(C++) Thinking Qt 1: general

January 18, 2018 · View on GitHub

This article shows one way to think about program architecture when using the Qt library. Because I use this program architecture also when using the Wt library, this article is very similar to Thinking Wt 1: general.

Downloads

Overview

In the first two paragraph I will explain the rationale behind the architecture proposed. The following paragraphs describes the steps in implementing this architecture in a top-down way, followed by a conclusion.

Why these guidelines?

Qt is 'a cross-platform application and UI framework'. I see many resemblances in its architecture with the Wt library, suitable for, among others, web applications. Due to their different running environments, Qt (desktop) and Wt (web) are mutually exclusive. Because I enjoy porting applications to as much platforms as possible, I have developed some applications that use both (but not simultaneously) a Qt and a Wt front-end. To be able to do this easily, I follow the guidelines presented in this article. Additionally, I like to reuse my own widgets and dialogs in multiple applications, which is only possible with a proper architecture.

Note that I follow the same guidelines for Wt front-ends.

Architecture

The architecture, from biggest to smallest, is: main, Qt::QApplication, dialog, widget:

 

  • Every C++ program has a single main function. The purpose of this main function is to create a Qt::QApplication and the first dialog. Optionally, main can parse the command-line arguments
  • The purpose of the Qt::QApplication is to manage signals. In this article I will not create a derived class of Qt::QApplication, but let main create the first dialog
  • A dialog is a collection of at least one widget. The purpose of a dialog is to respond to its widgets or creating other dialogs. A menu is an example of a dialog that creates other dialogs. A TicTacToe dialog would display a TicTacToe widget, but additionally shows the score, responding to a player winning and allowing the user to close it
  • A widget is a single visual element. The purpose of a widget is to repond to interaction and emitting signals when needed. For example, a TicTacToe widget responds to the clicking of the user and emits a signal when the game is finished. The signal might (or might not) be used by the dialog it is in

 

 

 

 

 

Implementing main

 

In this example, main creates a single Qt::WApplication, and creates the first (and only) dialog.

 

The most basic main function would only call WRun with a createApplication function that only returns a newly created Qt::WApplication:

 

#include <QtGui/QApplication> #include "qtdialog.h" int main(int argc, char *argv[]) {   QApplication a(argc, argv);   QtDialog w;   w.show();   return a.exec(); }

 

This code can be created by Qt Creator, by starting aGUI application and instead of using a Qt::QMainWindow called 'MainWindow', use a Qt::QDialog called 'QtDialog'.

 

 

 

 

 

 

Implementing the Qt::QApplication

 

signals. In this article I will not create a derived class of Qt::QApplication, but let main create the first dialog, as shown above.

 

 

 

 

 

Implementing the dialog

 

Because a dialog is, well, a dialog, QtDialog is a derived class of Qt::QDialog. QtDialog manages two widgets, but does not respond to their signals.

 

 

 

 

 

QtDialog.h

#ifndef QTDIALOG_H #define QTDIALOG_H #include <QDialog> struct QtWidget; class QtDialog : public QDialog {   Q_OBJECT public:   explicit QtDialog(QWidget *parent = 0); private:   QtWidget * const m_widget1;   QtWidget * const m_widget2; }; #endif // QTDIALOG_H

 

 

 

 

 

QtDialog.cpp

 

#include "qtdialog.h" #include "qtwidget.h" QtDialog::QtDialog(QWidget *parent)   : QDialog(parent),     m_widget1(new QtWidget(this)),     m_widget2(new QtWidget(this)) {   m_widget1->setGeometry( 0,0,32,32);   m_widget2->setGeometry(32,0,32,32); }

 

Note that this class must be split in a header (.h) file and an implementation (.cpp) file. Putting both QtDialog's declaration and definition in a single header (.h) file will result in the link error undefined reference to 'vtable for MyDialog'.

 

Because the pointers m_widget1 and m_widget2 are set to have their parent to 'this' in the QtDialog's constructor, these should not be deleted (doing so results in a double deletion).

 

 

 

 

 

Implementing the widget

 

A widget is a single visual element. In this example, QtWidget is a button (and thus a derived class of Qt::QPushButton), that displays how often it is clicked.

 

 

 

 

 

QtWidget.h

 

#ifndef QTWIDGET_H #define QTWIDGET_H #include <QPushButton> class QtWidget : public QPushButton {     Q_OBJECT public:   explicit QtWidget(QWidget *parent = 0);   void mousePressEvent(QMouseEvent *);   private:   int m_count; }; #endif // QTWIDGET_H

 

 

 

 

 

QtWidget.h

 

#include "qtwidget.h" QtWidget::QtWidget(QWidget *parent) :   QPushButton(parent),   m_count(0) { } void QtWidget::mousePressEvent(QMouseEvent *) {   ++m_count;   this->setText(QString::number(m_count)); }

 

 

 

 

 

Running the Qt application

 

Click 'Build -> Run' or CTRL-R to run the application.

 

 

 

 

 

Conclusion

 

In this article I have shown one of many Qt program architectures you can use, for a very basic application. In my humble opinion, this architecture makes sense, but I am open to discussion on this subject.