Qt Getting started with Qt Hello World

Example

In this example, we simply create and show a push button in a window frame on the desktop. The push button will have the label Hello world!

This represents the simplest possible Qt program.

First of all we need a project file:

helloworld.pro

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = helloworld
TEMPLATE = app

SOURCES += main.cpp

• QT is used to indicate what libraries (Qt modules) are being used in this project. Since our first app is a small GUI, we will need QtCore and QtGui. As Qt5 separate QtWidgets from QtGui, we need add greaterThan line in order to compile it with Qt5.
• TARGET is the name of the app or the library.
• TEMPLATE describes the type to build. It can be an application (app), a library (lib), or simply subdirectories (subdirs).
• SOURCES is a list of source code files to be used when building the project.

We also need the main.cpp containing a Qt application:

main.cpp

#include <QApplication>
#include <QPushButton>

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

    QPushButton button ("Hello world!");
    button.show();

    return a.exec(); // .exec starts QApplication and related GUI, this line starts 'event loop'    
}

• QApplication object. This object manages application-wide resources and is necessary to run any Qt program that has a GUI. It needs argv and args because Qt accepts a few command line arguments. When calling a.exec() the Qt event loop is launched.
• QPushButton object. The push button with the label Hello world!. The next line, button.show(), shows the push button on the screen in its own window frame.

Finally, to run the application, open a command prompt, and enter the directory in which you have the .cpp file of the program. Type the following shell commands to build the program.

qmake -project
qmake
make