What's the difference between QMainWindow, QWidget and QDialog?

Question

What's the difference between QMainWindow, QWidget and QDialog?

Solution 1

A QWidget is the base class for all drawable classes in Qt. Any QWidget-based class can be shown as a window by showing it when it has no parent.

A QDialog is based on QWidget, but designed to be shown as a window. It will always appear in a window, and has functions to make it work well with common buttons on dialogs (accept, reject, etc.).

QMainWindow is designed around common needs for a main window to have. It has predefined places for a menu bar, a status bar, a toolbar, and other widgets. It does not have any built-in allowances for buttons like QDialog does.

Solution 2

QWidget :is a base class for all other GUI elements in QtWidgets module. It can constitute a window by itself, or be part of a QLayout, or just a member of parent-child hierarchy.

QDialog :is usually used to - surprise! - display a temporary dialog when user input is required.

QMainWindow :is a convenience class that can be used as the main window of your application. It has some nice features built-in: a status bar, tool bars and a menu bar

Comments

[Lilian A. Moraru]

I searched for this question but that's not really the answer I was waiting for. In QtDesigner using a QMainWindow gives you a menu bar and a status bar. When you write the code yourself and use QMainWindow you don't have this things. Or your saying that you can do something like window.addToolBar("Toolbar"); and it will place it at a predefined place without any other headache?

[Caleb Huitt - cjhuitt]

@MoraruLilian: I don't actually use QMainWindow, so I don't know the particulars. As I said in my answer, I know it has places for them, so doing it in code might be as simple as you state.

[Lilian A. Moraru]

@Caleb_Huitt It does what I am saying. I wanted just clear up if that's what you are trying to say. Anyway, I would recommend always using QWidget instead of QMainWindow. You can get into problems, for example with setting another Layout. If you want to set a new layout in QMainWindow you would have to create a QWidget and set it as a central widget for QMainWin and just then in QWidget you could setLayout(QLayout *). While you could avoid additional headaches just by using QWidget. Sure, QMainWindow offers that easy setup of the ToolBar and StatusBar but you can do that yourself in QWidget...

[ComradeJoecool]

One reason we are looking to use QMainWindow instead of just the base class QWidget is that QMainWindow has QDockWidget support whereas QWidget alone does not

[mLstudent33]

if I have a base class QMainWindow called calculator.ui, calculator.cpp, calculator.h, what does the constructor for derived class scientific_calculatror look like?

[Caleb Huitt - cjhuitt]

@mLstudent33 That sounds like a good question for documentation, and if that doesn't help, for asking here as a primary question rather than in the comments. One example (for Qt5) can be seen at doc.qt.io/qt-5/qtwidgets-mainwindows-application-example.htm‌​l