Finally, we have compare-and-swap (CAS), sometimes called compare-and-exchange. It allows us to conditionally exchange a value if its previous value matches some expected one. In C and C++, CAS resembles the following, if it were executed atomically:
template bool atomic::compare_exchange_strong( T& expected, T desired) < if (*this == expected) < *this = desired; return true; >expected = *this; return false; > The compare_exchange_strong suffix may leave you wondering if there is a corresponding “weak” CAS. Indeed, there is. However, we will delve into that topic later in chapter Spurious LL/SC failures. Let’s say we have some long-running task that we might want to cancel. We’ll give it three states: idle, running, and cancelled, and write a loop that exits when it is cancelled.
enum class TaskState : int8_t < Idle, Running, Cancelled >; std::atomic ts; void taskLoop() < ts = TaskState::Running; while (ts == TaskState::Running) < // Do good work. >> If we want to cancel the task if it is running, but do nothing if it is idle, we could CAS: