Event-driven programming under Qt

The Qt Framework is an industrial-strength, cross-platform, and multi-platform GUI toolkit that runs on Windows, GNU Linux, macOS X, and other Mac systems. The toolkit has been compiled into embedded systems and mobile devices. The C++ Programming model has leveraged something called Meta Object Compiler (MOC), which will peruse the source code for directives (a bunch of macros and language extensions embedded in the source code) and generate appropriate additional source code to generate event handlers. So, before the C++ compiler gets the source code, the MOC pass has to run to generate legal ANSI C++ by removing those extra linguistic constructs specific to the Qt system. Consult the Qt documentation to learn more about this. The following simple Qt program will demonstrate the key aspects of Qt programming and its event processing system:

#include <qapplication.h>
#include <qdialog.h>
#include <qmessagebox.h>
#include <qobject.h>
#include <qpushbutton.h>

class MyApp : public QDialog {
  Q_OBJECT
public:
    MyApp(QObject* /*parent*/ = 0):
    button(this)
    {
      button.setText("Hello world!"); button.resize(100, 30);

      // When the button is clicked, run button_clicked
      connect(&button,
              &QPushButton::clicked, this, &MyApp::button_clicked);
    }

The macro Q_OBJECT is a directive to the MOC to generate an Event Dispatch table. When we connect the event source to an event sink, an entry will be given to the Event Dispatch table. The generated code will be compiled along with the C++ code to produce an executable:

public slots:
    void button_clicked() {
      QMessageBox box;
      box.setWindowTitle("Howdy");
      box.setText("You clicked the button");
      box.show();
      box.exec();
    }

protected:
  QPushButton button;
};

The language extension public slots will be stripped away by the MOC (after doing the job of source code generation) to a form compatible with the ANSI C/C++ compiler:

int main(int argc, char** argv) {
  QApplication app(argc, argv);
  MyApp myapp;
  myapp.show();
  return app.exec();
}

The preceding code snippet initializes the Qt application object and displays the main window. For all practical purposes, Qt is the most prominent application development framework for the C++ language and it also has got a good binding to the Python Programming language.