#include <cassert>

#include <coroutine>

#include <cstdio>

#include <exception>

// ===================================================================

// User Defined resumable and promise_types. They are generally the

// same for every case.

// ===================================================================

class resumable {

public:

struct promise_type;

using co_handle = std::coroutine_handle<promise_type>;

resumable(co_handle handle) : handle_(handle) { assert(handle); }

~resumable() { handle_.destroy(); }

resumable(resumable &) = delete;

resumable(resumable &&) = delete;

bool resume() {

if (not handle_.done()) {

handle_.resume();

}

return not handle_.done();

}

private:

co_handle handle_;

};

struct resumable::promise_type {

using co_handle = std::coroutine_handle<promise_type>;

auto get_return_object() { return co_handle::from_promise(*this); }

auto initial_suspend() { return std::suspend_always(); }

auto final_suspend() { return std::suspend_always(); }

void return_void() {}

void unhandled_exception() { std::terminate(); }

};

// ===================================================================

resumable foo() {

printf("<foo> a\n");

co_await std::suspend_always();

printf("<foo> b\n");

co_await std::suspend_always();

printf("<foo> c\n");

}

int main() {

// Coroutine is created in initial suspend state.

resumable r1 = foo();

r1.resume();

r1.resume();

r1.resume();

r1.resume();

return 0;

}