coro/corotasksub.hpp

The following code example is taken from the book
C++20 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2021
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <coroutine>
#include <exception>  // for terminate()

// coroutine interface to deal with a simple task
// - providing resume() to resume it
// - including resuming sub-coroutines if there is one
class [[nodiscard]] CoroTaskSub {
public:
  // initialize members for state and customization:
  struct promise_type;
  using CoroHdl = std::coroutine_handle<promise_type>;
private:
  CoroHdl hdl;            // native coroutine handle

public:
  struct promise_type {
    CoroHdl subHdl = nullptr;        // sub-coroutine (if any)

    auto get_return_object() {
      return CoroTaskSub{CoroHdl::from_promise(*this)};
    }
    auto initial_suspend() { return std::suspend_always{}; }
    void unhandled_exception() { std::terminate(); }
    void return_void() { }
    auto final_suspend() noexcept { return std::suspend_always{}; }
  };

  // special members (no copy or move):
  CoroTaskSub(auto h)
   : hdl{h} {             // store coroutine handle in interface
  }
  ~CoroTaskSub() {
    if (hdl) {
      hdl.destroy();      // destroy coroutine handle
    }
  }
  CoroTaskSub(const CoroTaskSub&) = delete;
  CoroTaskSub& operator=(const CoroTaskSub&) = delete;

  // API to resume the deepest (sub) coroutine not done yet
  // - returns whether there is still something to process
  bool resume() const {
    if (!hdl || hdl.done()) {
      return false;       // nothing (more) to process
    }
    // find deepest sub-coroutine not done yet:
    CoroHdl innerHdl = hdl;
    while (innerHdl.promise().subHdl) {
      if (innerHdl.promise().subHdl.done()) {
        innerHdl.promise().subHdl = nullptr; // remove sub-coroutine if done
        break;
      }
      innerHdl = innerHdl.promise().subHdl;
    }
    innerHdl.resume();         // RESUME
    return !hdl.done();
  }

  // Let co_await handle its operand as sub-coroutine:
  bool await_ready() { return false; }   // do not skip suspension
  void await_suspend(auto awaitHdl) {
    awaitHdl.promise().subHdl = hdl;     // store sub-coroutine and suspend
  }
  void await_resume() { }
};