Coroutines

Coroutines are special functions able to stop at any defined point of execution and resume later. Coroutines extend the language with the following new keywords:

• co_await suspends the execution of the coroutine.
• co_yield suspends the execution of the coroutine while also returning a value.
• co_return is similar to the regular return keyword; it finishes the coroutine and returns a value. Take a look at the following classic example:

generator<int> step_by_step(int n = 0) {
  while (true) {
    co_yield n++;
  }
}

A coroutine is associated with a promise object. The promise object stores and alerts the state of the coroutine. We will dive deeper into coroutines in Chapter 8, Concurrency and Multithreading.