Design Patterns: Elements of Reusable Object-Oriented Software (1994) is a software engineering book describing software design patterns. The book was written by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, with a foreword by Grady Booch. The book is divided into two parts, with the first two chapters exploring the capabilities and pitfalls of object-oriented programming, and the remaining chapters describing 23 classic software design patterns. The book includes examples in C++ and Smalltalk. It has been influential to the field of software engineering and is regarded as an important source for object-oriented design theory and practice. More than 500,000 copies have been sold in English and in 13 other languages. The authors are often referred to as the Gang of Four (GoF). History The book started at a birds of a feather (BoF) session at OOPSLA '90, "Towards an Architecture Handbook", run by Bruce Anderson, where Erich Gamma and Richard Helm met and discovered their common interest. They were later joined by Ralph Johnson and John Vlissides. The original publication date of the book was October 21, 1994 with a 1995 copyright, hence it is often cited with a 1995-year, despite being published in 1994. The book was first made available to the public at the OOPSLA meeting held in Portland, Oregon, in October 1994. In 2005 the ACM SIGPLAN awarded that year's Programming Languages Achievement Award to the authors, in recognition of the impact of their work "on programming practice and programming language design". As of March 2012, the book was in its 40th printing. Introduction Chapter 1 is a discussion of object-oriented design techniques, based on the authors' experience, which they believe would lead to good object-oriented software design, including: "Program to an interface, not an implementation." (Gang of Four 1995:18) Composition over inheritance: "Favor 'object composition' over 'class inheritance'." (Gang of Four 1995:20) The authors claim the following as advantages of interfaces over implementation: clients remain unaware of the specific types of objects they use, as long as the object adheres to the interface clients remain unaware of the classes that implement these objects; clients only know about the abstract class(es) defining the interface Use of an interface also leads to dynamic binding and polymorphism, which are central features of object-oriented programming. The authors refer to inheritance as white-box reuse, with white-box referring to visibility, because the internals of parent classes are often visible to