Essential C++ Design Patterns

Implement a thread-safe Singleton pattern using `std::call_once` to prevent race conditions during initialization.

Implement the Factory Method pattern using polymorphism to allow subclasses to decide which class to instantiate.

Implement the Builder pattern to construct complex objects step-by-step, separating construction from representation.

Implement the Adapter pattern to make an existing class's interface compatible with another.

Implement the Decorator pattern to dynamically add responsibilities to objects without affecting other objects.

Implement the Strategy pattern using polymorphism or templates to define a family of algorithms and make them interchangeable.

Implement the Observer pattern to create a one-to-many dependency between objects for event notification.

Implement the Command pattern to encapsulate a request as an object, enabling queuing and undoable operations.