condition variables

After the introduction to std::atomic_flag in my last post Synchronization with Atomics in C++20, I want to dive deeper. Today, I create a ping-pong game using condition variables, std::atomic_flag, and std::atomic<bool>. Let's play.

Sender/receiver workflows are quite common for threads. In such a workflow, the receiver is waiting for the sender's notification before it continues to work. There are various ways to implement these workflows. With C++11, you can use condition variables or promise/future pairs; with C++20, you can use atomics.

Today, I write a scary post about condition variables. You should be aware of these issues of condition variables. The C++ core guideline CP 42 just states: "Don't wait without a condition".

The remaining atomics - in contrast to std::atomic_flag - are partial or full specialisations of the class template std::atomic. Let's start with std::atomic<bool>.

In case you use promise and future to synchronize threads, they have much in common with condition variables. But most of the time, tasks are the better choice.

Condition variables allow us to synchronize threads via notifications. So, you can implement workflows like sender/receiver or producer/consumer. In such a workflow, the receiver is waiting for the sender's notification. If the receiver gets the notification, it continues its work.