Explains and applies the 23 Gang of Four (GoF) design patterns with idiomatic code examples in Python, C, C++, Java, C#, JavaScript, Rust, Go, PHP, and Swift. Use when users ask about design patterns (singleton, factory, builder, observer, strategy, decorator, adapter, etc.), when they need help choosing a pattern for a design problem, when refactoring code toward a pattern, or when they want a pattern example in a specific language.
