CMake: vcpkg-qt takes precedence over onlineinstaller-qt

[tuli]

Hi,

I am using the standard autogenerated CMake-template, with vpckg.
Thus I have this line at the very top:

set(CMAKE_TOOLCHAIN_FILE "C:/vcpkg/scripts/buildsystems/vcpkg.cmake" CACHE STRING "")

set(CMAKE_TOOLCHAIN_FILE "C:/vcpkg/scripts/buildsystems/vcpkg.cmake" CACHE STRING "")

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Everything works fine when I open this in QtCreater 8 and build it.
For Kits, I have 5.15 and 6.2 installed from the online installer. I configure both in QtCreator and can then switch between them in the build menu.


This works fine ... as long as no Qt is installed in vcpkg. If I install "qtbase" vcpkg package (which contains Qt 6.3), then the build will *always* use this Qt installation, no matter what I select in Qt Creator (even when I select Qt 5.15).


How can I resolve this situation so that CMake prefers the online installer Kits from QtCreator over the vcpkg package?

[d_stranz]

Somewhere, either in the CMakelists.txt file that Qt Creator makes for your project, or perhaps in the vcpkg.cmake toolchain file, there will be a find_package() command that loads the Qt references. This find_package() command takes an optional "version" argument that appears after the package name. There is also a "REQUIRED" argument which causes CMake to fail if the package can't be found.

There are some predefined constants that are passed in to the Qt package finder in CMake, one of which is QT_DEFAULT_MAJOR_VERSION. See the discussion here, especially the part about setting the value to either 5 or 6 before the find_package() command is called if you have mixed projects.

You will probably have to modify your CMakelists.txt file to define this appropriately. Qt Creator has a "%(Qt:Version)" variable which I assume changes with the kit, but I don't have any idea how to reference this from within CMake.

CMake can also take optional command line arguments that pass variables into CMake. I don't use Qt Creator much, especially not with CMake, so I do not know how Qt Creator invokes CMake. If you can specify a command line argument on a kit by kit basis, then that is where you could set a value for the QT_DEFAULT_MAJOR_VERSION variable.

vpckg

Thanks for this. I had no idea it existed and it looks like it could be useful. Sort of Microsoft's answer for the C++ world to Python's pip and conda I suppose.

[tuli]

> vcpkg

yep, it's great. I prefer it over conan. Unfortunately Qt package for Linux is barely usable, it ships only static version and has a bazillion dependencies on system-packages (at least that was the case a year ago).


I made some progress with my issue:

Default generated finding of Qt is this:

find_package(QT NAMES Qt6 Qt5 COMPONENTS Widgets REQUIRED)
find_package(Qt${QT_VERSION_MAJOR} COMPONENTS Widgets REQUIRED)

find_package(QT NAMES Qt6 Qt5 COMPONENTS Widgets REQUIRED)
find_package(Qt${QT_VERSION_MAJOR} COMPONENTS Widgets REQUIRED)

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Where the first findpackage() finds either Qt6 or Qt5 (prefers 6, if that doesnt exist looks for 5) and the second call does the actual importing/configuring.
I can (and do) force the major version to 5 by setting QT_VERSION_MAJOR to 5 and skipping the first call.


The reason why CMake prefers vcpkg's Qt package over QtCreators Kits is because the ONLY way QtCreator selects between Kits is setting CMAKE_PREFIX_PATH to the full path of the Kit. Hence if I select Qt5Kit in QtCreator find_package() only gets a path to Qt5, thus Qt6 cant be found and will default to Qt5.

There are two issues with that:

1. If vcpkg joins the party, CMAKE_PREFIX_PATH will contain paths to the Kit as well as to vcpkg packages. Now there is always a Qt6 found (the vcpkg one), and the path to the Qt5 Kit passed by QtCreator is ignored.

2. Actually QtCreator appends the Kit path to CMAKE_PREFIX_PATH, vcpkg is faster, and its path comes before the Kit path. Thus if both vcpkg and QtCreator offer a Qt6 package, vcpkg satisfies find_package() first.


Problem 2 can be worked around like so:

list(POP_BACK CMAKE_PREFIX_PATH XX)
list(PREPEND CMAKE_PREFIX_PATH ${XX})

list(POP_BACK CMAKE_PREFIX_PATH XX)
list(PREPEND CMAKE_PREFIX_PATH ${XX})

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Unfortunately I see no good way around Problem 1, besides very ugly string matching against the path to find out if the path is Qt5 or Qt6...
If someone sees a good solution, please let me know...

---

Added after 1 39 minutes:

Ah, QtCreator sets Qt5_DIR or Qt6_DIR variables, depending on which one you select. If the project is built directly from CMake then neither of them is set.
This should solve the second problem as well, I will check which variable is defined, and set QT_VERSION_MAJOR accordingly. If both are undefined I let find_package do its thing.

edit: Nope, it will happily set both variables if it finds both version.... the search continues ...

[d_stranz]

the search continues ...

You're already well past any help I can give, so good luck and please report back when you find a solution.

I have 3 CMake books: Mastering CMake, CMake Cookbook, and Modern CMake for C++. I'll look through those and see if anything pops out.

Edit:

find_package(Qt${QT_VERSION_MAJOR} COMPONENTS Widgets REQUIRED)

The link I posted above refers to a QT_DEFAULT_MAJOR_VERSION, not a QT_VERSION_MAJOR. Maybe using the former set to 5 or 6 will work?

[tuli]

QT_DEFAULT_MAJOR_VERSION only helps to select between Qt5 and Qt6, which can be achieved e.g. by changing the order in the find_package call too, and sadly doesnt help here.
I asked some other people and here are the results of this thread summarized:


1. detect if built from within QtCreator vs plain Cmake: CMAKE_PREFIX_PATH is set to the path to the Qt Kit *before* the first call to project() vs being empty otherwise.
2. detect if path is Qt5/Qt6: check for existence of e.g. Src/..../Qt6Config.cmake vs Qt5Config.cmake
3. select between Qt5/Qt6 with QT_DEFAULT_MAJOR_VERSION or QT_MAJOR_VERSION (or a variety of other ways)
4. it's not possible to ignore the vcpkg package in favor the QtCreator Kit. There are two solutions: a) maintain a separate vcpkg checkout that has no Qt package installed b) use vcpkg.json file, which basically does a) for you.

[d_stranz]

4. it's not possible to ignore the vcpkg package in favor the QtCreator Kit. There are two solutions: a) maintain a separate vcpkg checkout that has no Qt package installed b) use vcpkg.json file, which basically does a) for you.

Thanks for the update. It sounds like this is the way to go. Except for the convenience of having vcpkg manage the Qt build for you, is there a problem with not having a Qt package within vcpkg? If you are using Qt Creator for your builds, then selecting the right kit seems like it would solve the Qt version problem. (This being asked based on a lack of familiarity with both Qt Creator and vcpkg - I have used Visual Studio almost exclusively for literally decades and have manually managed project configurations).