Add websocket-chat-multi example:

This is the multi-threaded io_context version of the
CppCon2018 WebSocket chat server and client example.

example/websocket/server/chat-multi/shared_state.hpp

@@ -0,0 +1,48 @@
+//
+// Copyright (c) 2018 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)
+//
+// Official repository: https://github.com/vinniefalco/CppCon2018
+//
+
+#ifndef BOOST_BEAST_EXAMPLE_WEBSOCKET_CHAT_MULTI_SHARED_STATE_HPP
+#define BOOST_BEAST_EXAMPLE_WEBSOCKET_CHAT_MULTI_SHARED_STATE_HPP
+
+#include <boost/smart_ptr.hpp>
+#include <memory>
+#include <mutex>
+#include <string>
+#include <unordered_set>
+
+// Forward declaration
+class websocket_session;
+
+// Represents the shared server state
+class shared_state
+{
+    std::string const doc_root_;
+
+    // This mutex synchronizes all access to sessions_
+    std::mutex mutex_;
+
+    // Keep a list of all the connected clients
+    std::unordered_set<websocket_session*> sessions_;
+
+public:
+    explicit
+    shared_state(std::string doc_root);
+
+    std::string const&
+    doc_root() const noexcept
+    {
+        return doc_root_;
+    }
+
+    void join  (websocket_session* session);
+    void leave (websocket_session* session);
+    void send  (std::string message);
+};
+
+#endif