NAME

std::coroutine_traits - std::coroutine_traits

Synopsis

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

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