GoF Design Patterns Reference

A comprehensive reference for the Gang of Four design patterns. This skill provides language-agnostic guidance with pseudocode examples that can be translated to any programming language.

When This Skill Activates

This skill automatically activates when you:

• Ask about or need to implement a design pattern
• Need to choose between patterns for a design problem
• Are refactoring code and considering structural improvements
• Discuss architecture, decoupling, or extensibility
• Mention specific patterns: factory, singleton, observer, decorator, etc.

Quick Pattern Reference

Creational Patterns (Object Creation)

Pattern	Intent	Use When
Abstract Factory	Create families of related objects	Need platform/theme independence
Builder	Construct complex objects step-by-step	Object has many optional parts
Factory Method	Let subclasses decide which class to instantiate	Don't know concrete types ahead of time
Prototype	Clone existing objects	Object creation is expensive
Singleton	Ensure single instance	Need exactly one shared instance

Structural Patterns (Composition)

Pattern	Intent	Use When
Adapter	Convert interface to expected interface	Integrating incompatible interfaces
Bridge	Separate abstraction from implementation	Need to vary both independently
Composite	Treat individual and groups uniformly	Have tree structures
Decorator	Add responsibilities dynamically	Need flexible extension
Facade	Simplified interface to subsystem	Complex subsystem needs simple API
Flyweight	Share common state efficiently	Many similar objects needed
Proxy	Control access to object	Need lazy loading, access control, logging

Behavioral Patterns (Communication)

Pattern	Intent	Use When
Chain of Responsibility	Pass request along handler chain	Multiple handlers, unknown which handles
Command	Encapsulate request as object	Need undo, queue, or log operations
Interpreter	Define grammar and interpret sentences	Have a simple language to parse
Iterator	Sequential access without exposing internals	Need to traverse collections
Mediator	Centralize complex communications	Many objects communicate in complex ways
Memento	Capture and restore object state	Need undo/snapshot functionality
Observer	Notify dependents of state changes	One-to-many dependency
State	Alter behavior when state changes	Object behavior depends on state
Strategy	Encapsulate interchangeable algorithms	Need to swap algorithms at runtime
Template Method	Define skeleton, let subclasses fill in	Algorithm structure fixed, steps vary
Visitor	Add operations without changing classes	Need to add many operations to stable structure

Decision Guide

See Pattern Selection Guide for help choosing the right pattern.

How to Use This Reference

1. Choosing a pattern: Start with the decision guide or tables above
2. Learning a pattern: Read the full documentation with examples
3. Quick reminder: Use the tables above for at-a-glance reference
4. Implementation: Follow the pseudocode, adapt to your language

Language Translation Notes

The pseudocode in this reference uses these conventions:

• class

  for type definitions

• function

  for methods/functions

• ->

  for method calls on objects

• //

  for comments
• Type hints shown as

  name: Type

Translate to your language:

• PHP:

  class

  ,

  function

  ,

  ->

  ,

  //

  , type hints in docblocks or PHP 8+
• JavaScript/TypeScript:

  class

  ,

  function

  /arrow,

  .

  ,

  //

  , TS types
• Python:

  class

  ,

  def

  ,

  .

  ,

  #

  , type hints
• Java/C#: Direct mapping with

  new

  , generics

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Based on concepts from "Design Patterns: Elements of Reusable Object-Oriented Software" by Gamma, Helm, Johnson, and Vlissides (Gang of Four), 1994.