CMake error while linking

I'm quite new to CMake and I'm trying to use it to build a little KDE application. I've to use QTXml module, my program compile with no problem but during the linking phase, ld fail to find QXml components...

main.cpp

#include "test.h"

int main(int argc, char **argv)
{
    return 0;
}

test.h

#ifndef TEST_H
#define TEST_H

#include <QXmlResultItems>
#include <QString>
#include <QBuffer>
#include <QXmlQuery>

class test {
public:
    test(){}
    ~test(){}
    QXmlResultItems* find ( const QString& node, const QString& xpath );
private:
    QBuffer device_;
};

#endif // TEST_H

test.cpp

#include "test.h"

QXmlResultItems* test::find ( const QString& node, const QString& xpath )
{
    QXmlResultItems* result = new QXmlResultItems;
    QXmlQuery query;
    query.bindVariable ( "device",&device_ );
    query.setQuery ( "doc($device)/"+node+"/"+xpath );
    query.evaluateTo ( result );
    return result;
}

CMakeLists.cmake

project(qtcmakepb)

find_package(KDE4 REQUIRED)
include (KDE4Defaults)

include_directories( ${KDE4_INCLUDES} ${QT_INCLUDES} )
#Supposed to be useless because of KDE4 REQUIRED and ${QT_INCLUDES}
find_package(Qt4 COMPONENTS QtCore QtXml REQUIRED )


# In this CMakeLists.txt we define which files
# are used to compile the application
set(qtcmakepb_SRCS main.cpp test.cpp)


# Set the name of the application
kde4_add_executable(qtcmakepb ${qtcmakepb_SRCS})

# Select which libraries we need to link to
target_link_libraries(qtcmakepb ${KDE4_KDEUI_LIBS})
target_link_libraries(qtcmakepb ${QT_QTCORE_LIBS})
target_link_libraries(qtcmakepb ${QT_QTXML_LIBS})

# Tell cmake to install the application binary
install(TARGETS qtcmakepb ${INSTALL_TARGETS_DEFAULT_ARGS})

# Install the .desktop file
install( PROGRAMS qtcmakepb.desktop  DESTINATION ${XDG_APPS_INSTALL_DIR} )

output for make :

Linking CXX executable qtcmakepb
CMakeFiles/qtcmakepb.dir/test.o: In function `test::find(QString const&, QString const&)':
/home/zelwina/projects/QtCmakePb/src/test.cpp:5: undefined reference to `QXmlResultItems::QXmlResultItems()'
/home/zelwina/projects/QtCmakePb/src/test.cpp:6: undefined reference to `QXmlQuery::QXmlQuery()'
/home/zelwina/projects/QtCmakePb/src/test.cpp:7: undefined reference to `QXmlQuery::bindVariable(QString const&, QIODevice*)'
/home/zelwina/projects/QtCmakePb/src/test.cpp:8: undefined reference to `QXmlQuery::setQuery(QString const&, QUrl const&)'
/home/zelwina/projects/QtCmakePb/src/test.cpp:9: undefined reference to `QXmlQuery::evaluateTo(QXmlResultItems*) const'
/home/zelwina/projects/QtCmakePb/src/test.cpp:10: undefined reference to `QXmlQuery::~QXmlQuery()'
collect2: erreur: ld a retourné 1 code d'état d'exécution
make[2]: *** [src/qtcmakepb] Erreur 1
make[1]: *** [src/CMakeFiles/qtcmakepb.dir/all] Erreur 2
make: *** [all] Erreur 2

What am I doing wrong ?

Solution

To use CMake's FindQt4 module, do the following:

find_package(Qt4 COMPONENTS QtCore QtXml REQUIRED)
include(${QT_USE_FILE})
include_directories(${KDE4_INCLUDES} ${QT_INCLUDES})
target_link_libraries(qtcmakepb ${KDE4_KDEUI_LIBS} ${QT_LIBRARIES})

If you want to specify the individual include dirs and libraries, then replace the last 2 lines above with:

include_directories(${KDE4_INCLUDES}
                    ${QT_QTCORE_INCLUDE_DIR}
                    ${QT_QTXML_INCLUDE_DIR})
target_link_libraries(qtcmakepb
                      ${KDE4_KDEUI_LIBS}
                      ${QT_QTCORE_LIBRARY}
                      ${QT_QTXML_LIBRARY})

Your problem is that you're not calling include(${QT_USE_FILE}), and that QT_QTCORE_LIBS should be QT_QTCORE_LIBRARY (and similarly for QtXml library). Furthermore, you need to call include_directories after you've invoked the FindQt4 module and included the QT_USE_FILE.

For full info on the FindQt4 module provided with your version of CMake, run:

cmake --help-module FindQt4

EDIT

Turns out that the root cause is actually that the undefined functions are part of the QtXmlPatterns library, so the find_package call should include QtXmlPatterns in the list.

When this is done, the variables ${QT_QTXMLPATTERNS_INCLUDE_DIR} and ${QT_QTXMLPATTERNS_LIBRARY} are set by the call include(${QT_USE_FILE}) and can be added as required.

IF YOU'RE USING QT5

With Qt5, using CMake becomes simpler.

In order to include and link QtXml and QtXmlPatterns, you only need these lines:

find_package(Qt5Xml REQUIRED)
find_package(Qt5XmlPatterns REQUIRED)

And linking as follows:

target_link_libraries(qtcmakepb Qt5::Xml Qt5::XmlPatterns)