Concurrency support in C++20

C++20 added a few enhancements here and there to the existing concurrency support, but we are going to focus on the major new addition: coroutines. Coroutines, in general, are functions that can be interrupted and resumed. They are useful in several major applications: they can greatly simplify writing event-driven programs, they are almost unavoidable for work-stealing thread pools, and they make writing asynchronous I/O and other asynchronous code much easier.

The foundations of coroutines

There are two styles of coroutines: stackful and stackless. Stackful coroutines are also sometimes called fibers; they are similar to functions wherein their state is allocated on the stack. Stackless coroutines have no corresponding stack allocations, their state is stored on the heap. In general, stackful coroutines are more powerful and flexible, but stackless coroutines are significantly more efficient.

In this book, we will focus on stackless coroutines...