learnxinyminutes-docs/qt.md

category	name	filename	contributors
framework	Qt	learnqt.cpp	Aleksey Kholovchuk https://github.com/vortexxx192

Qt is a widely-known framework for developing cross-platform software that can be run on various software and hardware platforms with little or no change in the code, while having the power and speed of native applications. Though Qt was originally written in C++, there are its ports to other languages: PyQt, QtRuby, PHP-Qt, etc.

Qt is great for creating applications with graphical user interface (GUI). This tutorial is how to do it in C++.

/*
 * Let's start classically
 */

// all headers from Qt framework start with capital letter 'Q'
#include <QApplication>
#include <QLineEdit>

int main(int argc, char *argv[]) {
	 // create an object to manage application-wide resources
    QApplication app(argc, argv);

    // create line edit widget and show it on screen
    QLineEdit lineEdit("Hello world!");
    lineEdit.show();

    // start the application's event loop
    return app.exec();
}

GUI-related part of Qt is all about widgets and connections between them.

READ MORE ABOUT WIDGETS

/*
 * Let's create a label and a button.
 * A label should appear when a button is pressed.
 * 
 * Qt code is speaking for itself.
 */
 
#include <QApplication>
#include <QDialog>
#include <QVBoxLayout>
#include <QPushButton>
#include <QLabel>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QDialog dialogWindow;
    dialogWindow.show();
    
    // add vertical layout 
    QVBoxLayout layout;
    dialogWindow.setLayout(&layout);  

    QLabel textLabel("Thanks for pressing that button");
    layout.addWidget(&textLabel);
    textLabel.hide();

    QPushButton button("Press me");
    layout.addWidget(&button);
    
    // show hidden label when the button is pressed
    QObject::connect(&button, &QPushButton::pressed,
                     &textLabel, &QLabel::show);

    return app.exec();
}

Notice that QObject::connect part. This method is used to connect SIGNALS of one objects to SLOTS of another.

Signals are being emitted when certain things happen with objects, like pressed signal is emitted when user presses on QPushButton object.

Slots are actions that might be performed in response to received signals.

READ MORE ABOUT SLOTS AND SIGNALS

Next, let's learn that we can not only use standard widgets but also extend their behaviour using inheritance. Let's create a button and count how many times it was pressed. For this purpose we define our own class CounterLabel. It must be declared in separate file because of specific Qt architecture.

// counterlabel.hpp

#ifndef COUNTERLABEL
#define COUNTERLABEL

#include <QLabel>

class CounterLabel : public QLabel {
    Q_OBJECT  // Qt-defined macros that must be present in every custom widget

public:
    CounterLabel() : counter(0) {
        setText("Counter has not been increased yet");  // method of QLabel
    }

public slots:
    // action that will be called in response to button press
    void increaseCounter() {
        setText(QString("Counter value: %1").arg(QString::number(++counter)));
    }

private:
    int counter;
};

#endif // COUNTERLABEL

// main.cpp
// Almost the same as in previous example

#include <QApplication>
#include <QDialog>
#include <QVBoxLayout>
#include <QPushButton>
#include <QString>
#include "counterlabel.hpp"

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QDialog dialogWindow;
    dialogWindow.show();

    QVBoxLayout layout;
    dialogWindow.setLayout(&layout);

    CounterLabel counterLabel;
    layout.addWidget(&counterLabel);

    QPushButton button("Push me once more");
    layout.addWidget(&button);
    QObject::connect(&button, &QPushButton::pressed,
                     &counterLabel, &CounterLabel::increaseCounter);

    return app.exec();
}

That's it! Of course, Qt framework is much much larger than the part that was covered in this tutorial, so be ready to read and practice.

Further reading

• Qt 4.8 tutorials
• Qt 5 tutorials

Good luck and have fun!