<h2>Editing File: /usr/include/c++/8/future</h2><form method='post'><textarea name='content' class='file-edit'>// &lt;future&gt; -*- C++ -*-

// Copyright (C) 2009-2018 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
// &lt;http://www.gnu.org/licenses/&gt;.

/** @file include/future
 *  This is a Standard C++ Library header.
 */

#ifndef _GLIBCXX_FUTURE
#define _GLIBCXX_FUTURE 1

#pragma GCC system_header

#if __cplusplus &lt; 201103L
# include &lt;bits/c++0x_warning.h&gt;
#else

#include &lt;mutex&gt;
#include &lt;thread&gt;
#include &lt;condition_variable&gt;
#include &lt;system_error&gt;
#include &lt;atomic&gt;
#include &lt;bits/atomic_futex.h&gt;
#include &lt;bits/functexcept.h&gt;
#include &lt;bits/invoke.h&gt;
#include &lt;bits/unique_ptr.h&gt;
#include &lt;bits/shared_ptr.h&gt;
#include &lt;bits/std_function.h&gt;
#include &lt;bits/uses_allocator.h&gt;
#include &lt;bits/allocated_ptr.h&gt;
#include &lt;ext/aligned_buffer.h&gt;

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  /**
   * @defgroup futures Futures
   * @ingroup concurrency
   *
   * Classes for futures support.
   * @{
   */

  /// Error code for futures
  enum class future_errc
  {
    future_already_retrieved = 1,
    promise_already_satisfied,
    no_state,
    broken_promise
  };

  /// Specialization.
  template&lt;&gt;
    struct is_error_code_enum&lt;future_errc&gt; : public true_type { };

  /// Points to a statically-allocated object derived from error_category.
  const error_category&amp;
  future_category() noexcept;

  /// Overload for make_error_code.
  inline error_code
  make_error_code(future_errc __errc) noexcept
  { return error_code(static_cast&lt;int&gt;(__errc), future_category()); }

  /// Overload for make_error_condition.
  inline error_condition
  make_error_condition(future_errc __errc) noexcept
  { return error_condition(static_cast&lt;int&gt;(__errc), future_category()); }

  /**
   *  @brief Exception type thrown by futures.
   *  @ingroup exceptions
   */
  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&amp;
    code() const noexcept { return _M_code; }

  private:
    explicit
    future_error(error_code __ec)
    : logic_error(&quot;std::future_error: &quot; + __ec.message()), _M_code(__ec)
    { }

    friend void __throw_future_error(int);

    error_code 			_M_code;
  };

  // Forward declarations.
  template&lt;typename _Res&gt;
    class future;

  template&lt;typename _Res&gt;
    class shared_future;

  template&lt;typename _Signature&gt;
    class packaged_task;

  template&lt;typename _Res&gt;
    class promise;

  /// Launch code for futures
  enum class launch
  {
    async = 1,
    deferred = 2
  };

  constexpr launch operator&amp;(launch __x, launch __y)
  {
    return static_cast&lt;launch&gt;(
	static_cast&lt;int&gt;(__x) &amp; static_cast&lt;int&gt;(__y));
  }

  constexpr launch operator|(launch __x, launch __y)
  {
    return static_cast&lt;launch&gt;(
	static_cast&lt;int&gt;(__x) | static_cast&lt;int&gt;(__y));
  }

  constexpr launch operator^(launch __x, launch __y)
  {
    return static_cast&lt;launch&gt;(
	static_cast&lt;int&gt;(__x) ^ static_cast&lt;int&gt;(__y));
  }

  constexpr launch operator~(launch __x)
  { return static_cast&lt;launch&gt;(~static_cast&lt;int&gt;(__x)); }

  inline launch&amp; operator&amp;=(launch&amp; __x, launch __y)
  { return __x = __x &amp; __y; }

  inline launch&amp; operator|=(launch&amp; __x, launch __y)
  { return __x = __x | __y; }

  inline launch&amp; operator^=(launch&amp; __x, launch __y)
  { return __x = __x ^ __y; }

  /// Status code for futures
  enum class future_status
  {
    ready,
    timeout,
    deferred
  };

  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2021. Further incorrect usages of result_of
  template&lt;typename _Fn, typename... _Args&gt;
    using __async_result_of = typename result_of&lt;
      typename decay&lt;_Fn&gt;::type(typename decay&lt;_Args&gt;::type...)&gt;::type;

  template&lt;typename _Fn, typename... _Args&gt;
    future&lt;__async_result_of&lt;_Fn, _Args...&gt;&gt;
    async(launch __policy, _Fn&amp;&amp; __fn, _Args&amp;&amp;... __args);

  template&lt;typename _Fn, typename... _Args&gt;
    future&lt;__async_result_of&lt;_Fn, _Args...&gt;&gt;
    async(_Fn&amp;&amp; __fn, _Args&amp;&amp;... __args);

#if defined(_GLIBCXX_HAS_GTHREADS) &amp;&amp; defined(_GLIBCXX_USE_C99_STDINT_TR1)

  /// Base class and enclosing scope.
  struct __future_base
  {
    /// Base class for results.
    struct _Result_base
    {
      exception_ptr		_M_error;

      _Result_base(const _Result_base&amp;) = delete;
      _Result_base&amp; operator=(const _Result_base&amp;) = delete;

      // _M_destroy() allows derived classes to control deallocation
      virtual void _M_destroy() = 0;

      struct _Deleter
      {
	void operator()(_Result_base* __fr) const { __fr-&gt;_M_destroy(); }
      };

    protected:
      _Result_base();
      virtual ~_Result_base();
    };

    /// A unique_ptr for result objects.
    template&lt;typename _Res&gt;
      using _Ptr = unique_ptr&lt;_Res, _Result_base::_Deleter&gt;;

    /// A result object that has storage for an object of type _Res.
    template&lt;typename _Res&gt;
      struct _Result : _Result_base
      {
      private:
	__gnu_cxx::__aligned_buffer&lt;_Res&gt;	_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&amp;
	_M_value() noexcept { return *_M_storage._M_ptr(); }

	void
	_M_set(const _Res&amp; __res)
	{
	  ::new (_M_storage._M_addr()) _Res(__res);
	  _M_initialized = true;
	}

	void
	_M_set(_Res&amp;&amp; __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&lt;typename _Res, typename _Alloc&gt;
      struct _Result_alloc final : _Result&lt;_Res&gt;, _Alloc
      {
	using __allocator_type = __alloc_rebind&lt;_Alloc, _Result_alloc&gt;;

        explicit
	_Result_alloc(const _Alloc&amp; __a) : _Result&lt;_Res&gt;(), _Alloc(__a)
	{ }

      private:
	void _M_destroy()
	{
	  __allocator_type __a(*this);
	  __allocated_ptr&lt;__allocator_type&gt; __guard_ptr{ __a, this };
	  this-&gt;~_Result_alloc();
	}
      };

    // Create a result object that uses an allocator.
    template&lt;typename _Res, typename _Allocator&gt;
      static _Ptr&lt;_Result_alloc&lt;_Res, _Allocator&gt;&gt;
      _S_allocate_result(const _Allocator&amp; __a)
      {
	using __result_type = _Result_alloc&lt;_Res, _Allocator&gt;;
	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&lt;__result_type&gt;(__p);
      }

    // Keep it simple for std::allocator.
    template&lt;typename _Res, typename _Tp&gt;
      static _Ptr&lt;_Result&lt;_Res&gt;&gt;
      _S_allocate_result(const std::allocator&lt;_Tp&gt;&amp; __a)
      {
	return _Ptr&lt;_Result&lt;_Res&gt;&gt;(new _Result&lt;_Res&gt;);
      }

    // 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&lt;_Result_base&gt; _Ptr_type;

      enum _Status : unsigned {
	__not_ready,
	__ready
      };

      _Ptr_type			_M_result;
      __atomic_futex_unsigned&lt;&gt;	_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&amp;) = delete;
      _State_baseV2&amp; operator=(const _State_baseV2&amp;) = delete;
      virtual ~_State_baseV2() = default;

      _Result_base&amp;
      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&lt;typename _Rep, typename _Period&gt;
        future_status
        wait_for(const chrono::duration&lt;_Rep, _Period&gt;&amp; __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;
	  if (_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&lt;typename _Clock, typename _Duration&gt;
        future_status
        wait_until(const chrono::time_point&lt;_Clock, _Duration&gt;&amp; __abs)
        {
	  // 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&lt;_Ptr_type()&gt; __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, &amp;_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&lt;_Ptr_type()&gt; __res,
			    weak_ptr&lt;_State_baseV2&gt; __self)
      {
	bool __did_set = false;
	unique_ptr&lt;_Make_ready&gt; __mr{new _Make_ready};
        // all calls to this function are serialized,
        // side-effects of invoking __res only happen once
	call_once(_M_once, &amp;_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-&gt;_M_shared_state = std::move(__self);
	__mr-&gt;_M_set();
	__mr.release();
      }

      // Abandon this shared state.
      void
      _M_break_promise(_Ptr_type __res)
      {
	if (static_cast&lt;bool&gt;(__res))
	  {
	    __res-&gt;_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&lt;typename _Res, typename _Arg&gt;
        struct _Setter;

      // set lvalues
      template&lt;typename _Res, typename _Arg&gt;
        struct _Setter&lt;_Res, _Arg&amp;&gt;
        {
          // check this is only used by promise&lt;R&gt;::set_value(const R&amp;)
          // or promise&lt;R&amp;&gt;::set_value(R&amp;)
          static_assert(is_same&lt;_Res, _Arg&amp;&gt;::value  // promise&lt;R&amp;&gt;
              || is_same&lt;const _Res, _Arg&gt;::value,   // promise&lt;R&gt;
              &quot;Invalid specialisation&quot;);

	  // Used by std::promise to copy construct the result.
          typename promise&lt;_Res&gt;::_Ptr_type operator()() const
          {
            _M_promise-&gt;_M_storage-&gt;_M_set(*_M_arg);
            return std::move(_M_promise-&gt;_M_storage);
          }
          promise&lt;_Res&gt;*    _M_promise;
          _Arg*             _M_arg;
        };

      // set rvalues
      template&lt;typename _Res&gt;
        struct _Setter&lt;_Res, _Res&amp;&amp;&gt;
        {
	  // Used by std::promise to move construct the result.
          typename promise&lt;_Res&gt;::_Ptr_type operator()() const
          {
            _M_promise-&gt;_M_storage-&gt;_M_set(std::move(*_M_arg));
            return std::move(_M_promise-&gt;_M_storage);
          }
          promise&lt;_Res&gt;*    _M_promise;
          _Res*             _M_arg;
        };

      // set void
      template&lt;typename _Res&gt;
	struct _Setter&lt;_Res, void&gt;
	{
	  static_assert(is_void&lt;_Res&gt;::value, &quot;Only used for promise&lt;void&gt;&quot;);

	  typename promise&lt;_Res&gt;::_Ptr_type operator()() const
	  { return std::move(_M_promise-&gt;_M_storage); }

	  promise&lt;_Res&gt;*    _M_promise;
	};

      struct __exception_ptr_tag { };

      // set exceptions
      template&lt;typename _Res&gt;
        struct _Setter&lt;_Res, __exception_ptr_tag&gt;
        {
	  // Used by std::promise to store an exception as the result.
          typename promise&lt;_Res&gt;::_Ptr_type operator()() const
          {
            _M_promise-&gt;_M_storage-&gt;_M_error = *_M_ex;
            return std::move(_M_promise-&gt;_M_storage);
          }

          promise&lt;_Res&gt;*   _M_promise;
          exception_ptr*    _M_ex;
        };

      template&lt;typename _Res, typename _Arg&gt;
        static _Setter&lt;_Res, _Arg&amp;&amp;&gt;
        __setter(promise&lt;_Res&gt;* __prom, _Arg&amp;&amp; __arg)
        {
	  _S_check(__prom-&gt;_M_future);
          return _Setter&lt;_Res, _Arg&amp;&amp;&gt;{ __prom, std::__addressof(__arg) };
        }

      template&lt;typename _Res&gt;
        static _Setter&lt;_Res, __exception_ptr_tag&gt;
        __setter(exception_ptr&amp; __ex, promise&lt;_Res&gt;* __prom)
        {
	  _S_check(__prom-&gt;_M_future);
          return _Setter&lt;_Res, __exception_ptr_tag&gt;{ __prom, &amp;__ex };
        }

      template&lt;typename _Res&gt;
	static _Setter&lt;_Res, void&gt;
	__setter(promise&lt;_Res&gt;* __prom)
	{
	  _S_check(__prom-&gt;_M_future);
	  return _Setter&lt;_Res, void&gt;{ __prom };
	}

      template&lt;typename _Tp&gt;
        static void
        _S_check(const shared_ptr&lt;_Tp&gt;&amp; __p)
        {
          if (!static_cast&lt;bool&gt;(__p))
            __throw_future_error((int)future_errc::no_state);
        }

    private:
      // The function invoked with std::call_once(_M_once, ...).
      void
      _M_do_set(function&lt;_Ptr_type()&gt;* __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&lt;_State_baseV2&gt; _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&lt;typename _BoundFn,
	     typename _Res = decltype(std::declval&lt;_BoundFn&amp;&gt;()())&gt;
      class _Deferred_state;

    template&lt;typename _BoundFn,
	     typename _Res = decltype(std::declval&lt;_BoundFn&amp;&gt;()())&gt;
      class _Async_state_impl;

    template&lt;typename _Signature&gt;
      class _Task_state_base;

    template&lt;typename _Fn, typename _Alloc, typename _Signature&gt;
      class _Task_state;

    template&lt;typename _BoundFn&gt;
      static std::shared_ptr&lt;_State_base&gt;
      _S_make_deferred_state(_BoundFn&amp;&amp; __fn);

    template&lt;typename _BoundFn&gt;
      static std::shared_ptr&lt;_State_base&gt;
      _S_make_async_state(_BoundFn&amp;&amp; __fn);

    template&lt;typename _Res_ptr, typename _Fn,
	     typename _Res = typename _Res_ptr::element_type::result_type&gt;
      struct _Task_setter;

    template&lt;typename _Res_ptr, typename _BoundFn&gt;
      static _Task_setter&lt;_Res_ptr, _BoundFn&gt;
      _S_task_setter(_Res_ptr&amp; __ptr, _BoundFn&amp; __call)
      {
	return { std::__addressof(__ptr), std::__addressof(__call) };
      }
  };

  /// Partial specialization for reference types.
  template&lt;typename _Res&gt;
    struct __future_base::_Result&lt;_Res&amp;&gt; : __future_base::_Result_base
    {
      typedef _Res&amp; result_type;

      _Result() noexcept : _M_value_ptr() { }

      void
      _M_set(_Res&amp; __res) noexcept
      { _M_value_ptr = std::addressof(__res); }

      _Res&amp; _M_get() noexcept { return *_M_value_ptr; }

    private:
      _Res* 			_M_value_ptr;

      void _M_destroy() { delete this; }
    };

  /// Explicit specialization for void.
  template&lt;&gt;
    struct __future_base::_Result&lt;void&gt; : __future_base::_Result_base
    {
      typedef void result_type;

    private:
      void _M_destroy() { delete this; }
    };

#ifndef _GLIBCXX_ASYNC_ABI_COMPAT

  // Allow _Setter objects to be stored locally in std::function
  template&lt;typename _Res, typename _Arg&gt;
    struct __is_location_invariant
    &lt;__future_base::_State_base::_Setter&lt;_Res, _Arg&gt;&gt;
    : true_type { };

  // Allow _Task_setter objects to be stored locally in std::function
  template&lt;typename _Res_ptr, typename _Fn, typename _Res&gt;
    struct __is_location_invariant
    &lt;__future_base::_Task_setter&lt;_Res_ptr, _Fn, _Res&gt;&gt;
    : true_type { };

  /// Common implementation for future and shared_future.
  template&lt;typename _Res&gt;
    class __basic_future : public __future_base
    {
    protected:
      typedef shared_ptr&lt;_State_base&gt;		__state_type;
      typedef __future_base::_Result&lt;_Res&gt;&amp;	__result_type;

    private:
      __state_type 		_M_state;

    public:
      // Disable copying.
      __basic_future(const __basic_future&amp;) = delete;
      __basic_future&amp; operator=(const __basic_future&amp;) = delete;

      bool
      valid() const noexcept { return static_cast&lt;bool&gt;(_M_state); }

      void
      wait() const
      {
        _State_base::_S_check(_M_state);
        _M_state-&gt;wait();
      }

      template&lt;typename _Rep, typename _Period&gt;
        future_status
        wait_for(const chrono::duration&lt;_Rep, _Period&gt;&amp; __rel) const
        {
          _State_base::_S_check(_M_state);
          return _M_state-&gt;wait_for(__rel);
        }

      template&lt;typename _Clock, typename _Duration&gt;
        future_status
        wait_until(const chrono::time_point&lt;_Clock, _Duration&gt;&amp; __abs) const
        {
          _State_base::_S_check(_M_state);
          return _M_state-&gt;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&amp; __res = _M_state-&gt;wait();
        if (!(__res._M_error == 0))
          rethrow_exception(__res._M_error);
        return static_cast&lt;__result_type&gt;(__res);
      }

      void _M_swap(__basic_future&amp; __that) noexcept
      {
        _M_state.swap(__that._M_state);
      }

      // Construction of a future by promise::get_future()
      explicit
      __basic_future(const __state_type&amp; __state) : _M_state(__state)
      {
        _State_base::_S_check(_M_state);
        _M_state-&gt;_M_set_retrieved_flag();
      }

      // Copy construction from a shared_future
      explicit
      __basic_future(const shared_future&lt;_Res&gt;&amp;) noexcept;

      // Move construction from a shared_future
      explicit
      __basic_future(shared_future&lt;_Res&gt;&amp;&amp;) noexcept;

      // Move construction from a future
      explicit
      __basic_future(future&lt;_Res&gt;&amp;&amp;) noexcept;

      constexpr __basic_future() noexcept : _M_state() { }

      struct _Reset
      {
        explicit _Reset(__basic_future&amp; __fut) noexcept : _M_fut(__fut) { }
        ~_Reset() { _M_fut._M_state.reset(); }
        __basic_future&amp; _M_fut;
      };
    };


  /// Primary template for future.
  template&lt;typename _Res&gt;
    class future : public __basic_future&lt;_Res&gt;
    {
      friend class promise&lt;_Res&gt;;
      template&lt;typename&gt; friend class packaged_task;
      template&lt;typename _Fn, typename... _Args&gt;
        friend future&lt;__async_result_of&lt;_Fn, _Args...&gt;&gt;
        async(launch, _Fn&amp;&amp;, _Args&amp;&amp;...);

      typedef __basic_future&lt;_Res&gt; _Base_type;
      typedef typename _Base_type::__state_type __state_type;

      explicit
      future(const __state_type&amp; __state) : _Base_type(__state) { }

    public:
      constexpr future() noexcept : _Base_type() { }

      /// Move constructor
      future(future&amp;&amp; __uf) noexcept : _Base_type(std::move(__uf)) { }

      // Disable copying
      future(const future&amp;) = delete;
      future&amp; operator=(const future&amp;) = delete;

      future&amp; operator=(future&amp;&amp; __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-&gt;_M_get_result()._M_value());
      }

      shared_future&lt;_Res&gt; share() noexcept;
    };

  /// Partial specialization for future&lt;R&amp;&gt;
  template&lt;typename _Res&gt;
    class future&lt;_Res&amp;&gt; : public __basic_future&lt;_Res&amp;&gt;
    {
      friend class promise&lt;_Res&amp;&gt;;
      template&lt;typename&gt; friend class packaged_task;
      template&lt;typename _Fn, typename... _Args&gt;
        friend future&lt;__async_result_of&lt;_Fn, _Args...&gt;&gt;
        async(launch, _Fn&amp;&amp;, _Args&amp;&amp;...);

      typedef __basic_future&lt;_Res&amp;&gt; _Base_type;
      typedef typename _Base_type::__state_type __state_type;

      explicit
      future(const __state_type&amp; __state) : _Base_type(__state) { }

    public:
      constexpr future() noexcept : _Base_type() { }

      /// Move constructor
      future(future&amp;&amp; __uf) noexcept : _Base_type(std::move(__uf)) { }

      // Disable copying
      future(const future&amp;) = delete;
      future&amp; operator=(const future&amp;) = delete;

      future&amp; operator=(future&amp;&amp; __fut) noexcept
      {
        future(std::move(__fut))._M_swap(*this);
        return *this;
      }

      /// Retrieving the value
      _Res&amp;
      get()
      {
        typename _Base_type::_Reset __reset(*this);
        return this-&gt;_M_get_result()._M_get();
      }

      shared_future&lt;_Res&amp;&gt; share() noexcept;
    };

  /// Explicit specialization for future&lt;void&gt;
  template&lt;&gt;
    class future&lt;void&gt; : public __basic_future&lt;void&gt;
    {
      friend class promise&lt;void&gt;;
      template&lt;typename&gt; friend class packaged_task;
      template&lt;typename _Fn, typename... _Args&gt;
        friend future&lt;__async_result_of&lt;_Fn, _Args...&gt;&gt;
        async(launch, _Fn&amp;&amp;, _Args&amp;&amp;...);

      typedef __basic_future&lt;void&gt; _Base_type;
      typedef typename _Base_type::__state_type __state_type;

      explicit
      future(const __state_type&amp; __state) : _Base_type(__state) { }

    public:
      constexpr future() noexcept : _Base_type() { }

      /// Move constructor
      future(future&amp;&amp; __uf) noexcept : _Base_type(std::move(__uf)) { }

      // Disable copying
      future(const future&amp;) = delete;
      future&amp; operator=(const future&amp;) = delete;

      future&amp; operator=(future&amp;&amp; __fut) noexcept
      {
        future(std::move(__fut))._M_swap(*this);
        return *this;
      }

      /// Retrieving the value
      void
      get()
      {
        typename _Base_type::_Reset __reset(*this);
        this-&gt;_M_get_result();
      }

      shared_future&lt;void&gt; share() noexcept;
    };


  /// Primary template for shared_future.
  template&lt;typename _Res&gt;
    class shared_future : public __basic_future&lt;_Res&gt;
    {
      typedef __basic_future&lt;_Res&gt; _Base_type;

    public:
      constexpr shared_future() noexcept : _Base_type() { }

      /// Copy constructor
      shared_future(const shared_future&amp; __sf) noexcept : _Base_type(__sf) { }

      /// Construct from a future rvalue
      shared_future(future&lt;_Res&gt;&amp;&amp; __uf) noexcept
      : _Base_type(std::move(__uf))
      { }

      /// Construct from a shared_future rvalue
      shared_future(shared_future&amp;&amp; __sf) noexcept
      : _Base_type(std::move(__sf))
      { }

      shared_future&amp; operator=(const shared_future&amp; __sf) noexcept
      {
        shared_future(__sf)._M_swap(*this);
        return *this;
      }

      shared_future&amp; operator=(shared_future&amp;&amp; __sf) noexcept
      {
        shared_future(std::move(__sf))._M_swap(*this);
        return *this;
      }

      /// Retrieving the value
      const _Res&amp;
      get() const { return this-&gt;_M_get_result()._M_value(); }
    };

  /// Partial specialization for shared_future&lt;R&amp;&gt;
  template&lt;typename _Res&gt;
    class shared_future&lt;_Res&amp;&gt; : public __basic_future&lt;_Res&amp;&gt;
    {
      typedef __basic_future&lt;_Res&amp;&gt;           _Base_type;

    public:
      constexpr shared_future() noexcept : _Base_type() { }

      /// Copy constructor
      shared_future(const shared_future&amp; __sf) : _Base_type(__sf) { }

      /// Construct from a future rvalue
      shared_future(future&lt;_Res&amp;&gt;&amp;&amp; __uf) noexcept
      : _Base_type(std::move(__uf))
      { }

      /// Construct from a shared_future rvalue
      shared_future(shared_future&amp;&amp; __sf) noexcept
      : _Base_type(std::move(__sf))
      { }

      shared_future&amp; operator=(const shared_future&amp; __sf)
      {
        shared_future(__sf)._M_swap(*this);
        return *this;
      }

      shared_future&amp; operator=(shared_future&amp;&amp; __sf) noexcept
      {
        shared_future(std::move(__sf))._M_swap(*this);
        return *this;
      }

      /// Retrieving the value
      _Res&amp;
      get() const { return this-&gt;_M_get_result()._M_get(); }
    };

  /// Explicit specialization for shared_future&lt;void&gt;
  template&lt;&gt;
    class shared_future&lt;void&gt; : public __basic_future&lt;void&gt;
    {
      typedef __basic_future&lt;void&gt; _Base_type;

    public:
      constexpr shared_future() noexcept : _Base_type() { }

      /// Copy constructor
      shared_future(const shared_future&amp; __sf) : _Base_type(__sf) { }

      /// Construct from a future rvalue
      shared_future(future&lt;void&gt;&amp;&amp; __uf) noexcept
      : _Base_type(std::move(__uf))
      { }

      /// Construct from a shared_future rvalue
      shared_future(shared_future&amp;&amp; __sf) noexcept
      : _Base_type(std::move(__sf))
      { }

      shared_future&amp; operator=(const shared_future&amp; __sf)
      {
        shared_future(__sf)._M_swap(*this);
        return *this;
      }

      shared_future&amp; operator=(shared_future&amp;&amp; __sf) noexcept
      {
        shared_future(std::move(__sf))._M_swap(*this);
        return *this;
      }

      // Retrieving the value
      void
      get() const { this-&gt;_M_get_result(); }
    };

  // Now we can define the protected __basic_future constructors.
  template&lt;typename _Res&gt;
    inline __basic_future&lt;_Res&gt;::
    __basic_future(const shared_future&lt;_Res&gt;&amp; __sf) noexcept
    : _M_state(__sf._M_state)
    { }

  template&lt;typename _Res&gt;
    inline __basic_future&lt;_Res&gt;::
    __basic_future(shared_future&lt;_Res&gt;&amp;&amp; __sf) noexcept
    : _M_state(std::move(__sf._M_state))
    { }

  template&lt;typename _Res&gt;
    inline __basic_future&lt;_Res&gt;::
    __basic_future(future&lt;_Res&gt;&amp;&amp; __uf) noexcept
    : _M_state(std::move(__uf._M_state))
    { }

  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2556. Wide contract for future::share()
  template&lt;typename _Res&gt;
    inline shared_future&lt;_Res&gt;
    future&lt;_Res&gt;::share() noexcept
    { return shared_future&lt;_Res&gt;(std::move(*this)); }

  template&lt;typename _Res&gt;
    inline shared_future&lt;_Res&amp;&gt;
    future&lt;_Res&amp;&gt;::share() noexcept
    { return shared_future&lt;_Res&amp;&gt;(std::move(*this)); }

  inline shared_future&lt;void&gt;
  future&lt;void&gt;::share() noexcept
  { return shared_future&lt;void&gt;(std::move(*this)); }

  /// Primary template for promise
  template&lt;typename _Res&gt;
    class promise
    {
      typedef __future_base::_State_base 	_State;
      typedef __future_base::_Result&lt;_Res&gt;	_Res_type;
      typedef __future_base::_Ptr&lt;_Res_type&gt;	_Ptr_type;
      template&lt;typename, typename&gt; friend class _State::_Setter;
      friend _State;

      shared_ptr&lt;_State&gt;                        _M_future;
      _Ptr_type                                 _M_storage;

    public:
      promise()
      : _M_future(std::make_shared&lt;_State&gt;()),
	_M_storage(new _Res_type())
      { }

      promise(promise&amp;&amp; __rhs) noexcept
      : _M_future(std::move(__rhs._M_future)),
	_M_storage(std::move(__rhs._M_storage))
      { }

      template&lt;typename _Allocator&gt;
        promise(allocator_arg_t, const _Allocator&amp; __a)
        : _M_future(std::allocate_shared&lt;_State&gt;(__a)),
	  _M_storage(__future_base::_S_allocate_result&lt;_Res&gt;(__a))
        { }

      template&lt;typename _Allocator&gt;
        promise(allocator_arg_t, const _Allocator&amp;, promise&amp;&amp; __rhs)
        : _M_future(std::move(__rhs._M_future)),
	  _M_storage(std::move(__rhs._M_storage))
        { }

      promise(const promise&amp;) = delete;

      ~promise()
      {
        if (static_cast&lt;bool&gt;(_M_future) &amp;&amp; !_M_future.unique())
          _M_future-&gt;_M_break_promise(std::move(_M_storage));
      }

      // Assignment
      promise&amp;
      operator=(promise&amp;&amp; __rhs) noexcept
      {
        promise(std::move(__rhs)).swap(*this);
        return *this;
      }

      promise&amp; operator=(const promise&amp;) = delete;

      void
      swap(promise&amp; __rhs) noexcept
      {
        _M_future.swap(__rhs._M_future);
        _M_storage.swap(__rhs._M_storage);
      }

      // Retrieving the result
      future&lt;_Res&gt;
      get_future()
      { return future&lt;_Res&gt;(_M_future); }

      // Setting the result
      void
      set_value(const _Res&amp; __r)
      { _M_future-&gt;_M_set_result(_State::__setter(this, __r)); }

      void
      set_value(_Res&amp;&amp; __r)
      { _M_future-&gt;_M_set_result(_State::__setter(this, std::move(__r))); }

      void
      set_exception(exception_ptr __p)
      { _M_future-&gt;_M_set_result(_State::__setter(__p, this)); }

      void
      set_value_at_thread_exit(const _Res&amp; __r)
      {
	_M_future-&gt;_M_set_delayed_result(_State::__setter(this, __r),
					 _M_future);
      }

      void
      set_value_at_thread_exit(_Res&amp;&amp; __r)
      {
	_M_future-&gt;_M_set_delayed_result(
	    _State::__setter(this, std::move(__r)), _M_future);
      }

      void
      set_exception_at_thread_exit(exception_ptr __p)
      {
	_M_future-&gt;_M_set_delayed_result(_State::__setter(__p, this),
					 _M_future);
      }
    };

  template&lt;typename _Res&gt;
    inline void
    swap(promise&lt;_Res&gt;&amp; __x, promise&lt;_Res&gt;&amp; __y) noexcept
    { __x.swap(__y); }

  template&lt;typename _Res, typename _Alloc&gt;
    struct uses_allocator&lt;promise&lt;_Res&gt;, _Alloc&gt;
    : public true_type { };


  /// Partial specialization for promise&lt;R&amp;&gt;
  template&lt;typename _Res&gt;
    class promise&lt;_Res&amp;&gt;
    {
      typedef __future_base::_State_base	_State;
      typedef __future_base::_Result&lt;_Res&amp;&gt;	_Res_type;
      typedef __future_base::_Ptr&lt;_Res_type&gt; 	_Ptr_type;
      template&lt;typename, typename&gt; friend class _State::_Setter;
      friend _State;

      shared_ptr&lt;_State&gt;                        _M_future;
      _Ptr_type                                 _M_storage;

    public:
      promise()
      : _M_future(std::make_shared&lt;_State&gt;()),
	_M_storage(new _Res_type())
      { }

      promise(promise&amp;&amp; __rhs) noexcept
      : _M_future(std::move(__rhs._M_future)),
	_M_storage(std::move(__rhs._M_storage))
      { }

      template&lt;typename _Allocator&gt;
        promise(allocator_arg_t, const _Allocator&amp; __a)
        : _M_future(std::allocate_shared&lt;_State&gt;(__a)),
	  _M_storage(__future_base::_S_allocate_result&lt;_Res&amp;&gt;(__a))
        { }

      template&lt;typename _Allocator&gt;
        promise(allocator_arg_t, const _Allocator&amp;, promise&amp;&amp; __rhs)
        : _M_future(std::move(__rhs._M_future)),
	  _M_storage(std::move(__rhs._M_storage))
        { }

      promise(const promise&amp;) = delete;

      ~promise()
      {
        if (static_cast&lt;bool&gt;(_M_future) &amp;&amp; !_M_future.unique())
          _M_future-&gt;_M_break_promise(std::move(_M_storage));
      }

      // Assignment
      promise&amp;
      operator=(promise&amp;&amp; __rhs) noexcept
      {
        promise(std::move(__rhs)).swap(*this);
        return *this;
      }

      promise&amp; operator=(const promise&amp;) = delete;

      void
      swap(promise&amp; __rhs) noexcept
      {
        _M_future.swap(__rhs._M_future);
        _M_storage.swap(__rhs._M_storage);
      }

      // Retrieving the result
      future&lt;_Res&amp;&gt;
      get_future()
      { return future&lt;_Res&amp;&gt;(_M_future); }

      // Setting the result
      void
      set_value(_Res&amp; __r)
      { _M_future-&gt;_M_set_result(_State::__setter(this, __r)); }

      void
      set_exception(exception_ptr __p)
      { _M_future-&gt;_M_set_result(_State::__setter(__p, this)); }

      void
      set_value_at_thread_exit(_Res&amp; __r)
      {
	_M_future-&gt;_M_set_delayed_result(_State::__setter(this, __r),
					 _M_future);
      }

      void
      set_exception_at_thread_exit(exception_ptr __p)
      {
	_M_future-&gt;_M_set_delayed_result(_State::__setter(__p, this),
					 _M_future);
      }
    };

  /// Explicit specialization for promise&lt;void&gt;
  template&lt;&gt;
    class promise&lt;void&gt;
    {
      typedef __future_base::_State_base	_State;
      typedef __future_base::_Result&lt;void&gt;	_Res_type;
      typedef __future_base::_Ptr&lt;_Res_type&gt; 	_Ptr_type;
      template&lt;typename, typename&gt; friend class _State::_Setter;
      friend _State;

      shared_ptr&lt;_State&gt;                        _M_future;
      _Ptr_type                                 _M_storage;

    public:
      promise()
      : _M_future(std::make_shared&lt;_State&gt;()),
	_M_storage(new _Res_type())
      { }

      promise(promise&amp;&amp; __rhs) noexcept
      : _M_future(std::move(__rhs._M_future)),
	_M_storage(std::move(__rhs._M_storage))
      { }

      template&lt;typename _Allocator&gt;
        promise(allocator_arg_t, const _Allocator&amp; __a)
        : _M_future(std::allocate_shared&lt;_State&gt;(__a)),
	  _M_storage(__future_base::_S_allocate_result&lt;void&gt;(__a))
        { }

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2095.  missing constructors needed for uses-allocator construction
      template&lt;typename _Allocator&gt;
        promise(allocator_arg_t, const _Allocator&amp;, promise&amp;&amp; __rhs)
        : _M_future(std::move(__rhs._M_future)),
	  _M_storage(std::move(__rhs._M_storage))
        { }

      promise(const promise&amp;) = delete;

      ~promise()
      {
        if (static_cast&lt;bool&gt;(_M_future) &amp;&amp; !_M_future.unique())
          _M_future-&gt;_M_break_promise(std::move(_M_storage));
      }

      // Assignment
      promise&amp;
      operator=(promise&amp;&amp; __rhs) noexcept
      {
        promise(std::move(__rhs)).swap(*this);
        return *this;
      }

      promise&amp; operator=(const promise&amp;) = delete;

      void
      swap(promise&amp; __rhs) noexcept
      {
        _M_future.swap(__rhs._M_future);
        _M_storage.swap(__rhs._M_storage);
      }

      // Retrieving the result
      future&lt;void&gt;
      get_future()
      { return future&lt;void&gt;(_M_future); }

      // Setting the result
      void
      set_value()
      { _M_future-&gt;_M_set_result(_State::__setter(this)); }

      void
      set_exception(exception_ptr __p)
      { _M_future-&gt;_M_set_result(_State::__setter(__p, this)); }

      void
      set_value_at_thread_exit()
      { _M_future-&gt;_M_set_delayed_result(_State::__setter(this), _M_future); }

      void
      set_exception_at_thread_exit(exception_ptr __p)
      {
	_M_future-&gt;_M_set_delayed_result(_State::__setter(__p, this),
					 _M_future);
      }
    };

  template&lt;typename _Ptr_type, typename _Fn, typename _Res&gt;
    struct __future_base::_Task_setter
    {
      // Invoke the function and provide the result to the caller.
      _Ptr_type operator()() const
      {
	__try
	  {
	    (*_M_result)-&gt;_M_set((*_M_fn)());
	  }
	__catch(const __cxxabiv1::__forced_unwind&amp;)
	  {
	    __throw_exception_again; // will cause broken_promise
	  }
	__catch(...)
	  {
	    (*_M_result)-&gt;_M_error = current_exception();
	  }
	return std::move(*_M_result);
      }
      _Ptr_type*	_M_result;
      _Fn*		_M_fn;
    };

  template&lt;typename _Ptr_type, typename _Fn&gt;
    struct __future_base::_Task_setter&lt;_Ptr_type, _Fn, void&gt;
    {
      _Ptr_type operator()() const
      {
	__try
	  {
	    (*_M_fn)();
	  }
	__catch(const __cxxabiv1::__forced_unwind&amp;)
	  {
	    __throw_exception_again; // will cause broken_promise
	  }
	__catch(...)
	  {
	    (*_M_result)-&gt;_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&lt;typename _Res, typename... _Args&gt;
    struct __future_base::_Task_state_base&lt;_Res(_Args...)&gt;
    : __future_base::_State_base
    {
      typedef _Res _Res_type;

      template&lt;typename _Alloc&gt;
	_Task_state_base(const _Alloc&amp; __a)
	: _M_result(_S_allocate_result&lt;_Res&gt;(__a))
	{ }

      // Invoke the stored task and make the state ready.
      virtual void
      _M_run(_Args&amp;&amp;... __args) = 0;

      // Invoke the stored task and make the state ready at thread exit.
      virtual void
      _M_run_delayed(_Args&amp;&amp;... __args, weak_ptr&lt;_State_base&gt;) = 0;

      virtual shared_ptr&lt;_Task_state_base&gt;
      _M_reset() = 0;

      typedef __future_base::_Ptr&lt;_Result&lt;_Res&gt;&gt; _Ptr_type;
      _Ptr_type _M_result;
    };

  // Holds a packaged_task's stored task.
  template&lt;typename _Fn, typename _Alloc, typename _Res, typename... _Args&gt;
    struct __future_base::_Task_state&lt;_Fn, _Alloc, _Res(_Args...)&gt; final
    : __future_base::_Task_state_base&lt;_Res(_Args...)&gt;
    {
      template&lt;typename _Fn2&gt;
	_Task_state(_Fn2&amp;&amp; __fn, const _Alloc&amp; __a)
	: _Task_state_base&lt;_Res(_Args...)&gt;(__a),
	  _M_impl(std::forward&lt;_Fn2&gt;(__fn), __a)
	{ }

    private:
      virtual void
      _M_run(_Args&amp;&amp;... __args)
      {
	auto __boundfn = [&amp;] () -&gt; typename result_of&lt;_Fn&amp;(_Args&amp;&amp;...)&gt;::type {
	    return std::__invoke(_M_impl._M_fn, std::forward&lt;_Args&gt;(__args)...);
	};
	this-&gt;_M_set_result(_S_task_setter(this-&gt;_M_result, __boundfn));
      }

      virtual void
      _M_run_delayed(_Args&amp;&amp;... __args, weak_ptr&lt;_State_base&gt; __self)
      {
	auto __boundfn = [&amp;] () -&gt; typename result_of&lt;_Fn&amp;(_Args&amp;&amp;...)&gt;::type {
	    return std::__invoke(_M_impl._M_fn, std::forward&lt;_Args&gt;(__args)...);
	};
	this-&gt;_M_set_delayed_result(_S_task_setter(this-&gt;_M_result, __boundfn),
				    std::move(__self));
      }

      virtual shared_ptr&lt;_Task_state_base&lt;_Res(_Args...)&gt;&gt;
      _M_reset();

      struct _Impl : _Alloc
      {
	template&lt;typename _Fn2&gt;
	  _Impl(_Fn2&amp;&amp; __fn, const _Alloc&amp; __a)
	  : _Alloc(__a), _M_fn(std::forward&lt;_Fn2&gt;(__fn)) { }
	_Fn _M_fn;
      } _M_impl;
    };

  template&lt;typename _Signature, typename _Fn, typename _Alloc&gt;
    static shared_ptr&lt;__future_base::_Task_state_base&lt;_Signature&gt;&gt;
    __create_task_state(_Fn&amp;&amp; __fn, const _Alloc&amp; __a)
    {
      typedef typename decay&lt;_Fn&gt;::type _Fn2;
      typedef __future_base::_Task_state&lt;_Fn2, _Alloc, _Signature&gt; _State;
      return std::allocate_shared&lt;_State&gt;(__a, std::forward&lt;_Fn&gt;(__fn), __a);
    }

  template&lt;typename _Fn, typename _Alloc, typename _Res, typename... _Args&gt;
    shared_ptr&lt;__future_base::_Task_state_base&lt;_Res(_Args...)&gt;&gt;
    __future_base::_Task_state&lt;_Fn, _Alloc, _Res(_Args...)&gt;::_M_reset()
    {
      return __create_task_state&lt;_Res(_Args...)&gt;(std::move(_M_impl._M_fn),
						 static_cast&lt;_Alloc&amp;&gt;(_M_impl));
    }

  template&lt;typename _Task, typename _Fn, bool
	   = is_same&lt;_Task, typename decay&lt;_Fn&gt;::type&gt;::value&gt;
    struct __constrain_pkgdtask
    { typedef void __type; };

  template&lt;typename _Task, typename _Fn&gt;
    struct __constrain_pkgdtask&lt;_Task, _Fn, true&gt;
    { };

  /// packaged_task
  template&lt;typename _Res, typename... _ArgTypes&gt;
    class packaged_task&lt;_Res(_ArgTypes...)&gt;
    {
      typedef __future_base::_Task_state_base&lt;_Res(_ArgTypes...)&gt; _State_type;
      shared_ptr&lt;_State_type&gt;                   _M_state;

    public:
      // Construction and destruction
      packaged_task() noexcept { }

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2095.  missing constructors needed for uses-allocator construction
      template&lt;typename _Allocator&gt;
	packaged_task(allocator_arg_t, const _Allocator&amp; __a) noexcept
	{ }

      template&lt;typename _Fn, typename = typename
	       __constrain_pkgdtask&lt;packaged_task, _Fn&gt;::__type&gt;
	explicit
	packaged_task(_Fn&amp;&amp; __fn)
	: packaged_task(allocator_arg, std::allocator&lt;int&gt;(),
			std::forward&lt;_Fn&gt;(__fn))
	{ }

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2097.  packaged_task constructors should be constrained
      // 2407. [this constructor should not be] explicit
      template&lt;typename _Fn, typename _Alloc, typename = typename
	       __constrain_pkgdtask&lt;packaged_task, _Fn&gt;::__type&gt;
	packaged_task(allocator_arg_t, const _Alloc&amp; __a, _Fn&amp;&amp; __fn)
	: _M_state(__create_task_state&lt;_Res(_ArgTypes...)&gt;(
		    std::forward&lt;_Fn&gt;(__fn), __a))
	{ }

      ~packaged_task()
      {
        if (static_cast&lt;bool&gt;(_M_state) &amp;&amp; !_M_state.unique())
	  _M_state-&gt;_M_break_promise(std::move(_M_state-&gt;_M_result));
      }

      // No copy
      packaged_task(const packaged_task&amp;) = delete;
      packaged_task&amp; operator=(const packaged_task&amp;) = delete;

      template&lt;typename _Allocator&gt;
	packaged_task(allocator_arg_t, const _Allocator&amp;,
		      const packaged_task&amp;) = delete;

      // Move support
      packaged_task(packaged_task&amp;&amp; __other) noexcept
      { this-&gt;swap(__other); }

      template&lt;typename _Allocator&gt;
	packaged_task(allocator_arg_t, const _Allocator&amp;,
		      packaged_task&amp;&amp; __other) noexcept
	{ this-&gt;swap(__other); }

      packaged_task&amp; operator=(packaged_task&amp;&amp; __other) noexcept
      {
	packaged_task(std::move(__other)).swap(*this);
	return *this;
      }

      void
      swap(packaged_task&amp; __other) noexcept
      { _M_state.swap(__other._M_state); }

      bool
      valid() const noexcept
      { return static_cast&lt;bool&gt;(_M_state); }

      // Result retrieval
      future&lt;_Res&gt;
      get_future()
      { return future&lt;_Res&gt;(_M_state); }

      // Execution
      void
      operator()(_ArgTypes... __args)
      {
	__future_base::_State_base::_S_check(_M_state);
	_M_state-&gt;_M_run(std::forward&lt;_ArgTypes&gt;(__args)...);
      }

      void
      make_ready_at_thread_exit(_ArgTypes... __args)
      {
	__future_base::_State_base::_S_check(_M_state);
	_M_state-&gt;_M_run_delayed(std::forward&lt;_ArgTypes&gt;(__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-&gt;_M_reset();
      }
    };

  /// swap
  template&lt;typename _Res, typename... _ArgTypes&gt;
    inline void
    swap(packaged_task&lt;_Res(_ArgTypes...)&gt;&amp; __x,
	 packaged_task&lt;_Res(_ArgTypes...)&gt;&amp; __y) noexcept
    { __x.swap(__y); }

  template&lt;typename _Res, typename _Alloc&gt;
    struct uses_allocator&lt;packaged_task&lt;_Res&gt;, _Alloc&gt;
    : public true_type { };


  // Shared state created by std::async().
  // Holds a deferred function and storage for its result.
  template&lt;typename _BoundFn, typename _Res&gt;
    class __future_base::_Deferred_state final
    : public __future_base::_State_base
    {
    public:
      explicit
      _Deferred_state(_BoundFn&amp;&amp; __fn)
      : _M_result(new _Result&lt;_Res&gt;()), _M_fn(std::move(__fn))
      { }

    private:
      typedef __future_base::_Ptr&lt;_Result&lt;_Res&gt;&gt; _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, &amp;thread::join, &amp;_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&lt;typename _BoundFn, typename _Res&gt;
    class __future_base::_Async_state_impl final
    : public __future_base::_Async_state_commonV2
    {
    public:
      explicit
      _Async_state_impl(_BoundFn&amp;&amp; __fn)
      : _M_result(new _Result&lt;_Res&gt;()), _M_fn(std::move(__fn))
      {
	_M_thread = std::thread{ [this] {
	    __try
	      {
		_M_set_result(_S_task_setter(_M_result, _M_fn));
	      }
	    __catch (const __cxxabiv1::__forced_unwind&amp;)
	      {
		// make the shared state ready on thread cancellation
		if (static_cast&lt;bool&gt;(_M_result))
		  this-&gt;_M_break_promise(std::move(_M_result));
		__throw_exception_again;
	      }
        } };
      }

      // 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:
      typedef __future_base::_Ptr&lt;_Result&lt;_Res&gt;&gt; _Ptr_type;
      _Ptr_type _M_result;
      _BoundFn _M_fn;
    };

  template&lt;typename _BoundFn&gt;
    inline std::shared_ptr&lt;__future_base::_State_base&gt;
    __future_base::_S_make_deferred_state(_BoundFn&amp;&amp; __fn)
    {
      typedef typename remove_reference&lt;_BoundFn&gt;::type __fn_type;
      typedef _Deferred_state&lt;__fn_type&gt; __state_type;
      return std::make_shared&lt;__state_type&gt;(std::move(__fn));
    }

  template&lt;typename _BoundFn&gt;
    inline std::shared_ptr&lt;__future_base::_State_base&gt;
    __future_base::_S_make_async_state(_BoundFn&amp;&amp; __fn)
    {
      typedef typename remove_reference&lt;_BoundFn&gt;::type __fn_type;
      typedef _Async_state_impl&lt;__fn_type&gt; __state_type;
      return std::make_shared&lt;__state_type&gt;(std::move(__fn));
    }


  /// async
  template&lt;typename _Fn, typename... _Args&gt;
    future&lt;__async_result_of&lt;_Fn, _Args...&gt;&gt;
    async(launch __policy, _Fn&amp;&amp; __fn, _Args&amp;&amp;... __args)
    {
      std::shared_ptr&lt;__future_base::_State_base&gt; __state;
      if ((__policy &amp; launch::async) == launch::async)
	{
	  __try
	    {
	      __state = __future_base::_S_make_async_state(
		  std::thread::__make_invoker(std::forward&lt;_Fn&gt;(__fn),
					      std::forward&lt;_Args&gt;(__args)...)
		  );
	    }
#if __cpp_exceptions
	  catch(const system_error&amp; __e)
	    {
	      if (__e.code() != errc::resource_unavailable_try_again
		  || (__policy &amp; launch::deferred) != launch::deferred)
		throw;
	    }
#endif
	}
      if (!__state)
	{
	  __state = __future_base::_S_make_deferred_state(
	      std::thread::__make_invoker(std::forward&lt;_Fn&gt;(__fn),
					  std::forward&lt;_Args&gt;(__args)...));
	}
      return future&lt;__async_result_of&lt;_Fn, _Args...&gt;&gt;(__state);
    }

  /// async, potential overload
  template&lt;typename _Fn, typename... _Args&gt;
    inline future&lt;__async_result_of&lt;_Fn, _Args...&gt;&gt;
    async(_Fn&amp;&amp; __fn, _Args&amp;&amp;... __args)
    {
      return std::async(launch::async|launch::deferred,
			std::forward&lt;_Fn&gt;(__fn),
			std::forward&lt;_Args&gt;(__args)...);
    }

#endif // _GLIBCXX_ASYNC_ABI_COMPAT
#endif // _GLIBCXX_HAS_GTHREADS &amp;&amp; _GLIBCXX_USE_C99_STDINT_TR1

  // @} group futures
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

#endif // C++11

#endif // _GLIBCXX_FUTURE
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