@J-Hilk said in Trying to create a simple subclass: try deleting your shadow build project, maybe old build artifacts cause problems here @ffej2ffej As @J-Hilk says. It should not be like this, but it's a nasty lesson to learn with Qt building in practice: whenever you get an "inexplicable" compile/link message and you are "sure" your code is correct, start by deleting all the contents of the build output directory and rebuild from scratch, before puzzling further. This applies particularly when you have made a change in a header file to do with classes inheriting from QObject/changing whether they have Q_OBJECT macro, and similar....