How to draw on another widget?

Question

Qt

There is a Graph class that inherits QWidget , it creates a QWidget *full_window object , how to draw from the Graph class on the full_window widget.
Why is everything like this? - The Graph class is written for plotting graphs. It builds a miniature on itself, but if you double click on it, a full_window appears ( full_window->show() ), on which I want to draw a full-fledged graph.
Thought to do so
QPainter *full = new QPainter(full_window);
But in the console there are notifications like

QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1

Answer 1

Do not quite understand. If you have created a member widget. Then you just have to hide