QMetaObject: The MetaObject Pattern

QMetaObject The MetaObject Pattern

By abstracting the abstract data type itself, we achieve what is called a MetaObject. A MetaObject is an object that describes the structure of another object.[2]

[2] Meta, the latin root meaning about, is used for its literal definition here.

The MetaObject Pattern

The QMetaObject is Qt's implementation of the MetaObject pattern. It provides information about properties and methods of a QObject. The MetaObject pattern is sometimes known as the Reflection pattern.

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 desired classes.

As long as certain conditions apply,[3] each class derived from QObject will have a QMetaObject generated for it by moc, as shown in Figure 15.2. QObject has a member function that returns a pointer to the object's QMetaObject.

[3] 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.

Figure 15.2. MetaObjects

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

A QMetaObject can be used to invoke functions such as:

• 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 will discuss in this chapter

The signal and slot mechanism also relies on the QMetaObject.

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