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/client/sync-ssl/websocket_client_sync_ssl.cpp

@@ -73,6 +73,15 @@ int main(int argc, char** argv)
         // Perform the SSL handshake
         ws.next_layer().handshake(ssl::stream_base::client);
 
+        // Set a decorator to change the User-Agent of the handshake
+        ws.set_option(websocket::stream_base::decorator(
+            [](websocket::request_type& req)
+            {
+                req.set(http::field::user_agent,
+                    std::string(BOOST_BEAST_VERSION_STRING) +
+                        " websocket-client-coro");
+            }));
+
         // Perform the websocket handshake
         ws.handshake(host, "/");