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Qt allows you to develop a multi-platform application, that you can develop a GUI application on one platform and deploy to many others (if you avoid system-specific extra modules). Qt contains modules for creating widgets from elementary ones to OpenGL 3D, which I will discuss later. In addition it contains module with other functionalities an application may need such as SQL, XML, web channels, web views and web sockets.

Qt can be downloaded from https://www.qt.io/ or installed using the UNIX/Linux package installers.

In addition to the QT libraries and header files, there are 3 useful resources:

  • Qt Designer – A GUI tool for the generation of Python and C++ headers from a form graphically designed.
  • Qt Assistant – A reference to the Qt classes, Example, the QMake tutorial, etc.
  • qmake – an easy-to-use utility that creates the Makefile.

In the following sections, I will show how to create a project using the three tools:

Creating the Main Window

Qt Designer is an editor with which GUI designers and developers can communicate. With the designer you can create, save and edit Designer UI Files (*.ui) and create class files from them for Python and C++ programs. The QMake program discussed later can use the “.ui” file to generate a header file from it. The file can be edited by creating top level windows such as widows and dialog, and dragging and dropping widgets into them. You can also access and modify their properties; if you are a developer and have a Desiger UI file, you can find names of object in your program.

Starting:

As you start the QT Designer, the following dialog pops up:

If you don’t see the dialog, choose File->New from the editor’s menu bar, to make the dialog pop up.

Choose your top-level widget from the “templates/forms” menu, and click Create; a window will be created:

Now, from the right-hand Property Editor, you can change some properties, for example: let us change the window title:

Adding widgets and promoting them

Now, you can choose widgets from the left-hand side of the editors and drag-and-drop them into the window:

For example, let us add a push button, and change its QAbstractButton property “text” to “Push me!”.

One of the other properties we can change is the QObject property “objectName”, which will be the name of the appropriate field of class QPushButton in the created class.

By right-clicking inside the widget and choosing “Promote to..” from the context-menu, you can choose to use a class that inherits from the gadget’s class. Don’t forget to define the new class and its constructor and methods.

Previewing

You can view your design by clicking Form->Preview from the editor’s menu bar.

Saving the created class

If you don’t want the Make program to generate a header file, you can save it from the Qt Designer tool.

  1. Choose Form->View C++ Vode/View Python Code from the editor’s menu bar.
    a dialog will open.
  2. Click the “save” icon to save the created header file.

Adding Functionality

This section will describe a little Hello World program – in C++ – that uses a header file created wit QtDesigner. The purpose of the program is to print “Hello, world” to the standard output, when the “Push Me” button is clicked. To add the mouse event handling the class QPushButton has been promoted to Extended Button. This section will describe the program, and will help you train yourself using Qt Assistant.

The main function

To learn what a the main function of a Qt Application with widgets should look like:

  1. Start the Qt Assistant if not started yet.
  2. Click the Contents tab
  3. Click Qt Widgets (highlighted in the following image:

You” see a heading reading “Getting Started Programming with Qt Widgets”. Scroll down and see the contents of a main source line.

In the Hello World program the code is as follows:

 #include "ui_example.h"
 #include <iostream>

 using namespace std;

 ExtendedButton::ExtendedButton(QWidget *parent):QPushButton(parent) {
 }
 void ExtendedButton::mouseReleaseEvent(QMouseEvent *event){
     cout<<"Hello, world!"<<endl;
 }
 int main(int argc, char **argv){
     QApplication a(argc,argv);
     Ui_MainWindow mainObject;
     QMainWindow mainWindow;
     mainObject.setupUi(&mainWindow);
     mainWindow.show();
     return a.exec();
 }

The main file includes the constructor of the class ExtendedButton. Its role is to call the parent’s constructor. Another function implemented is mouseReleaseEvent, which is an event handler. You can learn from Qt Assistant, that the method mouseReleaseEvent is an event handler by clicking Qt Widget->QWidget in the right-hand Contents tab, and then click the link reading “events” under detailed description in the pages contents.

The class ExtendedButton is defined in “extendedbutton.h”. Following is the definition:

#include <QtWidgets/QPushButton>

 class ExtendedButton:public QPushButton {
     public:
         ExtendedButton(QWidget *parent=nullptr);
         void mouseReleaseEvent(QMouseEvent *event);
 };

The main object of the application is defined in “ui_example.h”, the file generated by “Qt Designer”, you better avoid changing it manually if you want to modify the “.ui” file from which it was generated. You’ll see a warning at the beginning. The role of the Class Ui_MainWindow is to contain main window (or central widget) and its underlying widgets as public members. The method setupUi binds them.

Creating the qmake file

“qmake” is an easy-to-use utility that generates a Makefile to be used by the make command to generate binaries, objects, libraries. etc. Qt Assistant includes a Qt Manual. Following is the content of the example’s qmake file named ‘qmake.pro’:

TARGET=executable
SOURCES+=main.cpp
HEADERS+=extendedbutton.h
FORMS+=example.ui
DESTDIR=bin
QT = core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

If you want the Make program to create the main header file, you can include the “.ui” file generated by Qt Designer in a variable named “FORMS”. The “make” program in turn will generate a header file from it with the prefix “ui_” added to the “.ui” file name. For example, from a file named “example.ui” the “make” program will generate a header file named “ui_example.h”. In this case, don’t add “ui_example.h” to the variable “HEADERS”.

If you want to know what to add to QT, go to the <Class Category>->C++ Classes and click the Detailed Description link.If you want to run qmake without command line arguments, call the qmake file ‘qmake.pro’.

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