Typical main.cpp file

Let’s first look at the example of main.cpp:

#include <QApplication>
#include <QLabel>

int main(int argc, char** argv)
{
  QApplication app(argc, argv);
  QLabel("Hello world!");
  lbl.show();
  return app.exec();
}

You can see now that it is lot like any other C++ program’s main file. It goes as following:

1. Declare inclusions. These can be user headers as well as precompiled Qt libraries, if any Qt classes are used directly in main function. In the above we use QLabel and QApplication, thus we declare them before using in main().

2. Create an instance of QApplication class. This object controls and maintains the application that we create. Only one instance of QApplication is allowed in one program.

3. Create an instance of QLabel. After creating Qt elements are usually invisible, so we have to call show() in order to make it visible. In this case QLabel object is the root object, which means that it will be created as an separate window, and the applicaion will exit once this window is closed.

4. Last line launches the execution of the application, which runs till the moment QApplication::exit() is called.

Note

Try to create two instances of QLabel inside one application. Are they shown in separate windows? If so, will the application exit if you close one of them? Now try to set the first label as a parent for the second label. Refer to QLabel members list. What happens now? Are both labels shown in one window?