When you create a QObject with another object as parent, the object will automatically add itself to the parent's children() list. QObjects organize themselves in object trees. The signals and slots system will be discussed at length in the next chapter.

The central feature in this model is a very powerful mechanism for seamless object communication called signals and slots. The QObject class is the base class of all Qt objects. The Qt Object Model and the QObject class For example, without a copy constructor, you can't use a subclass of QObject as the value to be stored in one of the container classes. The main consequence is that you should use pointers to QObject (or to your QObject subclass) where you might otherwise be tempted to use your QObject subclass as a value.

In fact, all Qt classes derived from QObject (direct or indirect) use this macro to declare their copy constructor and assignment operator to be private. Actually, they are declared, but in a private section with the macro Q_DISABLE_COPY(). QObject has neither a copy constructor nor an assignment operator this is by design.

It extends C++ with many dynamic features using a meta-object system. Identity type in turn derives from QObject. How this is done will be discussed later in the chapter, with the property system in general. This is useful when, for example, reading properties. Custom value types can be made known to the meta-object system by using the Q_DECLARE_METATYPE macro, which makes them storable in QVariant. Implicitly shared classes are both safe and efficient when passed as arguments, because only a pointer to the data is passed around, and the data is copied only if and when a function writes to it. Many of the Qt value types, such as QString and Qt Containers, also use implicit sharing (copy-on-write). The Meta-Object Compiler ( moc) will read the class definitions with the declared Q_OBJECT macro and produce the meta-object code.The Q_OBJECT macro is used to enable meta-object features when declared within a class definition.QObject is the base class that all objects in the meta-object system inherit from.The meta-object system is implemented with a three-part mechanism: Signals and slots is one of the most important concepts in Qt, and it will be discussed in the next chapter. Qt's meta-object system provides the signals and slots mechanism for inter-object communication, run-time type information (RTTI), and the dynamic property system.

Qt multiple signals to one slot