[Boost-users] Boost.Spirit Karma: Alternative generator

Hello, I am construction karma::rule<OutputIterator, std::string()> which I would like to be either a string or quoted string if the string contains spaces. Now I think that the rule should be something like: rule = (&true[...] << string) | (lit("\"") << string << lit("\"")); but I've failed to write proper semantic action. How could I obtain std::string object reference to write something like: (obj.find(" ") == std::string::npos) ? Or maybe is there a better way to do the same?