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The question is in bold at the bottom, the problem is also summarized by the distillation code fragment towards the end. I am trying to unify my type system (the type system does to and from from type to string) into a single component(as defined by Lakos). I am using boost::array , boost::variant , and boost::mpl , in order to achieve this. I want to have the parser and generator rules for my types unified in a variant. there is a undefined type, a int4(see below) type and a int8 type. The variant reads as variant<undefined, int4,int8> . int4 traits: struct rbl_int4_parser_rule_definition { typedef boost::spirit::qi::rule<std::string::iterator, rbl_int4()> rule_type; boost::spirit::qi::int_parser<rbl_int4> parser_int32_t; rule_type rule; rbl_int4_parser_rule_definition() { rule.name("rbl int4 rule"); rule = parser_int32_t; } }; template<> struct rbl_type_parser_rule<rbl_int4> { typedef rbl_int4_parser_rule_definition string_parser; }; the variant above starts out as undefined, and then I initialize the rules. I had a problem, which caused 50 pages of errors, and I have finally managed to track it down, Variant uses operator= during assignment and a boost::spirit::qi::int_parser<> cannot be assigned to another (operator=). To contrast, I don't have a problem with my undefined type: struct rbl_undefined_parser_rule_definition { typedef boost::spirit::qi::rule<std::string::iterator, void()> rule_type; rule_type rule; rbl_undefined_parser_rule_definition() { rule.name("undefined parse rule"); rule = boost::spirit::qi::eps;

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