Hello Qt

• The following example program creates and displays a simple window. It doesn't do anything other than display a line of text, but it gives you an idea of the fundamental requirements of a Qt program. The window is shown in Figure 2.

  Figure 2: A simple Qt program displaying text.

  
  • The file qapplication.h included on line 2 is almost always included in the same source file that contains the main() function.
  • This example uses a QLabel widget to display text, so it is necessary to also include qlabel.h. And a QString object is required to specify the text displayed by the QLabel object, so qstring.h is included on line 4.
  • Line 8 creates a QApplication object named app. The QApplication object is a container that will hold the top-level window (or set of windows) of an application. A top-level window is unique in that it never has a parent window in the application. Because the QApplication object takes over things and manages your application, there can only be one of these per program. Also, the creation of a QApplication object initializes the Qt system, so it must exist before any of the other Qt facilities are available.
  • A Qt program is a C++ program. This means that in order to start the program, a function named main() will be called by the operating system. And, like all C++ programs, command-line options may or may not be passed to the main() function. The command-line options are passed on to the Qt software as part of the initialization process, as shown on line 8.
  • The two command-line arguments, argc and argv, are used in the construction of app because some special flags and settings can be specified. For example, starting a Qt program with -geometry will specify the size and location of the window it displays. By altering the profile information that starts a program, a user can personalize a program's appearance.
  • A QLabel widget is created on line 9. A QLabel widget is simply a window that is capable of displaying a string of characters. The label is created with its specified parent widget as NULL because this label is to be the top-level window, and top-level windows have no parents. As it is created, the label contains no text, but it is provided text by being passed the QString object created on line 10.
  • The QString object is inserted into the QLabel with the call to setText() on line 11. The default action for a QLabel is to display the character string centered vertically and justified to the left, so the call to setAlignment() is made on line 12 to center the text both vertically and horizontally.
  • The call to setGeometry() on line 14 determines the location, height, and width of the label widget inside the QApplication window. For this example, the label is positioned at location (0,0), which is the upper-left corner of the main window. It is also instructed to be 180 pixels wide by 75 pixels high. Before anything is displayed, the main window will query the label to find out its size, and then the main window will set its own size to contain the label.
  • The call to show() on line 16 is necessary in order for the label to actually appear on the window. The show() function does not immediately display the widget, it only configures it so that it will be displayed when the time comes. The parent window - in this case, the QApplication window - assumes the task of displaying the label, but will only do so if there has been a call to the label's show() method. Another function, named hide(), can be used to cause a widget to disappear from the display.
  • The call to setMainWidget() on line 11 inserts the label into the main window. To keep this example simple, the QLabel object is used, but normally the widget will be some sort of compound widget that contains the collection of widgets, text, and other elements of the main window of an application.
  • Finally, a call is made to exec() on line 17. This function does not return until it is time for the program to cease execution. It returns an int value representing its completion status; and because we are not processing status codes, the value is simply returned to the system.
• Because the program is simple and consists of only one source file, the makefile that compiles it is quite simple:

   INCL= -I$(QTDIR)/include -I$(KDEDIR)/include
  CFLAGS= -pipe -O2 -fno-strength-reduce
  LFLAGS= -L$(QTDIR)/lib -L$(KDEDIR)/lib -L/usr/X11R6/lib
  LIBS= -lqt-mt -lX11 -lXext
  CC=g++
  helloworld: helloworld.o
          $(CC) $(LFLAGS) -o helloworld helloworld.o $(LIBS)
  helloworld.o: helloworld.cpp
  clean:
          rm -f helloworld
          rm -f helloworld.o
  .SUFFIXES: .cpp
  .cpp.o:
          $(CC) -c $(CFLAGS) $(INCL) -o $@ $<
  

• The makefile assumes that the environment variables QTDIR and KDEDIR are defined as the name of the installation directory of the Qt and KDE development systems. Normally, these two environment variables have their definitions configured when you install the software.