[cmake] Qt6 cmake errors

[jmackay2]

Describe the bug

When building with Qt6 on Ubuntu 24.04 with ROS installed, cmake produces the following error:

 CMake Error at /usr/lib/x86_64-linux-gnu/cmake/Qt6Core/Qt6CoreVersionlessTargets.cmake:42 (message):
   Some (but not all) targets in this export set were already defined.

   Targets Defined: Qt::Core

   Targets not yet defined: Qt::CorePrivate

 Call Stack (most recent call first):
   /usr/lib/x86_64-linux-gnu/cmake/Qt6Core/Qt6CoreConfig.cmake:65 (include)
   /usr/share/cmake-3.28/Modules/CMakeFindDependencyMacro.cmake:76 (find_package)
   /usr/lib/x86_64-linux-gnu/cmake/Qt6/QtPublicDependencyHelpers.cmake:108 (find_dependency)
   /usr/lib/x86_64-linux-gnu/cmake/Qt6Concurrent/Qt6ConcurrentDependencies.cmake:39 (_qt_internal_find_qt_dependencies)
   /usr/lib/x86_64-linux-gnu/cmake/Qt6Concurrent/Qt6ConcurrentConfig.cmake:50 (include)
   /usr/lib/x86_64-linux-gnu/cmake/Qt6/Qt6Config.cmake:167 (find_package)
   cmake/pcl_find_qt.cmake:28 (find_package)
   CMakeLists.txt:432 (include)

 Configuring incomplete, errors occurred!

Context

Building pcl with Qt6.

Expected behavior
CMake should be able to successfully find and link to Qt6 without the error.

Current Behavior

If you switch from WITH_QT=AUTO to WITH_QT=QT5 everything builds properly. Auto and QT6 produce the same error as seen above.

To Reproduce
Ubuntu 24.04 with ROS installed.
Install Qt6 from apt.
Try to run cmake for pcl with qt.

docker run -it --rm osrf/ros:rolling-desktop-full
git clone https://github.com/PointCloudLibrary/pcl.git
mkdir pcl/build
cd pcl/build
cmake ..

Your Environment (please complete the following information):

• OS: Ubuntu 24.04
• Compiler: CMake 3.28.3
• PCL Version: HEAD

[mvieth]

Hi, I have successfully built PCL with Qt6 a few times, so in principle, it should work. Which apt packages have you installed for Qt6? Perhaps you are missing a package? How did you install VTK? If you installed VTK also via apt, it will probably install and use Qt5, which will then likely create conflicts if PCL tries to use Qt6. So you probably have to uninstall VTK and Qt5, install Qt6, then compile VTK from source, then compile PCL from source. I will try to later look up the Qt6 apt packages I used.

[mvieth]

I was successful with these apt packages: qt6-base-dev libqt6widgets6 libqt6concurrent6 libqt6opengl6-dev libqt6openglwidgets6 qt6-declarative-dev qtchooser

[jmackay2]

Thank you for the quick reply @mvieth . I tried this on multiple computers, but all of them had ROS installed. VTK was installed with APT. Qt5 and Qt6 are both present on the machine. I will look at building VTK from source and see if it fixes my issue. I updated the reproduction instructions in the issue to give a minimal reproduce-able set of instructions for what I am seeing.

[mvieth]

So just to be sure I understand correctly: do you explicitly want PCL to use Qt6? Or do you not care whether it uses Qt5 or Qt6, it just happens that the default PCL setting (WITH_QT=AUTO) chooses Qt6 over Qt5 if both are installed? I am wondering if WITH_QT=AUTO should actually choose the Qt version that VTK also uses, for compatibility.

[jmackay2]

I confirmed that rebuilding VTK from source (using Qt6) does resolve the issue.

I was hopeful that PCL would use the newer Qt6 if it was installed. Now that I understand the issue, I agree with you. Since there is an issue using Qt6 unless VTK is also built with Qt6, I think the most reasonable thing would be for AUTO to use the same version as VTK so it builds successfully.