Add QMenuBar into QWidget

[Cucus]

At the beginning of the project, the bar wasn't necessary, but now they told me to do it. So I have a QWidget with all the project in.

[mcosta]

Create a QMainWindow and use your QWidget as centralWidget.

[gbstack]

Like mcosta said, using QMainWindow is a good choice.
But if you insisting using QWidget to add menu bar, this post may be helpful to you: Qt QWidget add menu bar