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include/beast/async_completion.hpp

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+//
+// Copyright (c) 2013-2016 Vinnie Falco (vinnie dot falco at gmail 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 BEAST_ASYNC_COMPLETION_HPP
+#define BEAST_ASYNC_COMPLETION_HPP
+
+#include <beast/type_check.hpp>
+#include <boost/asio/async_result.hpp>
+#include <boost/asio/handler_type.hpp>
+#include <type_traits>
+#include <utility>
+
+namespace beast {
+
+/** Completion helper for implementing the extensible asynchronous model.
+
+    This class template is used to transform caller-provided
+    completion tokens in calls to asynchronous initiation
+    functions. The transformation allows customization of
+    the return type of the initiating function, and the type
+    of the final handler.
+
+    @tparam CompletionToken A CompletionHandler, or a user defined type
+    with specializations for customizing the return type (for example,
+    `boost::asio::use_future` or `boost::asio::yield_context`).
+
+    @tparam Signature The callable signature of the final completion handler.
+
+    Usage:
+    @code
+    template<class CompletionToken>
+    auto
+    async_initfn(..., CompletionToken&& token)
+    {
+        async_completion<CompletionToken,
+            void(error_code, std::size_t)> completion(token);
+        ...
+        return completion.result.get();
+    }
+    @endcode
+
+    See <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3896.pdf">
+        Library Foundations For Asynchronous Operations</a>
+*/
+
+template <class CompletionToken, class Signature>
+struct async_completion
+{
+    /** The type of the final handler.
+
+        Objects of this type will be callable with the
+        specified signature.
+    */
+    using handler_type =
+        typename boost::asio::handler_type<
+            CompletionToken, Signature>::type;
+
+    /** Construct the completion helper.
+
+        @param token The completion token. Copies will be made as
+        required. If `CompletionToken` is movable, it may also be moved.
+    */
+    async_completion(std::remove_reference_t<CompletionToken>& token)
+        : handler(std::forward<CompletionToken>(token))
+        , result(handler)
+    {
+        static_assert(is_Handler<handler_type, Signature>::value,
+            "Handler requirements not met");
+    }
+
+    /** The final completion handler, callable with the specified signature. */
+    handler_type handler;
+
+    /** The return value of the asynchronous initiation function. */
+    boost::asio::async_result<handler_type> result;
+};
+
+} // beast
+
+#endif