In this article, we’ll start with a basic example using std::mutex, look at its limitations, and then introduce std::shared_mutex, a reader-writer mutex added in C++17. Even in 2026, with many new concurrency features available, std::shared_mutex is still a valuable and practical tool. Let’s jump in. A Simple Thread-Safe Counter with std::mutex   We’ll begin with a small example (a standard “hello world” for this type of mutexes): a counter object that multiple threads can access: