undefined reference errors using QtMultimedia in qt 6.8.3 c++

andi456

Hi,

for some reason the ld does not seem to find the references to the QtMultimedia objects, I'd like to use. In my .h file, I have the following two lines after #include <QtMultimedia/QAudioOutput> and #include <QtMultimedia/QMediaPlayer>:

QAudioOutput *aout;
QMediaPlayer *player;

the objects are then instantiated in my .cpp file like this:
player = new QMediaPlayer();
aout = new QAudioOutput();

I followed pretty much the instructions in from the audio overview.
My aim is rather simple: Playing a sound after some seconds. An example of the errors occuring:
Error: /home/andi/Projekte/qt/Countdown/qcountdown.cpp:13:(.text+0x557): undefined reference to `QAudioOutput::QAudioOutput(QObject)'*

Btw., I'm using a recent version of qtcreator on a gentoo linux box.

Kind regards,

Andreas

Christian Ehrlicher

You do not link against the QtMultmedia library: https://doc.qt.io/qt-6/qaudiooutput.html

andi456

If I try
#include <QAudioOutput>
the compiler tells me: no such file or directory. So how could I make the compiler or linker happy?

Christian Ehrlicher

I gave you a link where it's documented what you have to do when using qmake or cmake...

SGaist

@andi456 hi,

Including a header is not sufficient to "use" a library.

As pointed by @Christian-Ehrlicher, you have to tell the build system that you intend to use said libraries.

CJF the Elder

If you have to put forward slashes in a Qt include directive to find the file means you are hosed, or without forward slashes, it can't be found. QAudio is a module that requires you to edit the CMakeLists file and add the reference in two places. I had to do the same thing for Charts and SerialPort.

under the packages, heading i need serialport and charts:
find_package(QT NAMES Qt6 Qt5 REQUIRED COMPONENTS Widgets)
find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Widgets)
find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS SerialPort)
find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Charts)

and here i have to add the libs for linking:
target_link_libraries(myProject PRIVATE Qt${QT_VERSION_MAJOR}::Widgets)
target_link_libraries(myProject PRIVATE
Qt${QT_VERSION_MAJOR}::SerialPort
Qt${QT_VERSION_MAJOR}::PrintSupport
Qt${QT_VERSION_MAJOR}::Charts)

Christian Ehrlicher

@CJF-the-Elder that's exactly what's written in the docs I linked...

andi456

Thanks for the answers. I figured it out for the cmake build system.

For anyone who stumbles across a similar problem. I had to edit three lines in CMakeLists.txt, i.e., essentially add the keyword "Multimedia" to them:
find_package(QT NAMES Qt6 Qt5 REQUIRED COMPONENTS Widgets Multimedia)
find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Widgets Multimedia)

target_link_libraries(Countdown PRIVATE Qt${QT_VERSION_MAJOR}::Widgets Qt${QT_VERSION_MAJOR}::Multimedia)

After that compiler and linker do find the corresponding files.