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Meta Object System

The Meta Object System in Qt is responsible for the signal/slot mechanism for communication between objects, runtime type information and the dynamic property system.

It 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' source file or compiled and linked with the class' 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 implements certain other features in QObject:

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

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

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

• two functions setProperty() and property() to dynamically set and get object properties by name.

• a function metaObject() 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.