Object-oriented programming

Object-oriented programming (OOP) is a way of structuring code that centers on objects—bundles that combine data with the functions that operate on that data. In an OOP program, these objects communicate with each other. Because experts disagree on exactly which features define OOP, it’s often unclear whether a given language qualifies as object-oriented, and to what extent. Since programming paradigms aren’t mutually exclusive, a single language can belong to multiple categories, not just OOP.

The concept of “objects” in software first appeared in the late 1950s and early 1960s at MIT’s artificial intelligence group, where it referred to LISP atoms with defined attributes. Around the same time, Ivan Sutherland’s Sketchpad (1960–1961) used terms like “object” and “instance” in its glossary, though it focused on graphical interaction. In 1968, MIT’s AED-0, a version of ALGOL, linked data structures (called “plexes”) with procedures, foreshadowing what later became known as messages, methods, and member functions. During this period, data abstraction and modular programming were frequent topics of discussion.

Meanwhile, in Norway, Simula was developed between 1961 and 1967. It introduced core OOP ideas such as classes, inheritance, and dynamic binding. Simula was primarily used by researchers modeling physical systems, like ships moving through cargo ports, and is widely considered the first language to include the essential features of an object-oriented framework.

Influenced by both MIT’s work and Simula, Alan Kay began developing his own ideas in November 1966, eventually creating Smalltalk, a highly influential OOP language. By 1967, Kay was already using the phrase “object-oriented programming” in conversation. Though sometimes called the father of OOP, Kay has said his vision differs from how OOP is commonly practiced, and he has suggested that the computer science establishment didn’t fully adopt his concept. A 1976 MIT memo co-authored by Barbara Liskov listed Simula 67, CLU, and Alphard as object-oriented languages but did not mention Smalltalk.

In the 1970s, the first version of Smalltalk was built at Xerox PARC by Alan Kay, Dan Ingalls, and Adele Goldberg. Smalltalk-72 stood out for using objects at the language level and for its graphical development environment. It was a fully dynamic system, letting users create and modify classes while working. Much of OOP theory, such as multiple inheritance, was developed within the Smalltalk context.

OOP gained prominence in the late 1970s and 1980s. The Flavors object-oriented Lisp, started in 1979, introduced multiple inheritance and mixins. In August 1981, Byte Magazine featured Smalltalk and OOP, bringing these ideas to a broad audience. LOOPS, the object system for Interlisp-D, was influenced by Smalltalk and Flavors, with a paper published in 1982. The first OOPSLA conference in 1986 drew 1,000 attendees and helped consolidate Lisp object systems, eventually leading to the Common Lisp Object System. During the 1980s, some attempts were made to design processors with hardware support for objects in memory, such as the Intel iAPX 432 and the Linn Smart Rekursiv, but these were unsuccessful.

In the mid-1980s, new OOP languages emerged: Objective-C, C++, and Eiffel. Brad Cox developed Objective-C after using Smalltalk at ITT Inc. Bjarne Stroustrup created C++ based on his experience with Simula during his PhD. Bertrand Meyer designed Eiffel in 1985, focusing on software quality through a design by contract approach.

By the 1990s, OOP had become the dominant programming paradigm, especially as more languages adopted it, including Visual FoxPro 3.0, C++, and Delphi. Its popularity surged with graphical user interfaces, which used objects for buttons, menus, and other elements—Apple’s Cocoa framework for macOS, written in Objective-C, is a well-known example. OOP toolkits also boosted event-driven programming.

At ETH Zürich, Niklaus Wirth and his colleagues took a different approach with Modula-2 (1978) and Oberon (1987), which included object orientation, classes, and type checking across module boundaries. In Wirth’s design, inheritance is not obvious because he called it “type extension,” viewing the relationship from the parent down to the inheritor.

Many languages created before OOP was widespread have since been updated with object-oriented features, including Ada, BASIC, Fortran, Pascal, and COBOL.

The specific OOP features vary from language to language. Below are some common ones. Comparing OOP with other styles, such as relational programming, is tricky because there is no clear, universally accepted definition of OOP.

Encapsulation and information hiding can refer to several related ideas. Cohesion means keeping related fields and methods together. A field (also called an attribute or property) holds information (or state) as a variable.