View source for Development/Tutorials/First program

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{{TutorialBrowser|

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series=Beginner Tutorial|

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name=Hello World|

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pre=[http://mindview.net/Books/TICPP/ThinkingInCPP2e.html C++], [http://qt.nokia.com Qt], [[Getting_Started/Build|Building KDE]]|

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next=[[Development/Tutorials/Using_KXmlGuiWindow|Tutorial 2 - KXmlGuiWindow]]| 

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reading=[[Development/Tutorials/CMake|CMake]]
}}

==Abstract== <!--T:6-->

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

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[[image:Introtokdetutorial1-kf5.png|frame|center]]

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{{Tip|To get more information about any class you come across, you can use the ‘kde’ search engine. For example, to look for information about KMessageBox, just type "kde:kmessagebox" into Konqueror, Rekonq or KRunner, and you’ll be taken to the documentation.}}

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{{Tip|
You might want to use [[KDevelop|KDevelop]] or [[qtcreator|QtCreator]] as IDE for your projects.
}}

==The Code== <!--T:10-->

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All the code we need will be in one file, <tt>main.cpp</tt>. Create that file with the code below:
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<syntaxhighlight lang="cpp-qt">
#include <cstdlib>

#include <QApplication>
#include <QCommandLineParser>
#include <KAboutData>
#include <KLocalizedString>
#include <KMessageBox>

int main (int argc, char *argv[])
{
    QApplication app(argc, argv);
    KLocalizedString::setApplicationDomain("tutorial1");

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    <!--T:60-->
KAboutData aboutData(
                         // The program name used internally. (componentName)
                         QStringLiteral("tutorial1"),
                         // A displayable program name string. (displayName)
                         i18n("Tutorial 1"),
                         // The program version string. (version)
                         QStringLiteral("1.0"),
                         // Short description of what the app does. (shortDescription)
                         i18n("Displays a KMessageBox popup"),
                         // The license this code is released under
                         KAboutLicense::GPL,
                         // Copyright Statement (copyrightStatement = QString())
                         i18n("(c) 2015"),
                         // Optional text shown in the About box.
                         // Can contain any information desired. (otherText)
                         i18n("Some text..."),
                         // The program homepage string. (homePageAddress = QString())
                         QStringLiteral("http://example.com/"),
                         // The bug report email address
                         // (bugsEmailAddress = QLatin1String("submit@bugs.kde.org")
                         QStringLiteral("submit@bugs.kde.org"));
    aboutData.addAuthor(i18n("Name"), i18n("Task"), QStringLiteral("your@email.com"),
                         QStringLiteral("http://your.website.com"), QStringLiteral("OSC Username"));
    KAboutData::setApplicationData(aboutData);
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    QCommandLineParser parser;
    aboutData.setupCommandLine(&parser);
    parser.process(app);
    aboutData.processCommandLine(&parser);
    
    KGuiItem yesButton( i18n( "Hello" ), QString(),
                        i18n( "This is a tooltip" ),
                        i18n( "This is a WhatsThis help text." ) );

return 
        KMessageBox::questionYesNo 
        (0, i18n( "Hello World" ), i18n( "Hello" ), yesButton ) 
        == KMessageBox::Yes? EXIT_SUCCESS: EXIT_FAILURE;
}
</syntaxhighlight>

<translate>
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First we need to create a [http://doc.qt.io/qt-5/qapplication.html QApplication] object. This needs to be done exactly once in each program since it is needed for things such as [[Development/Tutorials/Localization/i18n|i18n]]. It also should be created before any other KDE or Qt object. A call to {{class|KLocalizedString}}::setApplicationDomain() is required to properly set the translation catalog and must be done before the next step happens.

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The first KDE specific object we create in this program is {{class|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. We then call {{class|KAboutData}}::setApplicationData() to initialize the properties of the [http://doc.qt.io/qt-5/qapplication.html QApplication] object.

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Then we come to [http://doc.qt.io/qt-5/qcommandlineparser.html QCommandLineParser]. 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 {{class|KAboutData}} object we created so we can use the <tt>--version</tt> or <tt>--author</tt> switches.

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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 {{class|KGuiItem}} object. The first argument in the {{class|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 <tt>QString()</tt>. We then 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.

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Now we have our item, we can create our popup. We call the <tt>{{class|KMessageBox}}::questionYesNo()</tt> 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 message box above the buttons. 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 <tt>KGuiItem yesButton</tt> we created.

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Note that all user-visible text is passed through the i18n() function; this is necessary for the UI to be translatable. More information on localization can be found in the [[Development/Tutorials/Localization/i18n|localization tutorial]].

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

== Build == <!--T:21-->

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You want to [[Development/Tutorials/CMake|use CMake]] for your build environment. You provide a file CMakeLists.txt, cmake uses this file to generate all Makefiles out of it.

=== CMakeLists.txt === <!--T:23-->

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Create a file named CMakeLists.txt in the same directory as main.cpp with this content:
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<syntaxhighlight lang="cmake">
cmake_minimum_required(VERSION 3.0)

project (tutorial1)

set(QT_MIN_VERSION "5.3.0")
set(KF5_MIN_VERSION "5.2.0")

find_package(ECM 1.0.0 REQUIRED NO_MODULE)
set(CMAKE_MODULE_PATH ${ECM_MODULE_PATH} ${ECM_KDE_MODULE_DIR} ${CMAKE_CURRENT_SOURCE_DIR}/cmake)

include(KDEInstallDirs)
include(KDECMakeSettings)
include(KDECompilerSettings NO_POLICY_SCOPE)
include(FeatureSummary)

# Find Qt modules
find_package(Qt5 ${QT_MIN_VERSION} CONFIG REQUIRED COMPONENTS 
    Core    # QCommandLineParser, QStringLiteral
    Widgets # QApplication 
)

# Find KDE modules
find_package(KF5 ${KF5_MIN_VERSION} REQUIRED COMPONENTS
    CoreAddons      # KAboutData
    I18n            # KLocalizedString
    WidgetsAddons   # KMessageBox
)

feature_summary(WHAT ALL INCLUDE_QUIET_PACKAGES FATAL_ON_MISSING_REQUIRED_PACKAGES)
    
set(tutorial1_SRCS main.cpp)

add_executable(tutorial1 ${tutorial1_SRCS})

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target_link_libraries(tutorial1
    Qt5::Widgets
    KF5::CoreAddons
    KF5::I18n
    KF5::WidgetsAddons
)

install(TARGETS tutorial1  ${INSTALL_TARGETS_DEFAULT_ARGS})
</syntaxhighlight>

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The <tt>find_package()</tt> function locates the package that you ask it for (in this case ECM, Qt5, or KF5) and sets some variables describing the location of the package's headers and libraries. ECM, or Extra CMake Modules, is required to import special CMake files and functions for building KDE applications.

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Here we try to find the modules for Qt 5 and KDE Frameworks 5 required to build our tutorial. The necessary files are included by CMake so that the compiler can see them at build time. Minimum version numbers are set at the very top of CMakeLists.txt file for easier reference.

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

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Then we use <tt>add_executable()</tt> to create an executable called <tt>tutorial1</tt> from the source files listed in our <tt>tutorial1_SRCS</tt> variable. Afterwards, we link our executable to the necessary libraries using <tt>target_link_libraries()</tt> function. The line starting with <tt>install</tt> writes a default "install" target into the Makefile.

=== Make And Run === <!--T:28-->

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To compile, link and install your program, you must have several software installed, e.g. cmake, make and gcc-c++, and the Qt 5 and KDE Frameworks development files. To be sure you have everything, best follow [[Getting_Started/Build/Environment|this install guide]].

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While you can run cmake directly inside the source code directory itself, it is a best practice, and actually enforced in some KDE software, to use a separate build directory and run cmake from there:
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<syntaxhighlight lang="bash">
mkdir build && cd build
</syntaxhighlight>
You can invoke CMake and make manually:
<syntaxhighlight lang="bash">
cmake .. && make
</syntaxhighlight>

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And launch it with:
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<syntaxhighlight lang="bash">
./tutorial1
</syntaxhighlight>

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==Moving On== <!--T:32-->

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Now you can move on to [[Development/Tutorials/Using_KXmlGuiWindow|using KXmlGuiWindow]].

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{{Tip||The source code on this page applies only the current KDE Frameworks 5 ("KF5") version. For the older KDE Development Platform ("KDE4"), See [[Development/Tutorials/First_program/KDE4]]}}

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[[Category:C++]]
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