When should Q_OBJECT be used?

The documentation states that:

The Q_OBJECT macro must appear in the private section of a class definition that declares its own signals and slots or that uses other services provided by Qt's meta-object system.

But exactly what does that mean? On which QObject-derived classes can I safely omit it? Will problems arise if you omit Q_OBJECT on a QObject-derived class, and then inherit from that one? Basically I would like a little more information on when I can omit it from my Qt classes.

Solution 1:

You should use the Q_OBJECT macro for any non-templated classes that derive from QObject.

Besides signals and slots, the Q_OBJECT macro provides the meta object information that is associated with given class.

As stated in the documentation:

we strongly recommend that all subclasses of QObject use the Q_OBJECT macro regardless of whether or not they actually use signals, slots, and properties.

Suppose we have the following class:

class Class : public QObject {
public:
  Class() {}
};

Without Q_OBJECT, the following metaobject system features (among others) will not work for Class:

1. qobject_cast<Class>() - due to missing metadata

2. QObject::tr() - due to missing metadata

3. slots and invokables first declared in Class, when invoked or looked up by name - none of QMetaObject methods will work for these methods, neither will the Qt 4 connect - due to missing metadata

4. signals - since moc won't generate their implementations and the code won't compile.

You can omit it, of course, but if you ever use these features, you'll need to remember to put the macro into the class's declaration. This is a rather brittle practice and best avoided. The savings are not worth it. So, don't wait - add the Q_OBJECT macro to every class that derives from QObject as a matter of coding policy.

The Q_OBJECT macro should never be used on classes that don't derive from QObject. To add invokables and properties to such classes, use the Q_GADGET macro instead.

Solution 2:

If you want to use signals/slots you MUST include the Q_OBJECT macro and derive the class from QObject.

Otherwise you can leave it out, but it doesn't do any harm to include it in all the Qt gui classes

Solution 3:

Well the first part is pretty clear as you probably already know.. signals and slots, the rest of the Meta-object system is a little lesser known. Perhaps one of the more useful features is dynamic properties. Although these have many uses, I used them to take advantage of Qt's animation system QPropertyAnimation.

There's a little more info about the meta-object system here: http://doc.qt.io/archives/4.6/metaobjects.html

I think the bottom line is, if you inherit from the QObject hierarchy, throw in the Q_OBJECT macro regardless. It's simple to do and will save you from some potentially baffling problems down the road.

Solution 4:

What @liaK said is correct (in short: you should always use the Q_OBJECT macro in any class that derives from QObject).

One thing that I haven't seen highlighted is that if you don't explicitly put the Q_OBJECT macro then using the sometimes very handy qobject_cast won't work!!!