binary '==' : no operator found which takes a left-hand operand of type 'const T2' and T2=config_iter so you need bool config_iter::operator == (const config_iter & other) const; struct config_iter { config_iter(const config::iterator iter) : iter_(iter) {} operator config::iterator() const { return iter_; } config::iterator operator() () const { return iter_; } //////ADD THIS////////////////////// bool operator == (const config_iter & other) const { return iter_ == other.iter_; } //////ADD THIS////////////////////// protected: config::iterator iter_; }; //////// SEE HERE /////////////////////////////////// 1>.\boost\variant\variant.hpp(781): error: binary '==' : no operator found which takes a left-hand operand of type 'const T2' (or there is no acceptable conversion) //////// SEE HERE /////////////////////////////////// 1> .\boost\blank.hpp(58): could be 'bool boost::operator ==(const boost::blank &,const boost::blank &)' 1> while trying to match the argument list '(const T2, const T2)' 1> .\boost\variant\variant.hpp(763) : see reference to function template instantiation 'bool boost::detail::variant::equal_comp::operator ()<T>(const T &,const T &) const' being compiled 1> with 1> [ 1> T=T2 1> ] 1> .\boost\variant\variant.hpp(832) : see reference to function template instantiation 'bool boost::detail::variant::comparer<Variant,Comp>::operator ()<const T>(const T &) const' being compiled 1> with 1> [ 1> Variant=boost::variant<std::string,long double,config_iter>, 1> Comp=boost::detail::variant::equal_comp, 1> T=T2 1> ] 1> .\boost\variant\detail\visitation_impl.hpp(130) : see reference to function template instantiation 'bool boost::detail::variant::invoke_visitor<Visitor>::internal_visit<const T>(T &,int)' being compiled 1> with 1> [ 1> Visitor=boost::detail::variant::comparer<boost::variant<std::string,long double,config_iter>,boost::detail::variant::equal_comp>, 1> T=T2 1> ] 1> .\boost\variant\detail\visitation_impl.hpp(173) : see reference to function template instantiation 'bool boost::detail::variant::visitation_impl_invoke_impl<Visitor,VoidPtrCV,T>(int,Visitor &,VoidPtrCV,T *,boost::mpl::true_)' being compiled 1> with 1> [ 1> Visitor=boost::detail::variant::invoke_visitor<boost::detail::variant::comparer<boost::variant<std::string,long double,config_iter>,boost::detail::variant::equal_comp>>, 1> VoidPtrCV=const void *, 1> T=T2 1> ] 1> .\boost\variant\detail\visitation_impl.hpp(256) : see reference to function template instantiation 'bool boost::detail::variant::visitation_impl_invoke<Visitor,VoidPtrCV,T2,NoBackupFlag>(int,Visitor &,VoidPtrCV,T *,NoBackupFlag,int)' being compiled 1> with 1> [ 1> Visitor=boost::detail::variant::invoke_visitor<boost::detail::variant::comparer<boost::variant<std::string,long double,config_iter>,boost::detail::variant::equal_comp>>, 1> VoidPtrCV=const void *, 1> NoBackupFlag=boost::variant<std::string,long double,config_iter>::has_fallback_type_, 1> T=T2 1> ] 1> .\boost\variant\variant.hpp(1776) : see reference to function template instantiation 'bool boost::detail::variant::visitation_impl<first_which,first_step,Visitor,VoidPtrCV,boost::variant<T0_,T1,T2>::has_fallback_type_>(const int,const int,Visitor &,VoidPtrCV,boost::mpl::false_,NoBackupFlag,Which *,step0 *)' being compiled 1> with 1> [ 1> Visitor=boost::detail::variant::invoke_visitor<boost::detail::variant::comparer<boost::variant<std::string,long double,config_iter>,boost::detail::variant::equal_comp>>, 1> VoidPtrCV=const void *, 1> T0_=std::string, 1> T1=long double, //////// SEE HERE /////////////////////////////////// 1> T2=config_iter, //////// SEE HERE /////////////////////////////////// 1> NoBackupFlag=boost::variant<std::string,long double,config_iter>::has_fallback_type_, 1> Which=first_which, 1> step0=first_step 1> ] // test program with config_iter and indentifier // doesn't compile if identifier::operator==() is in the program #include <boost/variant.hpp> #include <iterator> #include <string> #include <vector> #include <list> #include <map> struct config_iter; typedef boost::variant<std::string ,long double ,config_iter > var; typedef std::vector<var> value; typedef std::map<std::string,value> message; typedef std::map<std::string,message> config; struct config_iter { config_iter(const config::iterator iter) : iter_(iter) {} operator config::iterator() const { return iter_; } config::iterator operator() () const { return iter_; } protected: config::iterator iter_; }; struct identifier { identifier() : name_(""), version_({0}) { // std::cout << "identifier::identifier()" << std::endl; } identifier(const std::string & name, const value & version) : name_(name), version_(version) { // std::cout << "identifier::identifier(" << name << "," << version << ")" << std::endl; } bool operator == (const identifier & other) const { return name_ == other.name_ && version_ == other.version_; } private: // updated by constructor std::string name_; value version_; }; int main() { return 0; }