Update boost to 1.71 and add asio

boost/asio/detached.hpp

@@ -0,0 +1,64 @@
+//
+// detached.hpp
+// ~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2019 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 BOOST_ASIO_DETACHED_HPP
+#define BOOST_ASIO_DETACHED_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include <boost/asio/detail/config.hpp>
+#include <memory>
+
+#include <boost/asio/detail/push_options.hpp>
+
+namespace boost {
+namespace asio {
+
+/// Class used to specify that an asynchronous operation is detached.
+/**
+
+ * The detached_t class is used to indicate that an asynchronous operation is
+ * detached. That is, there is no completion handler waiting for the
+ * operation's result. A detached_t object may be passed as a handler to an
+ * asynchronous operation, typically using the special value
+ * @c boost::asio::detached. For example:
+
+ * @code my_socket.async_send(my_buffer, boost::asio::detached);
+ * @endcode
+ */
+class detached_t
+{
+public:
+  /// Constructor. 
+  BOOST_ASIO_CONSTEXPR detached_t()
+  {
+  }
+};
+
+/// A special value, similar to std::nothrow.
+/**
+ * See the documentation for boost::asio::detached_t for a usage example.
+ */
+#if defined(BOOST_ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION)
+constexpr detached_t detached;
+#elif defined(BOOST_ASIO_MSVC)
+__declspec(selectany) detached_t detached;
+#endif
+
+} // namespace asio
+} // namespace boost
+
+#include <boost/asio/detail/pop_options.hpp>
+
+#include <boost/asio/impl/detached.hpp>
+
+#endif // BOOST_ASIO_DETACHED_HPP