The 23 classic design patterns

In 1994 Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, the "Gang of Four", published Design Patterns: Elements of Reusable Object-Oriented Software with Addison-Wesley. It catalogues 23 patterns in three groups.

Creational (5)

Pattern	In one line
Abstract Factory	Create families of related objects without naming concrete classes.
Builder	Assemble a complex object step by step.
Factory Method	Let subclasses decide which class to instantiate.
Prototype	Create new objects by copying an existing one.
Singleton	Guarantee a class has exactly one instance.

Structural (7)

Pattern	In one line
Adapter	Make an incompatible interface fit the one clients expect.
Bridge	Separate an abstraction from its implementation.
Composite	Treat single objects and trees of objects the same way.
Decorator	Add behaviour to an object by wrapping it.
Facade	Offer one simple interface to a complex subsystem.
Flyweight	Share fine-grained objects to save memory.
Proxy	Stand in for another object to control access.

Behavioural (11)

Pattern	In one line
Chain of Responsibility	Pass a request along handlers until one deals with it.
Command	Turn a request into an object (undo, queues).
Interpreter	Evaluate sentences of a small language.
Iterator	Walk a collection without exposing its structure.
Mediator	Centralise how a group of objects talk to each other.
Memento	Capture and restore an object's state.
Observer	Notify dependents when something changes (Qt's signals and slots are a form of this).
State	Change behaviour when internal state changes.
Strategy	Swap interchangeable algorithms.
Template Method	Fix an algorithm's skeleton, let subclasses fill steps.
Visitor	Add operations to a structure without changing its classes.

The book's best-known advice: program to an interface, not an implementation, and prefer composition over inheritance. In 2005 the authors received the ACM SIGPLAN Programming Languages Achievement Award.

1. Wikipedia: Design Patterns