WebSocket Decorator is a socket option (API Change):

This changes the interface used to apply a decorator to the HTTP
request or response messages used to perform the WebSocket handshake
as follows:

* Add the `stream_base::decorator` option object

* Add `stream::set_option` overload to set the decorator from
  the option

* The decorator applies to all client and server handshakes
  performed on the stream after the option is set.

* Overloads of the following functions which accept a Decorator
  are deprecated:
  - accept, accept_ex
  - handshake, handshake_ex
  - async_accept, async_accept_ex
  - async_handshake, async_handshake_ex

Actions Required:

* Code which passes decorator to any `websocket::stream` member
  function should call `stream::set_option` instead with a newly
  constructed `stream_base::decorator` object containing the
  decorator. Alternatively, the macro `BOOST_BEAST_ALLOW_DEPRECATED`
  may be defined to 1.

example/websocket/server/sync/websocket_server_sync.cpp

@@ -39,6 +39,15 @@ do_session(tcp::socket& socket)
         // Construct the stream by moving in the socket
         websocket::stream<tcp::socket> ws{std::move(socket)};
 
+        // Set a decorator to change the Server of the handshake
+        ws.set_option(websocket::stream_base::decorator(
+            [](websocket::response_type& res)
+            {
+                res.set(http::field::server,
+                    std::string(BOOST_BEAST_VERSION_STRING) +
+                        " websocket-server-sync");
+            }));
+
         // Accept the websocket handshake
         ws.accept();