Why would QT_VERSION_MAJOR be empty?

Stokestack

We have a Qt 6 CMake-based project. In CMakeLists, we try to concatenate the Qt version like so:

Qt${QT_VERSION_MAJOR}::Core

This was failing, apparently because QT_VERSION_MAJOR is an empty string. Why would that be?

Paul Colby

QT_VERSION_MAJOR is set by find_package(QT ...), so either it failed, or you didn't call it.

Do you have something like this before you use Qt${QT_VERSION_MAJOR}::Core?

find_package(QT REQUIRED COMPONENTS Core NAMES Qt6 Qt5)
message(STATUS "Found Qt ${Qt${QT_VERSION_MAJOR}_VERSION}")

The message() is optional, and just for diagnostics, but I like to include it in my projects.

If your find_package() doesn't include REQUIRED, then you should also check the package (Qt) was actually found.

Cheers.