Update boost to 1.71 and add asio

boost/asio/handler_continuation_hook.hpp

@@ -0,0 +1,56 @@
+//
+// handler_continuation_hook.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_HANDLER_CONTINUATION_HOOK_HPP
+#define BOOST_ASIO_HANDLER_CONTINUATION_HOOK_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include <boost/asio/detail/config.hpp>
+
+#include <boost/asio/detail/push_options.hpp>
+
+namespace boost {
+namespace asio {
+
+/// Default continuation function for handlers.
+/**
+ * Asynchronous operations may represent a continuation of the asynchronous
+ * control flow associated with the current handler. The implementation can use
+ * this knowledge to optimise scheduling of the handler.
+ *
+ * Implement asio_handler_is_continuation for your own handlers to indicate
+ * when a handler represents a continuation.
+ *
+ * The default implementation of the continuation hook returns <tt>false</tt>.
+ *
+ * @par Example
+ * @code
+ * class my_handler;
+ *
+ * bool asio_handler_is_continuation(my_handler* context)
+ * {
+ *   return true;
+ * }
+ * @endcode
+ */
+inline bool asio_handler_is_continuation(...)
+{
+  return false;
+}
+
+} // namespace asio
+} // namespace boost
+
+#include <boost/asio/detail/pop_options.hpp>
+
+#endif // BOOST_ASIO_HANDLER_CONTINUATION_HOOK_HPP