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.
