Where is code file

[jacek]

The justification for such a pedantic approach is that the people doing these exercises are most likely beginners at C++, are also beginners at Qt, probably a Linux newbie

IMO one should learn one thing at a time. You are right that tutorial authors shouldn't overestimate the readers' knowledge, but you just can't learn 3 different things at the same time (and if you try to do that, you can't blame the author).

The error line [...] implies that QObject is not recognised, and the rest follows logically from whatever caused that.[...] Still looking at that.

Do you have #include <QObject> at the beginning of numberholder.h?