Meta Object System is a part of

Qt framework Qt (pronounced "cute") is cross-platform software for creating graphical user interfaces as well as cross-platform applications that run on various software and hardware platforms such as Linux, Windows, macOS, Android or embedded systems wit ...

core provided to support Qt extensions to

C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...

like signals/slots for inter-object communication, run-time type information, and the dynamic property system.

Architecture

The Meta object system consists of 3 things: QObject

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

, Q_OBJECT macro and a tool called moc (Meta-Object

Compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...

). QObject is the base class for all Qt classes, Q_OBJECT macro is used to enable meta-object features in classes and finally moc is a

preprocessor In computer science, a preprocessor (or precompiler) is a program that processes its input data to produce output that is used as input in another program. The output is said to be a preprocessed form of the input data, which is often used by so ...

that changes Q_OBJECT macro instances to

source code In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the wo ...

to enable meta object system mechanism in the class in which it is used. Using the meta object system has brought some criticism. In Qt documentation, several reasons have been given for the use of the meta object system, including benefits of code generation, dynamism of

GUI The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows users to interact with electronic devices through graphical icons and audio indicator such as primary notation, inste ...

s, automatic binding to scripting languages, not adding limitations and also reasonable performance in signal/slot implementation with moc. There are some efforts to make Qt needless of a preprocessor. These efforts include re-implementing Qt moc using libclang.

References

{{Qt Qt (software)