Meta-object System

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Meta Object System is a part of Qt framework core provided to support Qt extensions to C++ like signals/slots for inter-object communication, run-time type information, and the dynamic property system. [1]

C++ general purpose high-level programming language

C++ is a general-purpose programming language that was developed by Bjarne Stroustrup as an extension of the C language, or "C with Classes". It has imperative, object-oriented and generic programming features, while also providing facilities for low-level memory manipulation. It is almost always implemented as a compiled language, and many vendors provide C++ compilers, including the Free Software Foundation, Microsoft, Intel, and IBM, so it is available on many platforms.

Signals and slots is a language construct introduced in Qt for communication between objects which makes it easy to implement the observer pattern while avoiding boilerplate code. The concept is that GUI widgets can send signals containing event information which can be received by other widgets / controls using special functions known as slots. This is similar to C/C++ function pointers, but signal/slot system ensures the type-correctness of callback arguments.

In programming languages, a type system is a set of rules that assigns a property called type to the various constructs of a computer program, such as variables, expressions, functions or modules. These types formalize and enforce the otherwise implicit categories the programmer uses for algebraic data types, data structures, or other components. The main purpose of a type system is to reduce possibilities for bugs in computer programs by defining interfaces between different parts of a computer program, and then checking that the parts have been connected in a consistent way. This checking can happen statically, dynamically, or as a combination of static and dynamic checking. Type systems have other purposes as well, such as expressing business rules, enabling certain compiler optimizations, allowing for multiple dispatch, providing a form of documentation, etc.

Architecture

The Meta object system consists of 3 things: QObject class, Q_OBJECT macro and a tool called moc (Meta-Object Compiler). 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 that changes Q_OBJECT macro instances to C++ source code to enable meta object system mechanism in the class in which it is used. [2]

In object-oriented programming, a class is an extensible program-code-template for creating objects, providing initial values for state and implementations of behavior. In many languages, the class name is used as the name for the class, the name for the default constructor of the class, and as the type of objects generated by instantiating the class; these distinct concepts are easily conflated.

Macro (computer science) rule or pattern that specifies how a certain input sequence should be mapped to a replacement output sequence according to a defined procedure

A macro in computer science is a rule or pattern that specifies how a certain input sequence should be mapped to a replacement output sequence according to a defined procedure. The mapping process that instantiates (transforms) a macro use into a specific sequence is known as macro expansion. A facility for writing macros may be provided as part of a software application or as a part of a programming language. In the former case, macros are used to make tasks using the application less repetitive. In the latter case, they are a tool that allows a programmer to enable code reuse or even to design domain-specific languages.

A compiler is a computer program that translates computer code written in one programming language into another programming language. The name compiler is primarily used for programs that translate source code from a high-level programming language to a lower level language to create an executable program.

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 GUIs, automatic binding to scripting languages, not adding limitations and also reasonable performance in signal/slot implementation with moc. [3] There are some efforts to make Qt needless of a preprocessor. These efforts include re-implementing Qt moc using libclang. [4]

In computing, code generation is the process by which a compiler's code generator converts some intermediate representation of source code into a form that can be readily executed by a machine.

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Qt (software) Object-oriented framework for GUI creation

Qt is a free and open-source widget toolkit 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 with little or no change in the underlying codebase while still being a native application with native capabilities and speed. Qt is currently being developed by The Qt Company, a publicly listed company, and the Qt Project under open-source governance, involving individual developers and organizations working to advance Qt. Qt is available under both commercial licenses and open source GPL 2.0, GPL 3.0, and LGPL 3.0 licenses.

Templates are a feature of the C++ programming language that allows functions and classes to operate with generic types. This allows a function or class to work on many different data types without being rewritten for each one.

In computer science, a preprocessor is a program that processes its input data to produce output that is used as input to another program. The output is said to be a preprocessed form of the input data, which is often used by some subsequent programs like compilers. The amount and kind of processing done depends on the nature of the preprocessor; some preprocessors are only capable of performing relatively simple textual substitutions and macro expansions, while others have the power of full-fledged programming languages.

Template metaprogramming (TMP) is a metaprogramming technique in which templates are used by a compiler to generate temporary source code, which is merged by the compiler with the rest of the source code and then compiled. The output of these templates include compile-time constants, data structures, and complete functions. The use of templates can be thought of as compile-time execution. The technique is used by a number of languages, the best-known being C++, but also Curl, D, and XL.

Programming languages can be grouped by the number and types of paradigms supported.

The C preprocessor or cpp is the macro preprocessor for the C and C++ computer programming languages. The preprocessor provides the ability for the inclusion of header files, macro expansions, conditional compilation, and line control.

Metaprogramming is a programming technique in which computer programs have the ability to treat other programs as their data. It means that a program can be designed to read, generate, analyze or transform other programs, and even modify itself while running. In some cases, this allows programmers to minimize the number of lines of code to express a solution, in turn reducing development time. It also allows programs greater flexibility to efficiently handle new situations without recompilation.

PyQt

PyQt is a Python binding of the cross-platform GUI toolkit Qt, implemented as a Python plug-in. PyQt is free software developed by the British firm Riverbank Computing. It is available under similar terms to Qt versions older than 4.5; this means a variety of licenses including GNU General Public License (GPL) and commercial license, but not the GNU Lesser General Public License (LGPL). PyQt supports Microsoft Windows as well as various flavours of UNIX, including Linux and MacOS.

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Racket is a general-purpose, multi-paradigm programming language based on the Scheme dialect of Lisp. It is designed to be a platform for programming language design and implementation. Racket is also used for scripting, computer science education, and research.

libsigc++ is a C++ library for typesafe callbacks.

QtScript is a scripting engine that has been part of the Qt cross-platform application framework since version 4.3.0.

qmake is a utility that automates the generation of makefiles. Makefiles are used by the program make to build executable programs from source code; therefore qmake is a make-makefile tool, or makemake for short.

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Mono (software) computer software project

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Nim (programming language) programming language

Nim is an imperative, general-purpose, multi-paradigm, statically typed, systems, compiled programming language designed and developed by Andreas Rumpf. It is designed to be "efficient, expressive, and elegant", supporting metaprogramming, functional, message passing, procedural, and object-oriented programming styles by providing several features such as compile time code generation, algebraic data types, a foreign function interface (FFI) with C/C++ and compiling to C, C++, Objective-C and Javascript.

References

  1. ↑ "The Meta-Object System - QtCore 5.1". Qt Project. 2013-07-04. Retrieved 2013-07-04.
  2. ↑ "Using the Meta-Object Compiler (moc)". Qt Project. 2013-07-04. Retrieved 2013-07-04.
  3. ↑ "Why Does Qt Use Moc for Signals and Slots?". Qt Project. 2013-07-04. Retrieved 2013-07-04.
  4. ↑ "Re-implementing Qt moc using libclang". Olivier Goffart. 2013-07-04. Retrieved 2013-07-04.
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