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| Coroutine traits | ||||
coroutine_traits (C++20) | ||||
| Coroutine handle | ||||
(C++20) | ||||
| No-op coroutines | ||||
(C++20) | ||||
(C++20) | ||||
| Trivial awaitables | ||||
(C++20) | ||||
(C++20) | ||||
| Range generators | ||||
(C++23) |
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 typepromise_type same asR::promise_type if the qualified-id is valid and denotes a type. Otherwise, it has no such member.
Program-defined specializations ofcoroutine_traits must define a publicly accessible nested typepromise_type, otherwise the program is ill-formed.
Contents |
| R | - | return type of the coroutine |
| Args | - | parameter types of the coroutine, including theimplicit object parameter if the coroutine is a non-static member function |
| Name | Definition |
promise_type | R::promise_type if it is valid, or provided by program-defined specializations |
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...>{}; |
If the coroutine is a non-static member function, then the first type inArgs... is the type of the implicit object parameter, and the rest are parameter types of the function (if any).
Ifstd::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 ofcoroutine_traits dependent on program-defined types to avoid modification to return types.
#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 dependstruct 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()constnoexcept{return{};}std::suspend_never final_suspend()constnoexcept{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()constnoexcept{return{};}std::suspend_never final_suspend()constnoexcept{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()constnoexcept{usingnamespace 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_awaitstd::async([]{return6;});int b= co_awaitstd::async([]{return7;}); co_return a* b;} std::future<void> fail(as_coroutine){throwstd::runtime_error("bleah"); co_return;} int main(){std::cout<< compute({}).get()<<'\n'; try{ fail({}).get();}catch(conststd::runtime_error& e){std::cout<<"error: "<< e.what()<<'\n';}}
Output:
42error: bleah
