官方的手册：

QWidget Class Reference

The QWidget class is the base class of all user interface objects. 

Inherits: QObject and QPaintDevice.

Inherited by: ……, QDialog, ……,QMainWindow, ……

QDialog Class Reference

The QDialog class is the base class of dialog windows.

Inherits: QWidget.

QMainWindow Class Reference

The QMainWindow class provides a main application window.

Inherits: QWidget.

可以看到QMainWindow和QDialog都是继承自QWidget。