Meta Object System

Qt's Meta Object System provides the signals and slots mechanism for inter-object communication, runtime type information, and the dynamic property system.

The Meta Object System is based on three things:

1. the QObject class;
2. the Q_OBJECT macro inside the private section of the class declaration;
3. the Meta Object Compiler (moc).

The moc reads a C++ source file. If it finds one or more class declarations that contain the Q_OBJECT macro, it produces another C++ source file which contains the meta object code for the classes that contain the Q_OBJECT macro. This generated source file is either #included into the class's source file or compiled and linked with the class's implementation.

In addition to providing the signals and slots mechanism for communication between objects (the main reason for introducing the system), the meta object code provides additional features in QObject:

• the className() function that returns the class name as a string at runtime, without requiring native runtime type information (RTTI) support through the C++ compiler.

• the inherits() function that returns whether an object is an instance of a class that inherits a specified class within the QObject inheritance tree.

• the tr() and trUtf8() functions for string translation as used for internationalization.

• the setProperty() and property() functions for dynamically setting and getting object properties by name.

• the metaObject() function that returns the associated meta object for the class.

While it is possible to use QObject as a base class without the Q_OBJECT macro and without meta object code, neither signals and slots nor the other features described here will be available if the Q_OBJECT macro is not used. From the meta object system's point of view, a QObject subclass without meta code is equivalent to its closest ancestor with meta object code. This means for example, that className() will not return the actual name of your class, but the class name of this ancestor. We strongly recommend that all subclasses of QObject use the Q_OBJECT macro regardless of whether they actually use signals, slots and properties or not.