std::atomic is for data accessed from multiple threads without using mutexes (concurrent usage); volatile is for memory where reads and writes should not be optimised away (special memory).

Using std::promises and futures is useful skill to create one-shot communication between a detecting task and reacting task.

Future destructors normally just destroy the future’s data members, execept for the final future referring to a shared state for a non-deferred task launched via std::async, which blocks until the task completes.

Join-on-destruction can lead to difficult-to-debug performance anomalies; while detach-on-destruction can lead to difficult-to-debug undefined behavior.

The flexibility that default policy for std::async permits both async and sync task execution leads to uncertainty when accessing thread_locals, implies that the task may never execute, and affects program logic for timeout-based wait calls.