Development/Tutorials/First program

Hello World

Tutorial Series	Beginner Tutorial
Previous	C++, Qt, KDE4 development environment
What's Next	Tutorial 2 - KXmlGuiWindow
Further Reading	CMake

Abstract

Your first program shall greet the world with a friendly "Hello World", what else? For that, we will use a KMessageBox and customise one of the buttons.

Tip

To get more information about any class you come across, Konqueror offers a quick shortcut. So to look for information about KMessageBox, just type "kde:kmessagebox" into Konqueror and you'll be taken to the documentation.

Tip

You might want to use KDevelop for your projects, which does many nice things like code completition, easy access to API documentation or debugging support.

Read this tutorial to set up KDevelop correctly for this task. You probably want to check if the setup is working by testing opening an existing KDE 4 application with KDevelop first.

You still need to edit the CMake files by hand though.

The Code

All the code we need will be in one file, main.cpp. Create that file with the code below:

include <QString>
include <KApplication>
include <KAboutData>
include <KMessageBox>
include <KCmdLineArgs>

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

 KAboutData aboutData( "tutorial1", "Tutorial 1",
                       "1.0", "KMessageBox popup",
                       KAboutData::License_GPL, "(c) 2006" );
 KCmdLineArgs::init( argc, argv, &aboutData );
 KApplication app;
 KGuiItem guiItem( QString( "Hello" ), QString(),
                   QString( "this is a tooltip" ),
                   QString( "this is a whatsthis" ) );
 KMessageBox::questionYesNo( 0, "Hello World",
                             "Hello", guiItem );

} The first KDE specific code we come across in this program is KAboutData. This is the class used to store information about the program such as a short description, authors or license information. Pretty much every KDE application should use this class.

Then we come to KCmdLineArgs. This is the class one would use to specify command line switches to, for example, open the program with a specific file. However, in this tutorial, we simply initialise it with the KAboutData object we created so we can use the --version or --author switches.

On line 13 we create a KApplication object. This needs to be done exactly once in each program since it is needed for things such as i18n.

Now we've done all the necessary KDE setup, we can move on to doing interesting things with our application. We're going to create a popup box but we're going to customise one of the buttons. To do this customisation, we need to use a KGuiItem object. The first argument in the KGuiItem constructor is the text that will appear on the item (in our case, a button). Then we have an option of setting an icon for the button but we don't want one so we just give it QString(). Finally we set the tooltip (what appears when you hover over an item) and finally the "What's This?" (accessed through right-clicking or Shift-F1) text.

Now we have our item, we can create our popup. we call the KMessageBox::questionYesNo() function which, by default, creates a message box with a "Yes" and a "No" button. The second argument is the text that will appear in the middle of the popup box. The third is the caption the window will have and finally we set the KGuiItem for (what would normally be) the "Yes" button to the KGuiItem guiItem we created.

We're all done as far as the code is concerned. Now to build it and try it out.

Build

If you set up your environment as described in Getting_Started/Build/Unstable_Version, you can compile this code with

gcc main.cpp -o tutorial1 \
-I/home/kde-devel/qt-copy/include/Qt \
-I/home/kde-devel/qt-copy/include/QtCore \
-I/home/kde-devel/qt-copy/include \
-I/home/kde-devel/kde/include/KDE \
-I/home/kde-devel/kde/include \
-L/home/kde-devel/kde/lib \
-L/home/kde-devel/qt-copy/lib -lQtCore -lQtGui -lkdeui -lkdecore

and then run it with

dbus-launch ./tutorial1

Using CMake

If that worked, you may want to use CMake, just like the rest of KDE. This will automatically locate the libraries and headers for KDE, Qt etc. and will allow you to easily build your applications on other computers.

CMakeLists.txt

Create a file named CMakeLists.txt in the same directory as main.cpp with this content: project (tutorial1)

find_package(KDE4 REQUIRED) include_directories( ${KDE4_INCLUDES} )

set(tutorial1_SRCS main.cpp)

kde4_add_executable(tutorial1 ${tutorial1_SRCS}) target_link_libraries(tutorial1 ${KDE4_KDEUI_LIBS}) The find_package() function locates the package that you ask it for (in this case KDE4) and sets some variables describing the location of the package's headers and libraries. In this case we will use the KDE4_INCLUDES variable which contains the path to the KDE4 header files.

In order to allow the compiler to find these files, we pass that variable to the include_directories() function which adds the KDE4 headers to the header search path.

Next we create a variable called tutorial1_SRCS using the set() function. In this case we simply set it to the name of our only source file.

Then we use kde4_add_executable() to create an executable called tutorial1 from the source files listed in our tutorial1_SRCS variable. Finally we link our executable to the KDE4 kdeui library using target_link_libraries() and the KDE4_KDEUI_LIBS variable which was set by the find_package() function.

Make And Run

Again, if you set up your environment as described in Getting_Started/Build/Unstable_Version, you can compile this code with:

cmakekde

And launch it as:

./tutorial1.shell

Moving On

Now you can move on to using KXmlGuiWindow.