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diff --git a/lib/asio/detail/std_event.hpp b/lib/asio/detail/std_event.hpp
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+//
+// detail/std_event.hpp
+// ~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2018 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef ASIO_DETAIL_STD_EVENT_HPP
+#define ASIO_DETAIL_STD_EVENT_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+
+#if defined(ASIO_HAS_STD_MUTEX_AND_CONDVAR)
+
+#include <chrono>
+#include <condition_variable>
+#include "asio/detail/assert.hpp"
+#include "asio/detail/noncopyable.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+namespace detail {
+
+class std_event
+  : private noncopyable
+{
+public:
+  // Constructor.
+  std_event()
+    : state_(0)
+  {
+  }
+
+  // Destructor.
+  ~std_event()
+  {
+  }
+
+  // Signal the event. (Retained for backward compatibility.)
+  template <typename Lock>
+  void signal(Lock& lock)
+  {
+    this->signal_all(lock);
+  }
+
+  // Signal all waiters.
+  template <typename Lock>
+  void signal_all(Lock& lock)
+  {
+    ASIO_ASSERT(lock.locked());
+    (void)lock;
+    state_ |= 1;
+    cond_.notify_all();
+  }
+
+  // Unlock the mutex and signal one waiter.
+  template <typename Lock>
+  void unlock_and_signal_one(Lock& lock)
+  {
+    ASIO_ASSERT(lock.locked());
+    state_ |= 1;
+    bool have_waiters = (state_ > 1);
+    lock.unlock();
+    if (have_waiters)
+      cond_.notify_one();
+  }
+
+  // If there's a waiter, unlock the mutex and signal it.
+  template <typename Lock>
+  bool maybe_unlock_and_signal_one(Lock& lock)
+  {
+    ASIO_ASSERT(lock.locked());
+    state_ |= 1;
+    if (state_ > 1)
+    {
+      lock.unlock();
+      cond_.notify_one();
+      return true;
+    }
+    return false;
+  }
+
+  // Reset the event.
+  template <typename Lock>
+  void clear(Lock& lock)
+  {
+    ASIO_ASSERT(lock.locked());
+    (void)lock;
+    state_ &= ~std::size_t(1);
+  }
+
+  // Wait for the event to become signalled.
+  template <typename Lock>
+  void wait(Lock& lock)
+  {
+    ASIO_ASSERT(lock.locked());
+    unique_lock_adapter u_lock(lock);
+    while ((state_ & 1) == 0)
+    {
+      waiter w(state_);
+      cond_.wait(u_lock.unique_lock_);
+    }
+  }
+
+  // Timed wait for the event to become signalled.
+  template <typename Lock>
+  bool wait_for_usec(Lock& lock, long usec)
+  {
+    ASIO_ASSERT(lock.locked());
+    unique_lock_adapter u_lock(lock);
+    if ((state_ & 1) == 0)
+    {
+      waiter w(state_);
+      cond_.wait_for(u_lock.unique_lock_, std::chrono::microseconds(usec));
+    }
+    return (state_ & 1) != 0;
+  }
+
+private:
+  // Helper class to temporarily adapt a scoped_lock into a unique_lock so that
+  // it can be passed to std::condition_variable::wait().
+  struct unique_lock_adapter
+  {
+    template <typename Lock>
+    explicit unique_lock_adapter(Lock& lock)
+      : unique_lock_(lock.mutex().mutex_, std::adopt_lock)
+    {
+    }
+
+    ~unique_lock_adapter()
+    {
+      unique_lock_.release();
+    }
+
+    std::unique_lock<std::mutex> unique_lock_;
+  };
+
+  // Helper to increment and decrement the state to track outstanding waiters.
+  class waiter
+  {
+  public:
+    explicit waiter(std::size_t& state)
+      : state_(state)
+    {
+      state_ += 2;
+    }
+
+    ~waiter()
+    {
+      state_ -= 2;
+    }
+
+  private:
+    std::size_t& state_;
+  };
+
+  std::condition_variable cond_;
+  std::size_t state_;
+};
+
+} // namespace detail
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // defined(ASIO_HAS_STD_MUTEX_AND_CONDVAR)
+
+#endif // ASIO_DETAIL_STD_EVENT_HPP