structure with all the values as fields of that structure.

I should mention that it looks like NewWindow() creates a new instance of SettingsWindow each time it is called. All of these instances will persist until the MainWindow is destroyed.