Qt和C ++：双重继承与合成？

Question

My question is the following. I want to make a new widget which is a QTableWidget + a QMenuBar (with a particular configuration of the QTableWidget ). Currently, I have created a MyWidget which derives from QWidget and contains a QTableWidget + a QMenuBar . But is it possible to do the same thing by deriving MyWidget from QTableWidget and QMenuBar ? And if it is possible what the constructor looks like?

Answer 1

Since both QTableWidget and QMenuBar inherit from QWidget, I strongly recommend not using double inheritance. See, for example this question .

What your doing currently is generally considered the "right" way, at least in Qt. I suspect the main reason you want to use multiple inheritance instead of composition is that you might feel like you have to write a bunch of functions just so that people who are using your widget can, for example, add to the QMenuBar. Such a function might look like this:

QAction *MyWidget::addMenuToMyQMenuBar(QMenu *menu)
{
    return myMenu->addMenu(menu);
}

To me this is a bit silly, and instead I would recommend simply making an accessor function for your QMenuBar:

QMenuBar *MyWidget::menuBar()
{
    return myMenu;
}

Now someone using your class can just do

myWidget->menu()->addAction(someAction);

This style of coding occurs quite commonly in the Qt source code and I consider it much cleaner.

Answer 2

This sounds like composition will suit your needs.

Inheritance satisfies the is-a relationship (NewWidget is a QTableWidget and NewWidget is a QMenuBar?)
Composition satisfies the has-a relationship (NewWidget has a QTableWidget and NewWidget has a QMenuBar?)

Answer 3

It is impossible to inherit from two QWidget's at once. Each QWidget is also a QObject. MOC (Meta-Object compiler) cannot handle such classes.

Answer 4

Inheritance - The relationship is X isa Y

Composition - The relationship is X has a Y

There is your answer - what is the relationship between X and Y?