The coroutine abstraction
We will now take a step back and talk about coroutines in general and not just focus on the coroutines added to C++20. This will give you a better understanding of why coroutines are useful but also what types of coroutines there are and how they differ. If you are already familiar with stackful and stackless coroutines and how they are executed, you can skip this section and jump right to the next section, Coroutines in C++.
The coroutine abstraction has been around for more than 60 years and many languages have adopted some sort of coroutines into their syntax or standard libraries. This means that coroutines can denote slightly different things in different languages and environments. Since this is a book about C++, I will use the terminology used in the C++ standard.
Coroutines are very similar to subroutines. In C++, we don't have anything explicitly called subroutines; instead, we write functions (free functions or member functions, for...