Coroutines in C++

Introduction to coroutines in C++20

The coroutines added to C++20 are stackless coroutines. There are options to use stackful coroutines in C++ as well by using third-party libraries. The most well-known cross-platform library is Boost.Fiber. C++20 stackless coroutines introduce new language constructs, while Boost.Fiber is a library that can be used with C++11 and onward. We will not discuss stackful coroutines any further in this book but will instead focus on the stackless coroutines standardized in C++20.

The stackless coroutines in C++20 were designed with the following goals:

• Scalable in the sense that they add very little memory overhead. This makes it possible to have many more coroutines alive compared to the possible number of threads or stackful coroutines alive.
• Efficient context switching means that suspending and resuming a coroutine should be about as cheap as an ordinary function call.
• Highly flexible. C++ coroutines have more than 15 customization points, which gives application developers and library writers a lot of freedom to configure and shape coroutines as they like. Decisions about how coroutines are supposed to work can be determined by us developers rather than being hardcoded in a language specification. One example is whether a coroutine should be suspended directly after being called or continue executing to the first explicit suspend point. Such questions are usually hard-coded in other languages, but in

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