# std::coroutine_traits

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< cpp‎ | coroutine

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` must define a publicly accessible nested type `promise_type`, otherwise the program is ill-formed.

### 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

### Nested types

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

### Possible implementation

    namespace detail {
    template<class, class...>
    struct coroutine_traits_base {};

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

    template<class R, class... Args>
    struct coroutine_traits : detail::coroutine_traits_base<R, Args...> {};

### 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

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