Design Patterns

Design patterns represent classic solutions to recurring design problems, distilled from past experience. This volume covers the 21 core patterns from the Gang of Four (GoF), re-implementing them using modern C++.

Unlike traditional tutorials that follow a "Definition + UML + Example" format, our approach is to start from the most intuitive, primitive code and step-by-step derive each pattern. We clarify exactly what problem each pattern solves, why each evolutionary step is insufficient, and how—with C++17/20 features like std::variant, concepts, CRTP, and templates—we can achieve safer, zero-overhead versions of these classic patterns.

Key features throughout this volume: every pattern includes "let's verify this" real terminal output (not just theory on paper); common pitfalls are highlighted in ::: warning blocks; and wherever modern C++ can replace old idioms, we place "Classic vs. Modern" side-by-side for comparison.

The accompanying compilable project is located in the repository at code/volumn_codes/vol4/design-patterns/. Each pattern is a separate CMake sub-project. Run cmake -S . -B build && cmake --build build to build and run.

Creational

Singleton Pattern: From Comment Constraints to Meyer's Singleton→Builder Pattern→Factory Method & Abstract Factory→Prototype Pattern→

Structural

Adapter Pattern→Bridge Pattern (pImpl)→Decorator Pattern→Composite Pattern→Facade Pattern→Flyweight Pattern→Proxy Pattern→

Behavioral

Strategy Pattern→Command Pattern→State Machine Pattern→Memento Pattern→Visitor Pattern (variant + visit)→Observer Pattern→Iterator Pattern→Chain of Responsibility Pattern→Interpreter Pattern→Mediator Pattern→

Advanced: Generic and Template Patterns (Planned)

Beyond the classic GoF patterns, C++ possesses its own set of "template-level" design techniques. These serve as an extension of this volume and will be covered in the future:

• Policy-Based Design
• Type Erasure (underlying mechanism of std::function)
• CRTP (Static Polymorphism)
• Mixin and Compositional Design
• Tag Dispatching and if constexpr dispatch
• NVI (Non-Virtual Interface)
• Templates and DSL Construction