Qt5 -> Qt6 problem: hidden reference to Qt5?

Tom asso

I'm migrating from Qt5 to Qt6.7.2.
I have an app "qt-test" successfully built with Qt5. I've replaced references to "Qt5" with "Qt6"> Below is the qt-test CMakeLists.txt :

find_package(
  Qt6
  COMPONENTS Gui Widgets Quick
  REQUIRED)

set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)
set(CMAKE_AUTOUIC ON)

add_executable(qt-test main.cpp ToDoModel.cpp qml.qrc)

target_compile_options(qt-test PUBLIC "-Wunused-variable")

target_link_libraries(qt-test PRIVATE MBGui Qt6::Core Qt6::Quick
                      mbaux)

But now cmake gives this error:

CMake Error: The INTERFACE_QT_MAJOR_VERSION property of "Qt6::Core" does
not agree with the value of QT_MAJOR_VERSION already determined
for "qt-test".

How is QT_MAJOR_VERSION already determined for "qt-test"? I've done 'make clean' and removed CMakeCache.txt. I don't see explicit references to Qt5 in my source code. Where else should I look for references to Qt5?
Thanks!

Tom asso

Thanks everyone. I believe the problem is caused by linking my app with a third-party library that was itself built with Qt5.

Axel Spoerl

Please add this to your ``CMakeLists.txt```

message(STATUS "QT_MAJOR_VERSION=${QT_MAJOR_VERSION}")

My guess is, that it outputs "5". In that case, configure in an empty build directory and try again.

Tom asso

@Axel-Spoerl - cmake outputs the message

QT_MAJOR_VERSION=

I also print INTERFACE_QT_MAJOR_VERSION, which is also empty.
I tried rebuilding in an empty build directory, with the same result.
Thanks!

cristian-adam

cmake --trace-expand --trace-redirect=trace.txt and have a look through trace.txt and see what CMake is doing.

Tom asso

@cristian-adam - I ran cmake with --trace-expand, but I don't really know how to interpret the results.

One thing I did learn is that my ubuntu 22.04 system contains numerous Qt5 libraries in /usr/lib/ - but I don't dare to remove them as I think that ubuntu may depend on some of them:

$ find /usr/lib -name '*qt5*'
/usr/lib/x86_64-linux-gnu/metatypes/qt5core_metatypes.json
/usr/lib/x86_64-linux-gnu/metatypes/qt5gui_metatypes.json
/usr/lib/x86_64-linux-gnu/metatypes/qt5widgets_metatypes.json
/usr/lib/x86_64-linux-gnu/qtchooser/qt5.conf
/usr/lib/x86_64-linux-gnu/libqt5keychain.so.1
/usr/lib/x86_64-linux-gnu/libqca-qt5.so.2.3.4
/usr/lib/x86_64-linux-gnu/qt5
/usr/lib/x86_64-linux-gnu/qt5/plugins/designer/libpyqt5.so
/usr/lib/x86_64-linux-gnu/libqca-qt5.so.2
/usr/lib/x86_64-linux-gnu/libqt5keychain.so.0.13.2
/usr/lib/x86_64-linux-gnu/qca-qt5
/usr/lib/libqscintilla2_qt5.so.15.0
/usr/lib/libqwt-qt5.so.6.1.4
/usr/lib/libqwt-qt5.so.6
/usr/lib/qt5
/usr/lib/python3/dist-packages/matplotlib/backends/backend_qt5.py
/usr/lib/python3/dist-packages/matplotlib/backends/backend_qt5agg.py
/usr/lib/python3/dist-packages/matplotlib/backends/__pycache__/backend_qt5.cpython-310.pyc
/usr/lib/python3/dist-packages/matplotlib/backends/__pycache__/backend_qt5cairo.cpython-310.pyc
/usr/lib/python3/dist-packages/matplotlib/backends/__pycache__/backend_qt5agg.cpython-310.pyc
/usr/lib/python3/dist-packages/matplotlib/backends/backend_qt5cairo.py
/usr/lib/libqscintilla2_qt5.so.15
/usr/lib/libqscintilla2_qt5.so.15.0.0
/usr/lib/libqwt-qt5.so.6.1

Thanks!

cristian-adam

It's all about CMAKE_PREFIX_PATH and also CMAKE_IGNORE_PREFIX_PATH. This is how you control where CMake would look for packages.

You can add -D CMAKE_IGNORE_PREFIX_PATH=/usr/lib or -D CMAKE_IGNORE_PREFIX_PATH=/usr/lib/x86_64-linux-gnu to tell CMake to ignore looking into those directories.

In the trace log you can see what CMake is trying. If that's too much, you can look in the CMakeCache.txt to see what package found has an out of place directory.

Compiling on Linux where you can have system packages that can interfere, is more error prone.

Tom asso

Thanks everyone. I believe the problem is caused by linking my app with a third-party library that was itself built with Qt5.