Coroutine patterns in C++

Coroutines are a very recent addition to C++: they were introduced in C++20, and their present state is a foundation for building libraries and frameworks as opposed to features you should use in the application code directly. It is a complex feature with many subtle details, and it would take an entire chapter to explain what it does (there is a chapter like that in my book The Art of Writing Efficient Programs). Briefly, coroutines are functions that can suspend and resume themselves. They cannot be forced to suspend, a coroutine continues to execute until it suspends itself. They are used to implement what is known as cooperative multitasking, where multiple streams of execution voluntarily yield control to each other rather than being forcibly preempted by the OS.

Every execution pattern we saw in this chapter, and many more, can be implemented using coroutines. It is, however, too early to say whether this is going to become a common use of coroutines...