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
#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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