C++ Insights - C++ Insights E36

Source:

#include <coroutine>
#include <exception>  // std::terminate
#include <iostream>
#include <list>
#include <new>
#include <utility>

struct Task {
  struct promise_type {
    Task               get_return_object() noexcept { return {}; }
    std::suspend_never initial_suspend() noexcept { return {}; }
    std::suspend_never final_suspend() noexcept { return {}; }
    void               unhandled_exception() {}
  };
};

struct Scheduler {
  std::list<std::coroutine_handle<>> _tasks{};

bool schedule()
  {
    auto task = _tasks.front();
    _tasks.pop_front();

if(not task.done()) { task.resume(); }

return not _tasks.empty();
  }

auto suspend()
  {
    struct awaiter : std::suspend_always {
      Scheduler& _sched;

explicit awaiter(Scheduler& sched) : _sched{sched} {}
      void await_suspend(std::coroutine_handle<> coro) const noexcept { _sched._tasks.push_back(coro); }
    };

return awaiter{*this};
  }
};

Task taskA(Scheduler& sched)
{
  std::cout << "Hello, from task A\n";

co_await sched.suspend();

std::cout << "a is back doing work\n";

co_await sched.suspend();

std::cout << "a is back doing more work\n";
}

Task taskB(Scheduler& sched)
{
  std::cout << "Hello, from task B\n";

co_await sched.suspend();

std::cout << "b is back doing work\n";

co_await sched.suspend();

std::cout << "b is back doing more work\n";
}

int main()
{
  Scheduler scheduler{};

taskA(scheduler);
  taskB(scheduler);

while(scheduler.schedule()) {}
}

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