Outside Qt, code generators are common as well. It takes a user interface description in XML and creates C++ code that sets up the form. Another prominent example is the uic (User Interface Compiler). The moc is not the only code generator Qt is using. Typically moc is not called manually, but automatically by the build system, so it requires no additional effort by the programmer. The C++ source file generated by the moc must be compiled and linked with the implementation of the class (or it can be #included into the class's 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 those classes. It does so by generating additional C++ code which can be compiled by any standard C++ compiler. Qt's moc (Meta Object Compiler) provides a clean way to go beyond the compiled language's facilities.

This helps programmers get their design right from the very beginning, without even having to think about design patterns. People learning Qt find the syntax helps them understand and utilize the signals and slots concept - despite its highly abstract and generic nature. The syntax is intuitive, simple to use and easy to read. The syntax used for Qt's signals and slots has proved very successful in practice. Syntax isn't just sugar: the syntax we use to express our algorithms can significantly affect the readability and maintainability of our code. Here are some reasons why Qt uses the moc: Syntax Matters Code, unfortunately, is not meant to be published in books, but compiled with real-world compilers on real-world operating systems. But just because those things can be done, it does not necessarily mean doing them is the right design choice. Both templates and the C preprocessor can be stretched to do incredibly smart and mind boggling things. Things that you can hack with templates at the cost of code size, readability, portability, usability, extensability, robustness and ultimately design beauty. And then there is a gray area in between. A generic vector container class is easily expressible, even with partial specialisation for pointer types, while a function that sets up a graphical user interface based on an XML description given as a string is not expressible as a template. However, there are limitations: There are things that you can easily express with templates, and there are things that are impossible to express with templates. As such, templates are highly interesting to framework creators, and we do use advanced templates in many places in Qt. Templates are a builtin mechanism in C++ that allows the compiler to generate code on the fly, depending on the type of the arguments passed.

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