Standard library header <future> (C++11)

 

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Language support <cfloat> <climits> <compare>(C++20) <contracts>(C++26) <coroutine>(C++20) <csetjmp> <csignal> <cstdarg> <cstddef> <cstdint>(C++11) <cstdlib> <exception> <initializer_list>(C++11)    <limits> <new> <source_location>(C++20) <stdfloat>(C++23) <typeinfo> <version>(C++20) Concepts <concepts>(C++20) Diagnostics <cassert> <cerrno> <debugging>(C++26) <stacktrace>(C++23) <stdexcept> <system_error>(C++11) Memory management <memory> <memory_resource>(C++17) <scoped_allocator>(C++11) Metaprogramming <type_traits>(C++11) <ratio>(C++11)

General utilities <any>(C++17) <bitset> <bit>(C++20) <charconv>(C++17) <expected>(C++23) <format>(C++20) <functional> <optional>(C++17) <stdbit.h>(C++26) <tuple>(C++11) <typeindex>(C++11) <utility> <variant>(C++17) Containers <array>(C++11) <deque> <flat_map>(C++23) <flat_set>(C++23) <forward_list>(C++11) <hive>(C++26) <inplace_vector>(C++26)    <list> <map> <mdspan>(C++23) <queue> <set> <span>(C++20) <stack> <unordered_map>(C++11) <unordered_set>(C++11) <vector> Iterators <iterator> Ranges <generator>(C++23) <ranges>(C++20)

Algorithms <algorithm> <numeric> Strings <cctype> <cstring> <cuchar>(C++11) <cwchar> <cwctype> <string_view>(C++17) <string> Text processing <clocale> <codecvt>(C++11/17/26*) <locale> <regex>(C++11) <text_encoding>(C++26)    Numerics <cfenv>(C++11) <cmath> <complex> <linalg>(C++26) <numbers>(C++20) <random>(C++11) <simd>(C++26) <stdckdint.h>(C++26) <valarray> Time <chrono>(C++11) <ctime> C compatibility <ccomplex>(C++11/17/20*) <ciso646>(until C++20) <cstdalign>(C++11/17/20*) <cstdbool>(C++11/17/20*) <ctgmath>(C++11/17/20*)

Input/output <cinttypes>(C++11) <cstdio> <filesystem>(C++17) <fstream> <iomanip> <iosfwd> <iostream> <ios> <istream> <ostream> <print>(C++23) <spanstream>(C++23) <sstream> <streambuf> <strstream>(C++98/26*) <syncstream>(C++20) Concurrency support <atomic>(C++11) <barrier>(C++20) <condition_variable>(C++11) <future>(C++11) <hazard_pointer>(C++26) <latch>(C++20) <mutex>(C++11) <rcu>(C++26) <semaphore>(C++20) <shared_mutex>(C++14) <stdatomic.h>(C++23) <stop_token>(C++20) <thread>(C++11) Execution support <execution>(C++17)

 

This header is part of the thread support library.

Classes
promise (C++11)	stores a value for asynchronous retrieval (class template)
packaged_task (C++11)	packages a function to store its return value for asynchronous retrieval (class template)
future (C++11)	waits for a value that is set asynchronously (class template)
shared_future (C++11)	waits for a value (possibly referenced by other futures) that is set asynchronously (class template)
launch (C++11)	specifies the launch policy for std::async (enum)
future_status (C++11)	specifies the results of timed waits performed on std::future and std::shared_future (enum)
future_error (C++11)	reports an error related to futures or promises (class)
future_errc (C++11)	identifies the future error codes (enum)
std::uses_allocator<std::promise> (C++11)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator<std::packaged_task> (C++11)(until C++17)	specializes the std::uses_allocator type trait (class template specialization)
Functions
async (C++11)	runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template)
future_category (C++11)	identifies the future error category (function)
std::swap(std::promise) (C++11)	specializes the std::swap algorithm (function template)
std::swap(std::packaged_task) (C++11)	specializes the std::swap algorithm (function template)

Synopsis

namespace std {enumclass future_errc { broken_promise =/* implementation-defined */, future_already_retrieved =/* implementation-defined */, promise_already_satisfied =/* implementation-defined */, no_state =/* implementation-defined */};   enumclass launch :/* unspecified */{ async =/* unspecified */, deferred =/* unspecified */, /* implementation-defined */};   enumclass future_status { ready, timeout, deferred };   template<>struct is_error_code_enum<future_errc>:public true_type {}; error_code make_error_code(future_errc e)noexcept; error_condition make_error_condition(future_errc e)noexcept;   const error_category& future_category()noexcept;   class future_error;   template<class R>class promise;template<class R>class promise<R&>;template<>class promise<void>;   template<class R>void swap(promise<R>& x, promise<R>& y)noexcept;   template<class R, class Alloc>struct uses_allocator<promise<R>, Alloc>;   template<class R>class future;template<class R>class future<R&>;template<>class future<void>;   template<class R>class shared_future;template<class R>class shared_future<R&>;template<>class shared_future<void>;   template<class>class packaged_task;// not definedtemplate<class R, class... ArgTypes>class packaged_task<R(ArgTypes...)>;   template<class R, class... ArgTypes>void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&)noexcept;   template<class F, class... Args> future<invoke_result_t<decay_t<F>, decay_t<Args>...>> async(F&& f, Args&&... args);template<class F, class... Args> future<invoke_result_t<decay_t<F>, decay_t<Args>...>> async(launch policy, F&& f, Args&&... args);}

Class std::future_error

namespace std {class future_error :public logic_error {public:explicit future_error(future_errc e);   const error_code& code()constnoexcept;constchar* what()constnoexcept;   private: error_code ec_;// exposition only};}

Class template std::promise

namespace std {template<class R>class promise {public: promise();template<class Allocator> promise(allocator_arg_t, const Allocator& a); promise(promise&& rhs)noexcept; promise(const promise&)= delete; ~promise();   // assignment promise& operator=(promise&& rhs)noexcept; promise& operator=(const promise&)= delete;void swap(promise& other)noexcept;   // retrieving the result future<R> get_future();   // setting the resultvoid set_value(/* see description */);void set_exception(exception_ptr p);   // setting the result with deferred notificationvoid set_value_at_thread_exit(/* see description */);void set_exception_at_thread_exit(exception_ptr p);};   template<class R>void swap(promise<R>& x, promise<R>& y)noexcept;   template<class R, class Alloc>struct uses_allocator<promise<R>, Alloc>;}

Class template std::future

namespace std {template<class R>class future {public: future()noexcept; future(future&&)noexcept; future(const future&)= delete; ~future(); future& operator=(const future&)= delete; future& operator=(future&&)noexcept; shared_future<R> share()noexcept;   // retrieving the value/* see description */ get();   // functions to check statebool valid()constnoexcept;   void wait()const;template<class Rep, class Period> future_status wait_for(const chrono::duration<Rep, Period>& rel_time)const;template<class Clock, class Duration> future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time)const;};}

Class template std::shared_future

namespace std {template<class R>class shared_future {public: shared_future()noexcept; shared_future(const shared_future& rhs)noexcept; shared_future(future<R>&&)noexcept; shared_future(shared_future&& rhs)noexcept; ~shared_future(); shared_future& operator=(const shared_future& rhs)noexcept; shared_future& operator=(shared_future&& rhs)noexcept;   // retrieving the value/* see description */ get()const;   // functions to check statebool valid()constnoexcept;   void wait()const;template<class Rep, class Period> future_status wait_for(const chrono::duration<Rep, Period>& rel_time)const;template<class Clock, class Duration> future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time)const;};}

Class template std::packaged_task

namespace std {template<class>class packaged_task;// not defined   template<class R, class... ArgTypes>class packaged_task<R(ArgTypes...)>{public:// construction and destruction packaged_task()noexcept;template<class F>explicit packaged_task(F&& f); ~packaged_task();   // no copy packaged_task(const packaged_task&)= delete; packaged_task& operator=(const packaged_task&)= delete;   // move support packaged_task(packaged_task&& rhs)noexcept; packaged_task& operator=(packaged_task&& rhs)noexcept;void swap(packaged_task& other)noexcept;   bool valid()constnoexcept;   // result retrieval future<R> get_future();   // executionvoid operator()(ArgTypes... );void make_ready_at_thread_exit(ArgTypes...);   void reset();};   template<class R, class... ArgTypes> packaged_task(R (*)(ArgTypes...))-> packaged_task<R(ArgTypes...)>;   template<class F> packaged_task(F)-> packaged_task</* see description */>;   template<class R, class... ArgTypes>void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y)noexcept;}