future

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// <future> -*- C++ -*-
 
// Copyright (C) 2009-2026 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/** @file include/future
 *  This is a Standard C++ Library header.
 */
 
#ifndef _GLIBCXX_FUTURE
#define _GLIBCXX_FUTURE 1
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#include <bits/requires_hosted.h> // concurrency
#include <bits/functexcept.h>
 
#if __cplusplus < 201103L
# include <bits/c++0x_warning.h>
#else
 
#include <mutex>              // call_once
#include <condition_variable> // __at_thread_exit_elt
#include <system_error>
#include <bits/atomic_base.h> // atomic_flag
#include <bits/allocated_ptr.h>
#include <bits/atomic_futex.h>
#include <bits/exception_defines.h>
#include <bits/invoke.h>
#include <bits/unique_ptr.h>
#include <bits/shared_ptr.h>
#include <bits/std_function.h>
#include <bits/std_thread.h>
#include <ext/aligned_buffer.h>
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  /**
   * @defgroup futures Futures
   * @ingroup concurrency
   *
   * Futures and promises provide support for retrieving the result from
   * an asynchronous function, e.g. one that is running in another thread.
   * A `std::future` represents an asynchronous result that will become
   * ready at some later time. A consumer can wait on a future until the
   * result is ready to be accessed.
   *
   * @since C++11
   * @{
   */
 
  /// Error code for futures
  enum class future_errc
  {
    future_already_retrieved = 1,
    promise_already_satisfied,
    no_state,
    broken_promise
  };
 
  /// Specialization that allows `future_errc` to convert to `error_code`.
  template<>
    struct is_error_code_enum<future_errc> : public true_type { };
 
  /// Points to a statically-allocated object derived from error_category.
  [[__nodiscard__, __gnu__::__const__]]
  const error_category&
  future_category() noexcept;
 
  /// Overload of make_error_code for `future_errc`.
  [[__nodiscard__]]
  inline error_code
  make_error_code(future_errc __errc) noexcept
  { return error_code(static_cast<int>(__errc), future_category()); }
 
  /// Overload of make_error_condition for `future_errc`.
  [[__nodiscard__]]
  inline error_condition
  make_error_condition(future_errc __errc) noexcept
  { return error_condition(static_cast<int>(__errc), future_category()); }
 
  /**
   *  @brief Exception type thrown by futures.
   *  @ingroup exceptions
   *  @since C++11
   */
  class future_error : public logic_error
  {
  public:
    explicit
    future_error(future_errc __errc)
    : future_error(std::make_error_code(__errc))
    { }
 
    virtual ~future_error() noexcept;
 
    virtual const char*
    what() const noexcept;
 
    const error_code&
    code() const noexcept { return _M_code; }
 
  private:
    explicit
    future_error(error_code __ec)
    : logic_error("std::future_error: " + __ec.message()), _M_code(__ec)
    { }
 
    friend void __throw_future_error(int);
 
    error_code                  _M_code;
  };
 
  // Forward declarations.
  template<typename _Res>
    class future;
 
  template<typename _Res>
    class shared_future;
 
  template<typename _Signature>
    class packaged_task;
 
  template<typename _Res>
    class promise;
 
  /// Launch code for futures
  enum class launch
  {
    async = 1,
    deferred = 2
  };
 
  [[__nodiscard__]]
  constexpr launch operator&(launch __x, launch __y) noexcept
  {
    return static_cast<launch>(
        static_cast<int>(__x) & static_cast<int>(__y));
  }
 
  [[__nodiscard__]]
  constexpr launch operator|(launch __x, launch __y) noexcept
  {
    return static_cast<launch>(
        static_cast<int>(__x) | static_cast<int>(__y));
  }
 
  [[__nodiscard__]]
  constexpr launch operator^(launch __x, launch __y) noexcept
  {
    return static_cast<launch>(
        static_cast<int>(__x) ^ static_cast<int>(__y));
  }
 
  [[__nodiscard__]]
  constexpr launch operator~(launch __x) noexcept
  { return static_cast<launch>(~static_cast<int>(__x)); }
 
  _GLIBCXX14_CONSTEXPR
  inline launch& operator&=(launch& __x, launch __y) noexcept
  { return __x = __x & __y; }
 
  _GLIBCXX14_CONSTEXPR
  inline launch& operator|=(launch& __x, launch __y) noexcept
  { return __x = __x | __y; }
 
  _GLIBCXX14_CONSTEXPR
  inline launch& operator^=(launch& __x, launch __y) noexcept
  { return __x = __x ^ __y; }
 
  /// Status code for futures
  enum class future_status
  {
    ready,
    timeout,
    deferred
  };
 
  /// @cond undocumented
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2021. Further incorrect usages of result_of
  template<typename _Fn, typename... _Args>
    using __async_result_of = typename __invoke_result<
      typename decay<_Fn>::type, typename decay<_Args>::type...>::type;
  /// @endcond
 
  template<typename _Fn, typename... _Args>
    future<__async_result_of<_Fn, _Args...>>
    async(launch __policy, _Fn&& __fn, _Args&&... __args);
 
  template<typename _Fn, typename... _Args>
    future<__async_result_of<_Fn, _Args...>>
    async(_Fn&& __fn, _Args&&... __args);
 
#if defined(_GLIBCXX_HAS_GTHREADS)
 
  /// @cond undocumented
 
  /// Base class and enclosing scope.
  struct __future_base
  {
    /// Base class for results.
    struct _Result_base
    {
      exception_ptr             _M_error;
 
      _Result_base(const _Result_base&) = delete;
      _Result_base& operator=(const _Result_base&) = delete;
 
      // _M_destroy() allows derived classes to control deallocation
      virtual void _M_destroy() = 0;
 
      struct _Deleter
      {
        void operator()(_Result_base* __fr) const { __fr->_M_destroy(); }
      };
 
    protected:
      _Result_base();
      virtual ~_Result_base();
    };
 
    /// A unique_ptr for result objects.
    template<typename _Res>
      using _Ptr = unique_ptr<_Res, _Result_base::_Deleter>;
 
    /// A result object that has storage for an object of type _Res.
    template<typename _Res>
      struct _Result : _Result_base
      {
      private:
        __gnu_cxx::__aligned_buffer<_Res>       _M_storage;
        bool                                    _M_initialized;
 
      public:
        typedef _Res result_type;
 
        _Result() noexcept : _M_initialized() { }
 
        ~_Result()
        {
          if (_M_initialized)
            _M_value().~_Res();
        }
 
        // Return lvalue, future will add const or rvalue-reference
        _Res&
        _M_value() noexcept { return *_M_storage._M_ptr(); }
 
        void
        _M_set(const _Res& __res)
        {
          ::new (_M_storage._M_addr()) _Res(__res);
          _M_initialized = true;
        }
 
        void
        _M_set(_Res&& __res)
        {
          ::new (_M_storage._M_addr()) _Res(std::move(__res));
          _M_initialized = true;
        }
 
      private:
        void _M_destroy() { delete this; }
    };
 
    /// A result object that uses an allocator.
    template<typename _Res, typename _Alloc>
      struct _Result_alloc final : _Result<_Res>, _Alloc
      {
        using __allocator_type = __alloc_rebind<_Alloc, _Result_alloc>;
 
        explicit
        _Result_alloc(const _Alloc& __a) : _Result<_Res>(), _Alloc(__a)
        { }
 
      private:
        void _M_destroy()
        {
          __allocator_type __a(*this);
          __allocated_ptr<__allocator_type> __guard_ptr{ __a, this };
          this->~_Result_alloc();
        }
      };
 
    // Create a result object that uses an allocator.
    template<typename _Res, typename _Allocator>
      static _Ptr<_Result_alloc<_Res, _Allocator>>
      _S_allocate_result(const _Allocator& __a)
      {
        using __result_type = _Result_alloc<_Res, _Allocator>;
        typename __result_type::__allocator_type __a2(__a);
        auto __guard = std::__allocate_guarded(__a2);
        __result_type* __p = ::new((void*)__guard.get()) __result_type{__a};
        __guard = nullptr;
        return _Ptr<__result_type>(__p);
      }
 
    // Keep it simple for std::allocator.
    template<typename _Res, typename _Tp>
      static _Ptr<_Result<_Res>>
      _S_allocate_result(const std::allocator<_Tp>&)
      {
        return _Ptr<_Result<_Res>>(new _Result<_Res>);
      }
 
    // Base class for various types of shared state created by an
    // asynchronous provider (such as a std::promise) and shared with one
    // or more associated futures.
    class _State_baseV2
    {
      typedef _Ptr<_Result_base> _Ptr_type;
 
      enum _Status : unsigned {
        __not_ready,
        __ready
      };
 
      _Ptr_type                 _M_result;
      __atomic_futex_unsigned<> _M_status;
      atomic_flag               _M_retrieved = ATOMIC_FLAG_INIT;
      once_flag                 _M_once;
 
    public:
      _State_baseV2() noexcept : _M_result(), _M_status(_Status::__not_ready)
        { }
      _State_baseV2(const _State_baseV2&) = delete;
      _State_baseV2& operator=(const _State_baseV2&) = delete;
      virtual ~_State_baseV2() = default;
 
      _Result_base&
      wait()
      {
        // Run any deferred function or join any asynchronous thread:
        _M_complete_async();
        // Acquire MO makes sure this synchronizes with the thread that made
        // the future ready.
        _M_status._M_load_when_equal(_Status::__ready, memory_order_acquire);
        return *_M_result;
      }
 
      template<typename _Rep, typename _Period>
        future_status
        wait_for(const chrono::duration<_Rep, _Period>& __rel)
        {
          // First, check if the future has been made ready.  Use acquire MO
          // to synchronize with the thread that made it ready.
          if (_M_status._M_load(memory_order_acquire) == _Status::__ready)
            return future_status::ready;
 
          if (_M_is_deferred_future())
            return future_status::deferred;
 
          // Don't wait unless the relative time is greater than zero.
          if (__rel > __rel.zero()
              && _M_status._M_load_when_equal_for(_Status::__ready,
                                                  memory_order_acquire,
                                                  __rel))
            {
              // _GLIBCXX_RESOLVE_LIB_DEFECTS
              // 2100.  timed waiting functions must also join
              // This call is a no-op by default except on an async future,
              // in which case the async thread is joined.  It's also not a
              // no-op for a deferred future, but such a future will never
              // reach this point because it returns future_status::deferred
              // instead of waiting for the future to become ready (see
              // above).  Async futures synchronize in this call, so we need
              // no further synchronization here.
              _M_complete_async();
 
              return future_status::ready;
            }
          return future_status::timeout;
        }
 
      template<typename _Clock, typename _Duration>
        future_status
        wait_until(const chrono::time_point<_Clock, _Duration>& __abs)
        {
#if __cplusplus > 201703L
          static_assert(chrono::is_clock_v<_Clock>);
#endif
          // First, check if the future has been made ready.  Use acquire MO
          // to synchronize with the thread that made it ready.
          if (_M_status._M_load(memory_order_acquire) == _Status::__ready)
            return future_status::ready;
 
          if (_M_is_deferred_future())
            return future_status::deferred;
 
          if (_M_status._M_load_when_equal_until(_Status::__ready,
                                                 memory_order_acquire,
                                                 __abs))
            {
              // _GLIBCXX_RESOLVE_LIB_DEFECTS
              // 2100.  timed waiting functions must also join
              // See wait_for(...) above.
              _M_complete_async();
 
              return future_status::ready;
            }
          return future_status::timeout;
        }
 
      // Provide a result to the shared state and make it ready.
      // Calls at most once: _M_result = __res();
      void
      _M_set_result(function<_Ptr_type()> __res, bool __ignore_failure = false)
      {
        bool __did_set = false;
        // all calls to this function are serialized,
        // side-effects of invoking __res only happen once
        call_once(_M_once, &_State_baseV2::_M_do_set, this,
                  std::__addressof(__res), std::__addressof(__did_set));
        if (__did_set)
          // Use release MO to synchronize with observers of the ready state.
          _M_status._M_store_notify_all(_Status::__ready,
                                        memory_order_release);
        else if (!__ignore_failure)
          __throw_future_error(int(future_errc::promise_already_satisfied));
      }
 
      // Provide a result to the shared state but delay making it ready
      // until the calling thread exits.
      // Calls at most once: _M_result = __res();
      void
      _M_set_delayed_result(function<_Ptr_type()> __res,
                            weak_ptr<_State_baseV2> __self)
      {
        bool __did_set = false;
        unique_ptr<_Make_ready> __mr{new _Make_ready};
        // all calls to this function are serialized,
        // side-effects of invoking __res only happen once
        call_once(_M_once, &_State_baseV2::_M_do_set, this,
                  std::__addressof(__res), std::__addressof(__did_set));
        if (!__did_set)
          __throw_future_error(int(future_errc::promise_already_satisfied));
        __mr->_M_shared_state = std::move(__self);
        __mr->_M_set();
        __mr.release();
      }
 
      // Abandon this shared state.
      void
      _M_break_promise(_Ptr_type __res)
      {
        if (static_cast<bool>(__res))
          {
            __res->_M_error =
              make_exception_ptr(future_error(future_errc::broken_promise));
            // This function is only called when the last asynchronous result
            // provider is abandoning this shared state, so noone can be
            // trying to make the shared state ready at the same time, and
            // we can access _M_result directly instead of through call_once.
            _M_result.swap(__res);
            // Use release MO to synchronize with observers of the ready state.
            _M_status._M_store_notify_all(_Status::__ready,
                                          memory_order_release);
          }
      }
 
      // Called when this object is first passed to a future.
      void
      _M_set_retrieved_flag()
      {
        if (_M_retrieved.test_and_set())
          __throw_future_error(int(future_errc::future_already_retrieved));
      }
 
      template<typename _Res, typename _Arg>
        struct _Setter;
 
      // set lvalues
      template<typename _Res, typename _Arg>
        struct _Setter<_Res, _Arg&>
        {
          // check this is only used by promise<R>::set_value(const R&)
          // or promise<R&>::set_value(R&)
          static_assert(is_same<_Res, _Arg&>::value              // promise<R&>
                          || is_same<const _Res, _Arg>::value,   // promise<R>
                        "Invalid specialisation");
 
          // Used by std::promise to copy construct the result.
          typename promise<_Res>::_Ptr_type operator()() const
          {
            _M_promise->_M_storage->_M_set(*_M_arg);
            return std::move(_M_promise->_M_storage);
          }
          promise<_Res>*    _M_promise;
          _Arg*             _M_arg;
        };
 
      // set rvalues
      template<typename _Res>
        struct _Setter<_Res, _Res&&>
        {
          // Used by std::promise to move construct the result.
          typename promise<_Res>::_Ptr_type operator()() const
          {
            _M_promise->_M_storage->_M_set(std::move(*_M_arg));
            return std::move(_M_promise->_M_storage);
          }
          promise<_Res>*    _M_promise;
          _Res*             _M_arg;
        };
 
      // set void
      template<typename _Res>
        struct _Setter<_Res, void>
        {
          static_assert(is_void<_Res>::value, "Only used for promise<void>");
 
          typename promise<_Res>::_Ptr_type operator()() const noexcept
          { return std::move(_M_promise->_M_storage); }
 
          promise<_Res>*    _M_promise;
        };
 
      struct __exception_ptr_tag { };
 
      // set exceptions
      template<typename _Res>
        struct _Setter<_Res, __exception_ptr_tag>
        {
          // Used by std::promise to store an exception as the result.
          typename promise<_Res>::_Ptr_type operator()() const noexcept
          {
            _M_promise->_M_storage->_M_error = *_M_ex;
            return std::move(_M_promise->_M_storage);
          }
 
          promise<_Res>*   _M_promise;
          exception_ptr*    _M_ex;
        };
 
      template<typename _Res, typename _Arg>
        __attribute__((__always_inline__))
        static _Setter<_Res, _Arg&&>
        __setter(promise<_Res>* __prom, _Arg&& __arg) noexcept
        {
          return _Setter<_Res, _Arg&&>{ __prom, std::__addressof(__arg) };
        }
 
      template<typename _Res>
        __attribute__((__always_inline__))
        static _Setter<_Res, __exception_ptr_tag>
        __setter(exception_ptr& __ex, promise<_Res>* __prom) noexcept
        {
          __glibcxx_assert(__ex != nullptr); // LWG 2276
          return _Setter<_Res, __exception_ptr_tag>{ __prom, &__ex };
        }
 
      template<typename _Res>
        __attribute__((__always_inline__))
        static _Setter<_Res, void>
        __setter(promise<_Res>* __prom) noexcept
        {
          return _Setter<_Res, void>{ __prom };
        }
 
      template<typename _Tp>
        static void
        _S_check(const shared_ptr<_Tp>& __p)
        {
          if (!static_cast<bool>(__p))
            __throw_future_error((int)future_errc::no_state);
        }
 
    private:
      // The function invoked with std::call_once(_M_once, ...).
      void
      _M_do_set(function<_Ptr_type()>* __f, bool* __did_set)
      {
        _Ptr_type __res = (*__f)();
        // Notify the caller that we did try to set; if we do not throw an
        // exception, the caller will be aware that it did set (e.g., see
        // _M_set_result).
        *__did_set = true;
        _M_result.swap(__res); // nothrow
      }
 
      // Wait for completion of async function.
      virtual void _M_complete_async() { }
 
      // Return true if state corresponds to a deferred function.
      virtual bool _M_is_deferred_future() const { return false; }
 
      struct _Make_ready final : __at_thread_exit_elt
      {
        weak_ptr<_State_baseV2> _M_shared_state;
        static void _S_run(void*);
        void _M_set();
      };
    };
 
#ifdef _GLIBCXX_ASYNC_ABI_COMPAT
    class _State_base;
    class _Async_state_common;
#else
    using _State_base = _State_baseV2;
    class _Async_state_commonV2;
#endif
 
    template<typename _BoundFn,
             typename _Res = decltype(std::declval<_BoundFn&>()())>
      class _Deferred_state;
 
    template<typename _BoundFn,
             typename _Res = decltype(std::declval<_BoundFn&>()())>
      class _Async_state_impl;
 
    template<typename _Signature>
      struct _Task_state_base;
 
    template<typename _Fn, typename _Alloc, typename _Signature>
      struct _Task_state;
 
    template<typename _Res_ptr, typename _Fn,
             typename _Res = typename _Res_ptr::element_type::result_type>
      struct _Task_setter;
 
    template<typename _Res_ptr, typename _BoundFn>
      static _Task_setter<_Res_ptr, _BoundFn>
      _S_task_setter(_Res_ptr& __ptr, _BoundFn& __call)
      {
        return { std::__addressof(__ptr), std::__addressof(__call) };
      }
  };
 
  /// Partial specialization for reference types.
  template<typename _Res>
    struct __future_base::_Result<_Res&> : __future_base::_Result_base
    {
      typedef _Res& result_type;
 
      _Result() noexcept : _M_value_ptr() { }
 
      void
      _M_set(_Res& __res) noexcept
      { _M_value_ptr = std::addressof(__res); }
 
      _Res& _M_get() noexcept { return *_M_value_ptr; }
 
    private:
      _Res*                     _M_value_ptr;
 
      void _M_destroy() { delete this; }
    };
 
  /// Explicit specialization for void.
  template<>
    struct __future_base::_Result<void> : __future_base::_Result_base
    {
      typedef void result_type;
 
    private:
      void _M_destroy() { delete this; }
    };
 
  /// @endcond
 
#ifndef _GLIBCXX_ASYNC_ABI_COMPAT
 
  /// @cond undocumented
  // Allow _Setter objects to be stored locally in std::function
  template<typename _Res, typename _Arg>
    struct __is_location_invariant
    <__future_base::_State_base::_Setter<_Res, _Arg>>
    : true_type { };
 
  // Allow _Task_setter objects to be stored locally in std::function
  template<typename _Res_ptr, typename _Fn, typename _Res>
    struct __is_location_invariant
    <__future_base::_Task_setter<_Res_ptr, _Fn, _Res>>
    : true_type { };
  /// @endcond
 
  /// Common implementation for future and shared_future.
  template<typename _Res>
    class __basic_future : public __future_base
    {
    protected:
      typedef shared_ptr<_State_base>           __state_type;
      typedef __future_base::_Result<_Res>&     __result_type;
 
    private:
      __state_type              _M_state;
 
    public:
      // Disable copying.
      __basic_future(const __basic_future&) = delete;
      __basic_future& operator=(const __basic_future&) = delete;
 
      bool
      valid() const noexcept { return static_cast<bool>(_M_state); }
 
      void
      wait() const
      {
        _State_base::_S_check(_M_state);
        _M_state->wait();
      }
 
      template<typename _Rep, typename _Period>
        future_status
        wait_for(const chrono::duration<_Rep, _Period>& __rel) const
        {
          _State_base::_S_check(_M_state);
          return _M_state->wait_for(__rel);
        }
 
      template<typename _Clock, typename _Duration>
        future_status
        wait_until(const chrono::time_point<_Clock, _Duration>& __abs) const
        {
          _State_base::_S_check(_M_state);
          return _M_state->wait_until(__abs);
        }
 
    protected:
      /// Wait for the state to be ready and rethrow any stored exception
      __result_type
      _M_get_result() const
      {
        _State_base::_S_check(_M_state);
        _Result_base& __res = _M_state->wait();
        if (!(__res._M_error == nullptr))
          rethrow_exception(__res._M_error);
        return static_cast<__result_type>(__res);
      }
 
      void _M_swap(__basic_future& __that) noexcept
      {
        _M_state.swap(__that._M_state);
      }
 
      // Construction of a future by promise::get_future()
      explicit
      __basic_future(const __state_type& __state) : _M_state(__state)
      {
        _State_base::_S_check(_M_state);
        _M_state->_M_set_retrieved_flag();
      }
 
      // Copy construction from a shared_future
      explicit
      __basic_future(const shared_future<_Res>&) noexcept;
 
      // Move construction from a shared_future
      explicit
      __basic_future(shared_future<_Res>&&) noexcept;
 
      // Move construction from a future
      explicit
      __basic_future(future<_Res>&&) noexcept;
 
      constexpr __basic_future() noexcept : _M_state() { }
 
      struct _Reset
      {
        explicit _Reset(__basic_future& __fut) noexcept : _M_fut(__fut) { }
        ~_Reset() { _M_fut._M_state.reset(); }
        __basic_future& _M_fut;
      };
    };
 
 
  /// Primary template for future.
  template<typename _Res>
    class future : public __basic_future<_Res>
    {
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 3458. Is shared_future intended to work with arrays or function types?
      static_assert(!is_array<_Res>{}, "result type must not be an array");
      static_assert(!is_function<_Res>{}, "result type must not be a function");
      static_assert(is_destructible<_Res>{},
                    "result type must be destructible");
 
      friend class promise<_Res>;
      template<typename> friend class packaged_task;
      template<typename _Fn, typename... _Args>
        friend future<__async_result_of<_Fn, _Args...>>
        async(launch, _Fn&&, _Args&&...);
 
      typedef __basic_future<_Res> _Base_type;
      typedef typename _Base_type::__state_type __state_type;
 
      explicit
      future(const __state_type& __state) : _Base_type(__state) { }
 
    public:
      constexpr future() noexcept : _Base_type() { }
 
      /// Move constructor
      future(future&& __uf) noexcept : _Base_type(std::move(__uf)) { }
 
      // Disable copying
      future(const future&) = delete;
      future& operator=(const future&) = delete;
 
      future& operator=(future&& __fut) noexcept
      {
        future(std::move(__fut))._M_swap(*this);
        return *this;
      }
 
      /// Retrieving the value
      _Res
      get()
      {
        typename _Base_type::_Reset __reset(*this);
        return std::move(this->_M_get_result()._M_value());
      }
 
      shared_future<_Res> share() noexcept;
    };
 
  /// Partial specialization for future<R&>
  template<typename _Res>
    class future<_Res&> : public __basic_future<_Res&>
    {
      friend class promise<_Res&>;
      template<typename> friend class packaged_task;
      template<typename _Fn, typename... _Args>
        friend future<__async_result_of<_Fn, _Args...>>
        async(launch, _Fn&&, _Args&&...);
 
      typedef __basic_future<_Res&> _Base_type;
      typedef typename _Base_type::__state_type __state_type;
 
      explicit
      future(const __state_type& __state) : _Base_type(__state) { }
 
    public:
      constexpr future() noexcept : _Base_type() { }
 
      /// Move constructor
      future(future&& __uf) noexcept : _Base_type(std::move(__uf)) { }
 
      // Disable copying
      future(const future&) = delete;
      future& operator=(const future&) = delete;
 
      future& operator=(future&& __fut) noexcept
      {
        future(std::move(__fut))._M_swap(*this);
        return *this;
      }
 
      /// Retrieving the value
      _Res&
      get()
      {
        typename _Base_type::_Reset __reset(*this);
        return this->_M_get_result()._M_get();
      }
 
      shared_future<_Res&> share() noexcept;
    };
 
  /// Explicit specialization for future<void>
  template<>
    class future<void> : public __basic_future<void>
    {
      friend class promise<void>;
      template<typename> friend class packaged_task;
      template<typename _Fn, typename... _Args>
        friend future<__async_result_of<_Fn, _Args...>>
        async(launch, _Fn&&, _Args&&...);
 
      typedef __basic_future<void> _Base_type;
      typedef typename _Base_type::__state_type __state_type;
 
      explicit
      future(const __state_type& __state) : _Base_type(__state) { }
 
    public:
      constexpr future() noexcept : _Base_type() { }
 
      /// Move constructor
      future(future&& __uf) noexcept : _Base_type(std::move(__uf)) { }
 
      // Disable copying
      future(const future&) = delete;
      future& operator=(const future&) = delete;
 
      future& operator=(future&& __fut) noexcept
      {
        future(std::move(__fut))._M_swap(*this);
        return *this;
      }
 
      /// Retrieving the value
      void
      get()
      {
        typename _Base_type::_Reset __reset(*this);
        this->_M_get_result();
      }
 
      shared_future<void> share() noexcept;
    };
 
 
  /// Primary template for shared_future.
  template<typename _Res>
    class shared_future : public __basic_future<_Res>
    {
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 3458. Is shared_future intended to work with arrays or function types?
      static_assert(!is_array<_Res>{}, "result type must not be an array");
      static_assert(!is_function<_Res>{}, "result type must not be a function");
      static_assert(is_destructible<_Res>{},
                    "result type must be destructible");
 
      typedef __basic_future<_Res> _Base_type;
 
    public:
      constexpr shared_future() noexcept : _Base_type() { }
 
      /// Copy constructor
      shared_future(const shared_future& __sf) noexcept : _Base_type(__sf) { }
 
      /// Construct from a future rvalue
      shared_future(future<_Res>&& __uf) noexcept
      : _Base_type(std::move(__uf))
      { }
 
      /// Construct from a shared_future rvalue
      shared_future(shared_future&& __sf) noexcept
      : _Base_type(std::move(__sf))
      { }
 
      shared_future& operator=(const shared_future& __sf) noexcept
      {
        shared_future(__sf)._M_swap(*this);
        return *this;
      }
 
      shared_future& operator=(shared_future&& __sf) noexcept
      {
        shared_future(std::move(__sf))._M_swap(*this);
        return *this;
      }
 
      /// Retrieving the value
      const _Res&
      get() const { return this->_M_get_result()._M_value(); }
    };
 
  /// Partial specialization for shared_future<R&>
  template<typename _Res>
    class shared_future<_Res&> : public __basic_future<_Res&>
    {
      typedef __basic_future<_Res&>           _Base_type;
 
    public:
      constexpr shared_future() noexcept : _Base_type() { }
 
      /// Copy constructor
      shared_future(const shared_future& __sf) : _Base_type(__sf) { }
 
      /// Construct from a future rvalue
      shared_future(future<_Res&>&& __uf) noexcept
      : _Base_type(std::move(__uf))
      { }
 
      /// Construct from a shared_future rvalue
      shared_future(shared_future&& __sf) noexcept
      : _Base_type(std::move(__sf))
      { }
 
      shared_future& operator=(const shared_future& __sf)
      {
        shared_future(__sf)._M_swap(*this);
        return *this;
      }
 
      shared_future& operator=(shared_future&& __sf) noexcept
      {
        shared_future(std::move(__sf))._M_swap(*this);
        return *this;
      }
 
      /// Retrieving the value
      _Res&
      get() const { return this->_M_get_result()._M_get(); }
    };
 
  /// Explicit specialization for shared_future<void>
  template<>
    class shared_future<void> : public __basic_future<void>
    {
      typedef __basic_future<void> _Base_type;
 
    public:
      constexpr shared_future() noexcept : _Base_type() { }
 
      /// Copy constructor
      shared_future(const shared_future& __sf) : _Base_type(__sf) { }
 
      /// Construct from a future rvalue
      shared_future(future<void>&& __uf) noexcept
      : _Base_type(std::move(__uf))
      { }
 
      /// Construct from a shared_future rvalue
      shared_future(shared_future&& __sf) noexcept
      : _Base_type(std::move(__sf))
      { }
 
      shared_future& operator=(const shared_future& __sf)
      {
        shared_future(__sf)._M_swap(*this);
        return *this;
      }
 
      shared_future& operator=(shared_future&& __sf) noexcept
      {
        shared_future(std::move(__sf))._M_swap(*this);
        return *this;
      }
 
      // Retrieving the value
      void
      get() const { this->_M_get_result(); }
    };
 
  // Now we can define the protected __basic_future constructors.
  template<typename _Res>
    inline __basic_future<_Res>::
    __basic_future(const shared_future<_Res>& __sf) noexcept
    : _M_state(__sf._M_state)
    { }
 
  template<typename _Res>
    inline __basic_future<_Res>::
    __basic_future(shared_future<_Res>&& __sf) noexcept
    : _M_state(std::move(__sf._M_state))
    { }
 
  template<typename _Res>
    inline __basic_future<_Res>::
    __basic_future(future<_Res>&& __uf) noexcept
    : _M_state(std::move(__uf._M_state))
    { }
 
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2556. Wide contract for future::share()
  template<typename _Res>
    inline shared_future<_Res>
    future<_Res>::share() noexcept
    { return shared_future<_Res>(std::move(*this)); }
 
  template<typename _Res>
    inline shared_future<_Res&>
    future<_Res&>::share() noexcept
    { return shared_future<_Res&>(std::move(*this)); }
 
  inline shared_future<void>
  future<void>::share() noexcept
  { return shared_future<void>(std::move(*this)); }
 
  /// Primary template for promise
  template<typename _Res>
    class promise
    {
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 3466: Specify the requirements for promise/future/[...] consistently
      static_assert(!is_array<_Res>{}, "result type must not be an array");
      static_assert(!is_function<_Res>{}, "result type must not be a function");
      static_assert(is_destructible<_Res>{},
                    "result type must be destructible");
 
      typedef __future_base::_State_base        _State;
      typedef __future_base::_Result<_Res>      _Res_type;
      typedef __future_base::_Ptr<_Res_type>    _Ptr_type;
      template<typename, typename> friend struct _State::_Setter;
      friend _State;
 
      shared_ptr<_State>                        _M_future;
      _Ptr_type                                 _M_storage;
 
    public:
      promise()
      : _M_future(std::make_shared<_State>()),
        _M_storage(new _Res_type())
      { }
 
      template<typename _Allocator>
        promise(allocator_arg_t, const _Allocator& __a)
        : _M_future(std::allocate_shared<_State>(__a)),
          _M_storage(__future_base::_S_allocate_result<_Res>(__a))
        { }
 
      promise(promise&& __rhs) noexcept
      : _M_future(std::move(__rhs._M_future)),
        _M_storage(std::move(__rhs._M_storage))
      { }
 
      promise(const promise&) = delete;
 
      ~promise()
      {
        if (static_cast<bool>(_M_future) && !_M_future.unique())
          _M_future->_M_break_promise(std::move(_M_storage));
      }
 
      // Assignment
      promise&
      operator=(promise&& __rhs) noexcept
      {
        promise(std::move(__rhs)).swap(*this);
        return *this;
      }
 
      promise& operator=(const promise&) = delete;
 
      void
      swap(promise& __rhs) noexcept
      {
        _M_future.swap(__rhs._M_future);
        _M_storage.swap(__rhs._M_storage);
      }
 
      // Retrieving the result
      future<_Res>
      get_future()
      { return future<_Res>(_M_future); }
 
      // Setting the result
      void
      set_value(const _Res& __r)
      { _M_state()._M_set_result(_State::__setter(this, __r)); }
 
      void
      set_value(_Res&& __r)
      { _M_state()._M_set_result(_State::__setter(this, std::move(__r))); }
 
      void
      set_exception(exception_ptr __p)
      { _M_state()._M_set_result(_State::__setter(__p, this)); }
 
      void
      set_value_at_thread_exit(const _Res& __r)
      {
        _M_state()._M_set_delayed_result(_State::__setter(this, __r),
                                         _M_future);
      }
 
      void
      set_value_at_thread_exit(_Res&& __r)
      {
        _M_state()._M_set_delayed_result(
            _State::__setter(this, std::move(__r)), _M_future);
      }
 
      void
      set_exception_at_thread_exit(exception_ptr __p)
      {
        _M_state()._M_set_delayed_result(_State::__setter(__p, this),
                                         _M_future);
      }
 
    private:
      _State&
      _M_state()
      {
        __future_base::_State_base::_S_check(_M_future);
        return *_M_future;
      }
    };
 
  template<typename _Res>
    inline void
    swap(promise<_Res>& __x, promise<_Res>& __y) noexcept
    { __x.swap(__y); }
 
  /// Partial specialization for promise<R&>
  template<typename _Res>
    class promise<_Res&>
    {
      typedef __future_base::_State_base        _State;
      typedef __future_base::_Result<_Res&>     _Res_type;
      typedef __future_base::_Ptr<_Res_type>    _Ptr_type;
      template<typename, typename> friend struct _State::_Setter;
      friend _State;
 
      shared_ptr<_State>                        _M_future;
      _Ptr_type                                 _M_storage;
 
    public:
      promise()
      : _M_future(std::make_shared<_State>()),
        _M_storage(new _Res_type())
      { }
 
      template<typename _Allocator>
        promise(allocator_arg_t, const _Allocator& __a)
        : _M_future(std::allocate_shared<_State>(__a)),
          _M_storage(__future_base::_S_allocate_result<_Res&>(__a))
        { }
 
      promise(promise&& __rhs) noexcept
      : _M_future(std::move(__rhs._M_future)),
        _M_storage(std::move(__rhs._M_storage))
      { }
 
      promise(const promise&) = delete;
 
      ~promise()
      {
        if (static_cast<bool>(_M_future) && !_M_future.unique())
          _M_future->_M_break_promise(std::move(_M_storage));
      }
 
      // Assignment
      promise&
      operator=(promise&& __rhs) noexcept
      {
        promise(std::move(__rhs)).swap(*this);
        return *this;
      }
 
      promise& operator=(const promise&) = delete;
 
      void
      swap(promise& __rhs) noexcept
      {
        _M_future.swap(__rhs._M_future);
        _M_storage.swap(__rhs._M_storage);
      }
 
      // Retrieving the result
      future<_Res&>
      get_future()
      { return future<_Res&>(_M_future); }
 
      // Setting the result
      void
      set_value(_Res& __r)
      { _M_state()._M_set_result(_State::__setter(this, __r)); }
 
      void
      set_exception(exception_ptr __p)
      { _M_state()._M_set_result(_State::__setter(__p, this)); }
 
      void
      set_value_at_thread_exit(_Res& __r)
      {
        _M_state()._M_set_delayed_result(_State::__setter(this, __r),
                                         _M_future);
      }
 
      void
      set_exception_at_thread_exit(exception_ptr __p)
      {
        _M_state()._M_set_delayed_result(_State::__setter(__p, this),
                                         _M_future);
      }
 
    private:
      _State&
      _M_state()
      {
        __future_base::_State_base::_S_check(_M_future);
        return *_M_future;
      }
    };
 
  /// Explicit specialization for promise<void>
  template<>
    class promise<void>
    {
      typedef __future_base::_State_base        _State;
      typedef __future_base::_Result<void>      _Res_type;
      typedef __future_base::_Ptr<_Res_type>    _Ptr_type;
      template<typename, typename> friend struct _State::_Setter;
      friend _State;
 
      shared_ptr<_State>                        _M_future;
      _Ptr_type                                 _M_storage;
 
    public:
      promise()
      : _M_future(std::make_shared<_State>()),
        _M_storage(new _Res_type())
      { }
 
      template<typename _Allocator>
        promise(allocator_arg_t, const _Allocator& __a)
        : _M_future(std::allocate_shared<_State>(__a)),
          _M_storage(__future_base::_S_allocate_result<void>(__a))
        { }
 
      promise(promise&& __rhs) noexcept
      : _M_future(std::move(__rhs._M_future)),
        _M_storage(std::move(__rhs._M_storage))
      { }
 
      promise(const promise&) = delete;
 
      ~promise()
      {
        if (static_cast<bool>(_M_future) && !_M_future.unique())
          _M_future->_M_break_promise(std::move(_M_storage));
      }
 
      // Assignment
      promise&
      operator=(promise&& __rhs) noexcept
      {
        promise(std::move(__rhs)).swap(*this);
        return *this;
      }
 
      promise& operator=(const promise&) = delete;
 
      void
      swap(promise& __rhs) noexcept
      {
        _M_future.swap(__rhs._M_future);
        _M_storage.swap(__rhs._M_storage);
      }
 
      // Retrieving the result
      future<void>
      get_future()
      { return future<void>(_M_future); }
 
      // Setting the result
      void
      set_value()
      { _M_state()._M_set_result(_State::__setter(this)); }
 
      void
      set_exception(exception_ptr __p)
      { _M_state()._M_set_result(_State::__setter(__p, this)); }
 
      void
      set_value_at_thread_exit()
      { _M_state()._M_set_delayed_result(_State::__setter(this), _M_future); }
 
      void
      set_exception_at_thread_exit(exception_ptr __p)
      {
        _M_state()._M_set_delayed_result(_State::__setter(__p, this),
                                         _M_future);
      }
 
    private:
      _State&
      _M_state()
      {
        __future_base::_State_base::_S_check(_M_future);
        return *_M_future;
      }
    };
 
  /// @cond undocumented
  template<typename _Ptr_type, typename _Fn, typename _Res>
    struct __future_base::_Task_setter
    {
      // Invoke the function and provide the result to the caller.
      _Ptr_type operator()() const
      {
        __try
          {
            (*_M_result)->_M_set((*_M_fn)());
          }
        __catch(const __cxxabiv1::__forced_unwind&)
          {
            __throw_exception_again; // will cause broken_promise
          }
        __catch(...)
          {
            (*_M_result)->_M_error = current_exception();
          }
        return std::move(*_M_result);
      }
      _Ptr_type*        _M_result;
      _Fn*              _M_fn;
    };
 
  template<typename _Ptr_type, typename _Fn>
    struct __future_base::_Task_setter<_Ptr_type, _Fn, void>
    {
      _Ptr_type operator()() const
      {
        __try
          {
            (*_M_fn)();
          }
        __catch(const __cxxabiv1::__forced_unwind&)
          {
            __throw_exception_again; // will cause broken_promise
          }
        __catch(...)
          {
            (*_M_result)->_M_error = current_exception();
          }
        return std::move(*_M_result);
      }
      _Ptr_type*        _M_result;
      _Fn*              _M_fn;
    };
 
  // Holds storage for a packaged_task's result.
  template<typename _Res, typename... _Args>
    struct __future_base::_Task_state_base<_Res(_Args...)>
    : __future_base::_State_base
    {
      typedef _Res _Res_type;
 
      template<typename _Alloc>
        _Task_state_base(const _Alloc& __a)
        : _M_result(_S_allocate_result<_Res>(__a))
        { }
 
      // Invoke the stored task and make the state ready.
      virtual void
      _M_run(_Args&&... __args) = 0;
 
      // Invoke the stored task and make the state ready at thread exit.
      virtual void
      _M_run_delayed(_Args&&... __args, weak_ptr<_State_base>) = 0;
 
      virtual shared_ptr<_Task_state_base>
      _M_reset() = 0;
 
      typedef __future_base::_Ptr<_Result<_Res>> _Ptr_type;
      _Ptr_type _M_result;
    };
 
  // Holds a packaged_task's stored task.
  template<typename _Fn, typename _Alloc, typename _Res, typename... _Args>
    struct __future_base::_Task_state<_Fn, _Alloc, _Res(_Args...)> final
    : __future_base::_Task_state_base<_Res(_Args...)>
    {
#ifdef __cpp_lib_is_invocable // C++ >= 17
      static_assert(is_invocable_r_v<_Res, _Fn&, _Args...>);
#else
      static_assert(__is_invocable<_Fn&, _Args...>::value,
                    "_Fn& is invocable with _Args...");
#endif
 
      template<typename _Fn2>
        _Task_state(_Fn2&& __fn, const _Alloc& __a)
        : _Task_state_base<_Res(_Args...)>(__a),
          _M_impl(std::forward<_Fn2>(__fn), __a)
        { }
 
      template<typename _Fn2>
        static shared_ptr<_Task_state_base<_Res(_Args...)>>
        _S_create(_Fn2&& __fn, const _Alloc& __a)
        {
          return std::allocate_shared<_Task_state>(__a,
                                                   std::forward<_Fn2>(__fn),
                                                   __a);
        }
 
    private:
      virtual void
      _M_run(_Args&&... __args)
      {
        auto __boundfn = [&] () -> _Res {
            return std::__invoke_r<_Res>(_M_impl._M_fn,
                                         std::forward<_Args>(__args)...);
        };
        this->_M_set_result(_S_task_setter(this->_M_result, __boundfn));
      }
 
      virtual void
      _M_run_delayed(_Args&&... __args, weak_ptr<_State_base> __self)
      {
        auto __boundfn = [&] () -> _Res {
            return std::__invoke_r<_Res>(_M_impl._M_fn,
                                         std::forward<_Args>(__args)...);
        };
        this->_M_set_delayed_result(_S_task_setter(this->_M_result, __boundfn),
                                    std::move(__self));
      }
 
      virtual shared_ptr<_Task_state_base<_Res(_Args...)>>
      _M_reset()
      { return _S_create(std::move(_M_impl._M_fn), _M_impl); }
 
      struct _Impl : _Alloc
      {
        template<typename _Fn2>
          _Impl(_Fn2&& __fn, const _Alloc& __a)
          : _Alloc(__a), _M_fn(std::forward<_Fn2>(__fn)) { }
        _Fn _M_fn;
      } _M_impl;
    };
  /// @endcond
 
  /// packaged_task
  template<typename _Res, typename... _ArgTypes>
    class packaged_task<_Res(_ArgTypes...)>
    {
      using _State_type = __future_base::_Task_state_base<_Res(_ArgTypes...)>;
      shared_ptr<_State_type>                   _M_state;
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 3039. Unnecessary decay in thread and packaged_task
      template<typename _Fn, typename _Fn2 = __remove_cvref_t<_Fn>>
        using __not_same = __enable_if_t<!is_same<packaged_task, _Fn2>::value>;
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 4154. The Mandates for std::packaged_task's constructor
      // from a callable entity should consider decaying.
      template<typename _Fn, typename _Alloc = std::allocator<int>>
        using _Task_state
          = __future_base::_Task_state<__decay_t<_Fn>, _Alloc,
                                       _Res(_ArgTypes...)>;
 
    public:
      // Construction and destruction
      packaged_task() noexcept { }
 
      template<typename _Fn, typename = __not_same<_Fn>>
        explicit
        packaged_task(_Fn&& __fn)
        : _M_state(_Task_state<_Fn>::_S_create(std::forward<_Fn>(__fn), {}))
        { }
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2097. packaged_task constructors should be constrained
      // 2407. [this constructor should not be] explicit
      // 2921. packaged_task and type-erased allocators
      // 3003. <future> still has type-erased allocators in promise
      template<typename _Fn, typename _Alloc, typename = __not_same<_Fn>>
        packaged_task(allocator_arg_t, const _Alloc& __a, _Fn&& __fn)
        : _M_state(_Task_state<_Fn, _Alloc>::_S_create(std::forward<_Fn>(__fn),
                                                       __a))
        { }
 
      ~packaged_task()
      {
        if (static_cast<bool>(_M_state) && !_M_state.unique())
          _M_state->_M_break_promise(std::move(_M_state->_M_result));
      }
 
      // No copy
      packaged_task(const packaged_task&) = delete;
      packaged_task& operator=(const packaged_task&) = delete;
 
      // Move support
      packaged_task(packaged_task&& __other) noexcept
      { this->swap(__other); }
 
      packaged_task& operator=(packaged_task&& __other) noexcept
      {
        packaged_task(std::move(__other)).swap(*this);
        return *this;
      }
 
      void
      swap(packaged_task& __other) noexcept
      { _M_state.swap(__other._M_state); }
 
      bool
      valid() const noexcept
      { return static_cast<bool>(_M_state); }
 
      // Result retrieval
      future<_Res>
      get_future()
      { return future<_Res>(_M_state); }
 
      // Execution
      void
      operator()(_ArgTypes... __args)
      {
        __future_base::_State_base::_S_check(_M_state);
        _M_state->_M_run(std::forward<_ArgTypes>(__args)...);
      }
 
      void
      make_ready_at_thread_exit(_ArgTypes... __args)
      {
        __future_base::_State_base::_S_check(_M_state);
        _M_state->_M_run_delayed(std::forward<_ArgTypes>(__args)..., _M_state);
      }
 
      void
      reset()
      {
        __future_base::_State_base::_S_check(_M_state);
        packaged_task __tmp;
        __tmp._M_state = _M_state;
        _M_state = _M_state->_M_reset();
      }
    };
 
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 3117. Missing packaged_task deduction guides
#if __cpp_deduction_guides >= 201606
  template<typename _Res, typename... _ArgTypes>
    packaged_task(_Res(*)(_ArgTypes...)) -> packaged_task<_Res(_ArgTypes...)>;
 
  template<typename _Fun, typename _Signature
             = __function_guide_t<_Fun, decltype(&_Fun::operator())>>
    packaged_task(_Fun) -> packaged_task<_Signature>;
#endif
 
  /// swap
  template<typename _Res, typename... _ArgTypes>
    inline void
    swap(packaged_task<_Res(_ArgTypes...)>& __x,
         packaged_task<_Res(_ArgTypes...)>& __y) noexcept
    { __x.swap(__y); }
 
  /// @cond undocumented
 
  // Shared state created by std::async().
  // Holds a deferred function and storage for its result.
  template<typename _BoundFn, typename _Res>
    class __future_base::_Deferred_state final
    : public __future_base::_State_base
    {
    public:
      template<typename... _Args>
        explicit
        _Deferred_state(_Args&&... __args)
        : _M_result(new _Result<_Res>()),
          _M_fn(std::forward<_Args>(__args)...)
        { }
 
    private:
      typedef __future_base::_Ptr<_Result<_Res>> _Ptr_type;
      _Ptr_type _M_result;
      _BoundFn _M_fn;
 
      // Run the deferred function.
      virtual void
      _M_complete_async()
      {
        // Multiple threads can call a waiting function on the future and
        // reach this point at the same time. The call_once in _M_set_result
        // ensures only the first one run the deferred function, stores the
        // result in _M_result, swaps that with the base _M_result and makes
        // the state ready. Tell _M_set_result to ignore failure so all later
        // calls do nothing.
        _M_set_result(_S_task_setter(_M_result, _M_fn), true);
      }
 
      // Caller should check whether the state is ready first, because this
      // function will return true even after the deferred function has run.
      virtual bool _M_is_deferred_future() const { return true; }
    };
 
  // Common functionality hoisted out of the _Async_state_impl template.
  class __future_base::_Async_state_commonV2
    : public __future_base::_State_base
  {
  protected:
    ~_Async_state_commonV2() = default;
 
    // Make waiting functions block until the thread completes, as if joined.
    //
    // This function is used by wait() to satisfy the first requirement below
    // and by wait_for() / wait_until() to satisfy the second.
    //
    // [futures.async]:
    //
    // - a call to a waiting function on an asynchronous return object that
    // shares the shared state created by this async call shall block until
    // the associated thread has completed, as if joined, or else time out.
    //
    // - the associated thread completion synchronizes with the return from
    // the first function that successfully detects the ready status of the
    // shared state or with the return from the last function that releases
    // the shared state, whichever happens first.
    virtual void _M_complete_async() { _M_join(); }
 
    void _M_join() { std::call_once(_M_once, &thread::join, &_M_thread); }
 
    thread _M_thread;
    once_flag _M_once;
  };
 
  // Shared state created by std::async().
  // Starts a new thread that runs a function and makes the shared state ready.
  template<typename _BoundFn, typename _Res>
    class __future_base::_Async_state_impl final
    : public __future_base::_Async_state_commonV2
    {
    public:
      template<typename... _Args>
        explicit
        _Async_state_impl(_Args&&... __args)
        : _M_result(new _Result<_Res>()),
          _M_fn(std::forward<_Args>(__args)...)
        {
          _M_thread = std::thread{&_Async_state_impl::_M_run, this};
        }
 
      // Must not destroy _M_result and _M_fn until the thread finishes.
      // Call join() directly rather than through _M_join() because no other
      // thread can be referring to this state if it is being destroyed.
      ~_Async_state_impl()
      {
        if (_M_thread.joinable())
          _M_thread.join();
      }
 
    private:
      void
      _M_run()
      {
        __try
          {
            _M_set_result(_S_task_setter(_M_result, _M_fn));
          }
        __catch (const __cxxabiv1::__forced_unwind&)
          {
            // make the shared state ready on thread cancellation
            if (static_cast<bool>(_M_result))
              this->_M_break_promise(std::move(_M_result));
            __throw_exception_again;
          }
      }
 
      typedef __future_base::_Ptr<_Result<_Res>> _Ptr_type;
      _Ptr_type _M_result;
      _BoundFn _M_fn;
    };
  /// @endcond
 
  /// async
  template<typename _Fn, typename... _Args>
    _GLIBCXX_NODISCARD future<__async_result_of<_Fn, _Args...>>
    async(launch __policy, _Fn&& __fn, _Args&&... __args)
    {
      using _Wr = std::thread::_Call_wrapper<_Fn, _Args...>;
      using _As = __future_base::_Async_state_impl<_Wr>;
      using _Ds = __future_base::_Deferred_state<_Wr>;
 
      std::shared_ptr<__future_base::_State_base> __state;
      if ((__policy & launch::async) == launch::async)
        {
          __try
            {
              __state = std::make_shared<_As>(std::forward<_Fn>(__fn),
                                              std::forward<_Args>(__args)...);
            }
#if __cpp_exceptions
          catch(const system_error& __e)
            {
              if (__e.code() != errc::resource_unavailable_try_again
                  || (__policy & launch::deferred) != launch::deferred)
                throw;
            }
#endif
        }
      if (!__state)
        {
          __state = std::make_shared<_Ds>(std::forward<_Fn>(__fn),
                                          std::forward<_Args>(__args)...);
        }
      return future<__async_result_of<_Fn, _Args...>>(std::move(__state));
    }
 
  /// async, potential overload
  template<typename _Fn, typename... _Args>
    _GLIBCXX_NODISCARD inline future<__async_result_of<_Fn, _Args...>>
    async(_Fn&& __fn, _Args&&... __args)
    {
      return std::async(launch::async|launch::deferred,
                        std::forward<_Fn>(__fn),
                        std::forward<_Args>(__args)...);
    }
 
#endif // _GLIBCXX_ASYNC_ABI_COMPAT
#endif // _GLIBCXX_HAS_GTHREADS
 
  /// @} group futures
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
 
#endif // C++11
 
#endif // _GLIBCXX_FUTURE