Merge commit '1dd2f4645226bd269f2407d5ed431acc3f66e7a6' as 'Sources/ASIO'

Sources/ASIO/asio/include/asio/handler_invoke_hook.hpp

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+//
+// handler_invoke_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 ASIO_HANDLER_INVOKE_HOOK_HPP
+#define ASIO_HANDLER_INVOKE_HOOK_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+
+/** @defgroup asio_handler_invoke asio::asio_handler_invoke
+ *
+ * @brief Default invoke function for handlers.
+ *
+ * Completion handlers for asynchronous operations are invoked by the
+ * io_context associated with the corresponding object (e.g. a socket or
+ * deadline_timer). Certain guarantees are made on when the handler may be
+ * invoked, in particular that a handler can only be invoked from a thread that
+ * is currently calling @c run() on the corresponding io_context object.
+ * Handlers may subsequently be invoked through other objects (such as
+ * io_context::strand objects) that provide additional guarantees.
+ *
+ * When asynchronous operations are composed from other asynchronous
+ * operations, all intermediate handlers should be invoked using the same
+ * method as the final handler. This is required to ensure that user-defined
+ * objects are not accessed in a way that may violate the guarantees. This
+ * hooking function ensures that the invoked method used for the final handler
+ * is accessible at each intermediate step.
+ *
+ * Implement asio_handler_invoke for your own handlers to specify a custom
+ * invocation strategy.
+ *
+ * This default implementation invokes the function object like so:
+ * @code function(); @endcode
+ * If necessary, the default implementation makes a copy of the function object
+ * so that the non-const operator() can be used.
+ *
+ * @par Example
+ * @code
+ * class my_handler;
+ *
+ * template <typename Function>
+ * void asio_handler_invoke(Function function, my_handler* context)
+ * {
+ *   context->strand_.dispatch(function);
+ * }
+ * @endcode
+ */
+/*@{*/
+
+/// Default handler invocation hook used for non-const function objects.
+template <typename Function>
+inline void asio_handler_invoke(Function& function, ...)
+{
+  function();
+}
+
+/// Default handler invocation hook used for const function objects.
+template <typename Function>
+inline void asio_handler_invoke(const Function& function, ...)
+{
+  Function tmp(function);
+  tmp();
+}
+
+/*@}*/
+
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // ASIO_HANDLER_INVOKE_HOOK_HPP