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---
category: tool
tool: Qt Framework
language: c++
filename: learnqt.cpp
contributors:
- ["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](https://learnxinyminutes.com/docs/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++*.
```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](http://doc.qt.io/qt-5/qtwidgets-index.html)
```c++
/*
* 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](http://doc.qt.io/qt-5/signalsandslots.html)
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.
```c++
// 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
```
```c++
// 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](http://doc.qt.io/qt-4.8/tutorials.html)
- [Qt 5 tutorials](http://doc.qt.io/qt-5/qtexamplesandtutorials.html)
Good luck and have fun!
|
