A Lock-Free std::atomic< std::shared_ptr >

By Timur Doumler

std::shared_ptr is a standard smart pointer utility widely used in modern C++. A commonly overlooked property of std::shared_ptr is that while its control block is thread-safe, the shared_ptr itself isn’t. To fix this, C++20 introduced std::atomic<std::shared_ptr>. However, existing implementations are typically not lock-free, rendering std::atomic<std::shared_ptr> useless for low-latency and real-time applications.

What would it take to implement a lock-free atomic shared_ptr? In this talk, we first discuss the motivation and use case, review how std::shared_ptr works and the history of std::atomic<std::shared_ptr>. We then look at existing implementations and different implementation strategies. Finally, we present a new implementation of a lock-free atomic shared_ptr which is portable to multiple platforms.