boost::log::expressions::format_named_scope
// In header: <boost/log/expressions/formatters/named_scope.hpp> template<typename T, typename FallbackPolicyT, typename TagT, template< typename > class ActorT, typename CharT> format_named_scope_actor< FallbackPolicyT, CharT, ActorT > format_named_scope(attribute_actor< T, FallbackPolicyT, TagT, ActorT > const & placeholder, const CharT * element_format);