Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. It was the main programming language supported by Apple for macOS, iOS, and their respective application programming interfaces (APIs), Cocoa and Cocoa Touch, until the introduction of Swift in 2014.
The programming language Objective-C was originally developed in the early 1980s. It was selected as the main language used by NeXT for its NeXTSTEP operating system, from which macOS and iOS are derived. Portable Objective-C programs that do not use the Cocoa or Cocoa Touch libraries, or those using parts that may be ported or reimplemented for other systems, can also be compiled for any system supported by GNU Compiler Collection (GCC) or Clang.
Objective-C source code 'implementation' program files usually have .m filename extensions, while Objective-C 'header/interface' files have .h extensions, the same as C header files. Objective-C++ files are denoted with a .mm file extension.
Objective-C is a thin layer atop C, and is a "strict superset" of C, meaning that it is possible to compile any C program with an Objective-C compiler and to freely include C language code within an Objective-C class. Objective-C derives its object syntax from Smalltalk. All syntax for non-object-oriented operations (including primitive variables, pre-processing, expressions, function declarations, and function calls) are identical to those of C, while the syntax for object-oriented features is an implementation of Smalltalk-style messaging.
Objective-C is a strict superset of C, meaning that any valid C code is also valid Objective-C code. Object-oriented programming is done using object-oriented versions of C's struct and union constructs, which are called classes and which include the ability to define methods and instance variables. In most cases, software written in Objective-C can be compiled without modification on older versions of the operating system.
Objective-C source code 'implementation' program files usually have .m filename extensions, while Objective-C 'header/interface