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

 

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Standard library headers

Language support <cfloat> <climits> <compare> (C++20) <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) <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) <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) <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 {
  enum class future_errc {
    broken_promise = /* implementation-defined */,
    future_already_retrieved = /* implementation-defined */,
    promise_already_satisfied = /* implementation-defined */,
    no_state = /* implementation-defined */
  };
 
  enum class launch : /* unspecified */ {
    async = /* unspecified */,
    deferred = /* unspecified */,
    /* implementation-defined */
  };
 
  enum class 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 defined
  template<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() const noexcept;
    const char*       what() const noexcept;
 
  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 result
    void set_value(/* see description */);
    void set_exception(exception_ptr p);
 
    // setting the result with deferred notification
    void 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 state
    bool valid() const noexcept;
 
    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 state
    bool valid() const noexcept;
 
    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() const noexcept;
 
    // result retrieval
    future<R> get_future();
 
    // execution
    void 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;
}