a.async_auth(step, data, h)

void

step is async_mqtt5::auth_step_e that specifies current authentication stage.

data is std::string, server's authentication data.

h is boost::asio::any_completion_handler with signature void(async_mqtt5::error_code ec, std::string client_data). If ec is non-trivial, authentication is aborted.

a.method()

std::string_view, authentication method