12.1. QMetaObject - The Meta Object Pattern

12.1. QMetaObject - The Meta Object Pattern
	Chapter 12. Meta Objects, Properties, and Reflective Programming

[ fromfile: reflection.xml id: metaobjects ]

A meta object is an object that describes the structure of another object.^[88]

A class that has a MetaObject supports reflection. This is a feature found in many object-oriented languages. It does not exist in C++, but Qt's MetaObject compiler, moc, generates the code to support this for specially equipped QObjects.

As long as certain conditions apply,^[89] each class derived from QObject has a QMetaObject generated for it by moc, similar to those we have for Customer and Address, shown in Figure 12.1. QObject has a member function that returns a pointer to the object's QMetaObject. The prototype of that function is

QMetaObject* QObject::metaObject () const [virtual]

You can use these methods of QMetaObject to get information about a QObject:

className(), which returns the class name as a const char*
superClass(), which returns a pointer to the QMetaObject of the base class if there is one (or 0 if there is not)
methodCount(), which returns the number of member functions of the class
Several other useful functions that we discuss in this chapter

The signal and slot mechanism also relies on the QMetaObject. Figure 12.1 shows the inheritance relationships between the Qt base classes, derived QObjects, and the moc-generated meta classes for them.

Figure 12.1. MetaObjects

By using the QMetaObject and QMetaProperty, you can write code that is generic enough to handle all self-describing classes.

^[88] Meta, the Latin root meaning about, is used for its literal definition here.

^[89]Each class must be defined in a header file, listed in the project file's HEADERS, and must include the Q_OBJECT macro in its class definition.


Chapter 12. Meta Objects, Properties, and Reflective Programming		12.2. Type Identification and qobject_cast