Our first console application is really cool, isn't it? Let's continue exploring further. This time, let's write a simple GUI-based Hello World program. The procedure will remain almost the same, except for some minor changes in main.cpp. Refer to the following for the complete code:

Wait a minute. Let me explain the need for QApplication in line number 23 and line number 29. Every Qt GUI application must have exactly one instance of the QApplication instance. QApplication provides support for command-line switches for our application, hence the need to supply the argument count (argc) and the argument value (argv). GUI-based applications are event-driven, so they must respond to events or, to be precise, signals in the Qt world. In line number 29, the exec function starts the event loop, which ensures the application waits for user interactions until the user closes the window. The idea is that all the user events will be received by the QApplication instance in an event queue, which will then be notified to its Child widgets. The event queue ensures all the events deposited in the queue are handled in the same sequence that they occur, that is, first in, first out (FIFO).

In case you are curious to check what would happen if you comment line 29, the application will still compile and run but you may not see any window. The reason being the main thread or the main function creates an instance of QWidget in line number 25, which is the window that we see when we launch the application.

In line number 27, the window instance is displayed, but in the absence of line number 29, the main function will terminate the application immediately without giving a chance for you to check your first Qt GUI application. It's worth trying, so go ahead and see what happens with and without line number 29.

Let's generate Makefile, as shown in the following screenshot:

Now let's try to compile our project with the make utility, as shown in the following screenshot:

Interesting, right? Our brand new Qt GUI program fails to compile. Did you notice the fatal error? No big deal; let's understand why this happened. The reason is that we have not yet linked the Qt Widgets module, as the QApplication class is part of the Qt Widgets module. In that case, you may wonder how your first Hello World program compiled without any issue. In our first program, the QDebug class was part of the QtCore module that got linked implicitly, whereas other modules had to be linked explicitly. Let's see how to get this done:

We need to add QT += widgets to the Ex2.pro file so that the qmake utility understands that it needs to link Qt Widgets's shared object (the .so file) in Linux, also known as the Dynamic Link Library (the .dll file) in Windows, while creating the final executable. Once this is taken care of, we must qmake so that Makefile could reflect the new change in our Ex2.pro file, as demonstrated in the following screenshot:

Cool. Let's check our first GUI-based Qt app now. In my system, the application output looks as shown in Figure 5.12; you should get a similar output as well if all goes well at your end:

It would be nice if we set the title of our window as Hello Qt, right? Let's do this right away:

Add the code presented at line number 26 to ensure you build your project with the make utility before you test your new change: