Atomic Smart Pointers

A std::shared_ptr consists of a control block and its resource. The control block is thread-safe, but access to the resource is not. This means modifying the reference counter is an atomic operation and you have the guarantee that the resource is deleted exactly once. These are the guarantees std::shared_ptr gives you.

The importance of being thread-safe

Before I start, I want to make a short detour. This detour should only emphasise how important it is that the std::shared_ptr has well-defined multithreading semantic. At first glance, use of a std::shared_ptr does not appear to be a sensible choice for multithreaded code. It is by definition shared and mutable and is the ideal candidate for data races and hence for undefined behaviour. On the other hand, there is the guideline in modern C++: Don’t touch memory. This means use smart pointers in multithreading programs.

The proposal N4162 for atomic smart pointers directly addresses the deficiencies...