Update boost to 1.71 and add asio

boost/asio/use_awaitable.hpp

@@ -0,0 +1,73 @@
+//
+// use_awaitable.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_USE_AWAITABLE_HPP
+#define BOOST_ASIO_USE_AWAITABLE_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include <boost/asio/detail/config.hpp>
+
+#if defined(BOOST_ASIO_HAS_CO_AWAIT) || defined(GENERATING_DOCUMENTATION)
+
+#include <boost/asio/awaitable.hpp>
+
+#include <boost/asio/detail/push_options.hpp>
+
+namespace boost {
+namespace asio {
+
+/// A completion token that represents the currently executing coroutine.
+/**
+ * The @c use_awaitable_t class, with its value @c use_awaitable, is used to
+ * represent the currently executing coroutine. This completion token may be
+ * passed as a handler to an asynchronous operation. For example:
+ *
+ * @code awaitable<void> my_coroutine()
+ * {
+ *   std::size_t n = co_await my_socket.async_read_some(buffer, use_awaitable);
+ *   ...
+ * } @endcode
+ *
+ * When used with co_await, the initiating function (@c async_read_some in the
+ * above example) suspends the current coroutine. The coroutine is resumed when
+ * the asynchronous operation completes, and the result of the operation is
+ * returned.
+ */
+template <typename Executor = executor>
+struct use_awaitable_t
+{
+  BOOST_ASIO_CONSTEXPR use_awaitable_t()
+  {
+  }
+};
+
+/// A completion token object that represents the currently executing coroutine.
+/**
+ * See the documentation for boost::asio::use_awaitable_t for a usage example.
+ */
+#if defined(BOOST_ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION)
+constexpr use_awaitable_t<> use_awaitable;
+#elif defined(BOOST_ASIO_MSVC)
+__declspec(selectany) use_awaitable_t<> use_awaitable;
+#endif
+
+} // namespace asio
+} // namespace boost
+
+#include <boost/asio/detail/pop_options.hpp>
+
+#include <boost/asio/impl/use_awaitable.hpp>
+
+#endif // defined(BOOST_ASIO_HAS_CO_AWAIT) || defined(GENERATING_DOCUMENTATION)
+
+#endif // BOOST_ASIO_USE_AWAITABLE_HPP