Q_OBJECT macro and meta-object code

Question

This link: http://doc.trolltech.com/4.5/moc.html#moc says

The moc tool reads a C++ header file. If it finds one or more class declarations that contain the Q_OBJECT macro, it produces a C++ source file containing the meta-object code for those classes.

What is a meta object code?

EDIT 1

How to know in which classes I should write the Q_OBJECT? One example is the signals and slots, any other cases where that needs to be used?

Answer 1

You can read the article Qt internals and Reversing to get in depth knowledge about Qt and its moc compiler (meta objet compiler). In summary a meta object is created by Qt's moc compiler to add extra information to a class like signal/slot mechanism etc.

Answer 2

Meta objects enhance programming languages by creating new or manipulate existing objects. They provide functionalities a language does not actually have by itself. The Meta Objects are interpreted either by compile time or run time. In Qt and C++ it is done during compile time by the Meta Object Compiler (moc).

An example case is the usage of the signal/slot concept.

Answer 3

A meta object code in Qt environment is a C++ source file that is an expanded version of the C++ source file where you've put Q_OBJECT (and/or other related macros). The meta object code will have your implementation plus some other extra (meta) code so that signal and slots mechanism work.

Answer 4

A meta-object contains meta-information about an object like its name and a textual description of its signals and slots. This make it possible to call signal by "name". See the documentation about QMetaObject and this article.