coro/awaiter.cpp

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 "awaiter.hpp"

#include <iostream>
#include <string>
#include <coroutine>

// coroutine interface to deal with a simple task
// - providing resume() to resume it
class CoroTask {
// initialize members for state and customization:
public:
  struct promise_type;
private:
  using CoroHdl = std::coroutine_handle<promise_type>;
  CoroHdl hdl;          // native coroutine handle

public:
  struct promise_type {
    auto get_return_object() {       // init and return the coroutine interface
      return CoroTask{CoroHdl::from_promise(*this)};
    }
    auto initial_suspend() {         // initial suspend point
      return std::suspend_always{};  // - suspend immediately
    }
    void unhandled_exception() {     // deal with exceptions
      std::terminate();              // - terminate the program
    }
    void return_void() {             // deal with the end or co_return;
    }
    auto final_suspend() noexcept {  // final suspend point
      return std::suspend_always{};  // - suspend immediately
    }
  };

  // constructor and destructor:
  CoroTask(auto h)
   : hdl{h} {           // store coroutine handle in interface
  }
  ~CoroTask() {
    if (hdl) {
      hdl.destroy();    // destroy coroutine handle
    }
  }
  // don't copy or move:
  CoroTask(const CoroTask&) = delete;
  CoroTask& operator=(const CoroTask&) = delete;

  // API to resume the coroutine
  // - returns whether there is still something to process
  bool resume() const {
    if (!hdl || hdl.done()) {
      return false;     // nothing (more) to process
    }
    hdl.resume();       // RESUME (blocks until suspended again or end)
    return !hdl.done();
  }
};

CoroTask coro(int max)
{
  std::cout << "  CORO start\n";

  for (int val = 1; val <= max; ++val) {
    std::cout << "  CORO " << val << '\n';
    co_await Awaiter{};  // SUSPEND with our own awaiter
  }

  std::cout << "  CORO end\n";
}

int main()
{
  // start coroutine:
  auto coTask = coro(2);
  std::cout << "started\n";

  // loop to resume the coroutine until it is done:
  std::cout << "loop\n";
  while (coTask.resume()) {  // RESUME
    std::cout << " suspended\n";
  }

  std::cout << "done\n";
}