Structural design patterns in C++ focus on organizing classes and objects to form larger structures while keeping these structures flexible and efficient. These patterns help in defining how different components in a system are interconnected and provide mechanisms for composing objects in various ways to achieve desired functionality.

1 Adapter Pattern:

The Adapter pattern allows the interface of an existing class to be used as another interface. It is particularly useful when the interface provided by a class does not match the interface required by the client code.

Example: Adapting a legacy class with an outdated interface to work with a modern interface without modifying its source code.

2 Decorator Pattern:

The Decorator pattern attaches additional responsibilities to an object dynamically. It provides a flexible alternative to subclassing for extending functionality.

Example: Adding new features or behavior to an object at runtime by wrapping it with one or more decorator objects.

3 Composite Pattern:

The Composite pattern composes objects into tree structures to represent part-whole hierarchies. It allows clients to treat individual objects and compositions of objects uniformly.

Example: Representing a hierarchical structure like a file system, where a directory can contain files and subdirectories, which in turn can contain more files and subdirectories.

4 Proxy Pattern:

The Proxy pattern provides a surrogate or placeholder for another object to control access to it. It is useful for implementing lazy initialization, access control, logging, or monitoring of the actual object.

Example: Implementing a proxy for a resource-intensive object to delay its creation until it is actually needed, thereby improving performance.

5 Bridge Pattern:

The Bridge pattern decouples an abstraction from its implementation, allowing the two to vary independently. It provides a way to separate abstraction (interface) from implementation (implementation details).

Example: Creating a platform-independent abstraction for a graphical shape and providing different implementations for rendering on different platforms (e.g., Windows, Linux, macOS).

6 Flyweight Pattern:

The Flyweight pattern minimizes memory usage or computational expenses by sharing as much as possible with similar objects. It is useful when dealing with large numbers of objects with similar properties.

Example: Creating a text editor where each character in the document is represented by a flyweight object to conserve memory.