coroutine example

coro.cpp

#include <coroutine>
#include <iostream>
#include <optional>
#include <string>
#include <unordered_map>

struct task
{
   struct promise_type {
       std::unique_ptr<char const, decltype([](auto msg){ std::cout << msg; })>
            on_exit{"releasing promise_type\n"};
      std::suspend_always initial_suspend() const { std::cout << "initial_suspend()\n"; return {}; }
      std::suspend_always final_suspend() const noexcept { return {}; }
      //void return_void() { std::cout << "return_void\n"; }
      void unhandled_exception() {}
      task get_return_object() {
          std::cout << "get_return_object()\n";
          return {promise_ptr(this)};
      }
      std::optional<std::string> result;
      void return_value(std::string const& value) {
          std::cout << "return_value("<< value << ")\n";
          result = value;
      }

   };
   using promise_ptr = std::unique_ptr<promise_type, decltype([](auto* p){
          std::coroutine_handle<promise_type>::from_promise(*p).destroy();
          })>;
   promise_ptr promise;
   void run() {
      std::coroutine_handle<promise_type>::from_promise(*promise.get()).resume();
   }
   std::string get_result() const { return promise->result.value_or("<not set>"); }
};

struct awaiter;
using outstanding_type = std::unordered_map<std::string, awaiter*>;

struct awaiter
{
    std::string       name;
    outstanding_type* outstanding;
    std::coroutine_handle<> h{};
    int                     result{};
    void complete(int r) {
        result = r;
        h.resume();
    }
    bool await_ready() const { return false; }
    void await_suspend(std::coroutine_handle<> h) {
        this->h = h;
        (*outstanding)[name] = this;
    }
    int await_resume() {
        return result;
    }
};

auto f(auto* outstanding, std::string name) -> task
{
   //std::cout << "inside coroutine\n";
   //co_await std::suspend_always{};
   //std::cout << "after second suspend\n";

   auto result = co_await awaiter{name, outstanding};
   std::cout << "awaiter-result=" << result << "\n";;

   co_return std::string("hello");
}

int main()
{
   outstanding_type outstanding;

   std::cout << "before f\n";
   task t = f(&outstanding, "first");
   task t2 = f(&outstanding, "second");
   std::cout << "after f\n";
   t.run();
   t2.run();
   std::cout << "after 1st run()\n";

   outstanding["second"]->complete(42);
   outstanding["first"]->complete(17);

   std::cout << "coro return=" << t.get_result() << "\n";
}

Makefile.txt

CXXFLAGS = -W -Wall -std=c++20 -fsanitize=address
default: coro
	./coro