std::coroutine_traits

Defined in header `<coroutine>`
template< class R, class... Args > struct coroutine_traits;		(since C++20)

Determines the promise type from the return type and parameter types of a coroutine. The standard library implementation provides a publicly accessible member type promise_type same as R::promise_type if the qualified-id is valid and denotes a type. Otherwise, it has no such member.

Program-defined specializations of coroutine_traits shall define a publicly accessible member type promise_type; otherwise, the behavior is undefined.

Template parameters

R	-	return type of the coroutine
Args	-	parameter types of the coroutine, including the implicit object parameter if the coroutine is a non-static member function

Member types

Type	Definition
`promise_type`	`R::promise_type` if it is valid, or provided by program-defined specializations

Possible implementation

template<class, class...>
struct coroutine_traits {};
 
template<class R, class... Args>
requires requires { typename R::promise_type; }
struct coroutine_traits<R, Args...> {
  using promise_type = typename R::promise_type;
};

Notes

If the coroutine is a non-static member function, then the first type in Args... is the type of the implicit object parameter, and the rest are parameter types of the function (if any).

If std::coroutine_traits<R, Args...>::promise_type does not exist or is not a class type, the corresponding coroutine definition is ill-formed.

Users may define explicit or partial specializations of coroutine_traits dependent on program-defined types to avoid modification to return types.

Example

Run this code

#include <chrono>
#include <coroutine>
#include <exception>
#include <future>
#include <iostream>
#include <thread>
#include <type_traits>
 
// A program-defined type on which the coroutine_traits specializations below depend
struct as_coroutine {};
 
// Enable the use of std::future<T> as a coroutine type
// by using a std::promise<T> as the promise type.
template <typename T, typename... Args>
requires(!std::is_void_v<T> && !std::is_reference_v<T>)
struct std::coroutine_traits<std::future<T>, as_coroutine, Args...> {
  struct promise_type : std::promise<T> {
    std::future<T> get_return_object() noexcept {
      return this->get_future();
    }
 
    std::suspend_never initial_suspend() const noexcept { return {}; }
    std::suspend_never final_suspend() const noexcept { return {}; }
 
    void return_value(const T &value)
    noexcept(std::is_nothrow_copy_constructible_v<T>) {
      this->set_value(value);
    }
    void return_value(T &&value)
    noexcept(std::is_nothrow_move_constructible_v<T>) {
      this->set_value(std::move(value));
    }
    void unhandled_exception() noexcept {
      this->set_exception(std::current_exception());
    }
  };
};
 
// Same for std::future<void>.
template <typename... Args>
struct std::coroutine_traits<std::future<void>, as_coroutine, Args...> {
  struct promise_type : std::promise<void> {
    std::future<void> get_return_object() noexcept {
      return this->get_future();
    }
 
    std::suspend_never initial_suspend() const noexcept { return {}; }
    std::suspend_never final_suspend() const noexcept { return {}; }
 
    void return_void() noexcept {
      this->set_value();
    }
    void unhandled_exception() noexcept {
      this->set_exception(std::current_exception());
    }
  };
};
 
// Allow co_await'ing std::future<T> and std::future<void>
// by naively spawning a new thread for each co_await.
template <typename T>
auto operator co_await(std::future<T> future) noexcept
requires(!std::is_reference_v<T>) {
  struct awaiter : std::future<T> {
    bool await_ready() const noexcept {
      using namespace std::chrono_literals;
      return this->wait_for(0s) != std::future_status::timeout;
    }
    void await_suspend(std::coroutine_handle<> cont) const {
      std::thread([this, cont] {
        this->wait();
        cont();
      }).detach();
    }
    T await_resume() { return this->get(); }
  };
  return awaiter{std::move(future)};
}
 
// Utilize the infrastructure we have established.
std::future<int> compute(as_coroutine) {
  int a = co_await std::async([] { return 6; });
  int b = co_await std::async([] { return 7; });
  co_return a * b;
}
 
std::future<void> fail(as_coroutine) {
  throw std::runtime_error("bleah");
  co_return;
}
 
int main() {
  std::cout << compute({}).get() << '\n';
 
  try {
    fail({}).get();
  } catch (const std::runtime_error &e) {
    std::cout << "error: " << e.what() << '\n';
  }
}

Output:

42
error: bleah

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries