c++ - boost::variant & STL containers - Am I doing it wrong? -
i'm scratching head on one. i'm trying use boost::variant store vector of values of type determined @ run time. vector's internal type doesn't change once known. trying create interface code can add value vector , later retrieve values vector. code not compile. approach wrong? should using polymorphism job? code follows (does not compile):
template <typename t> class record_value_visitor : public boost::static_visitor<void> { public: record_value_visitor(t v) : _val(v) {} void operator()(vector<t>& vec) const { vec.push_back(this->_val); } private: t _val; }; class monitor { public: monitor(int type) { switch (type) { case 1: this->_values = vector<int>(); break; case 2: this->_values = vector<string>(); break; default: throw new exception("invalid type"); } this->_type = type; } void add_value(int val) { if ( this->_type != 1 ) throw new runtime_error("invalid type add_value()"); boost::apply_visitor( record_value_visitor<int>(val), this->_values); } void add_value(string val) { if ( this->_type != 2 ) throw new runtime_error("invalid type add_value()"); boost::apply_visitor( record_value_visitor<string>(val), this->_values); } private: int _type; boost::variant< vector<int>, vector<double>, vector<string> > _values; }; int main(int argc, char* argv[]) { monitor int_mon(1); monitor str_mon(2); int_mon.add_value(4); str_mon.add_value(string("foo")); return 0; }
edit: add error message
1>c:\boost_1_46_1\boost\variant\variant.hpp(832): error c2664: 'void record_value_visitor<t>::operator ()(const std::vector<_ty> &) const' : cannot convert parameter 1 't1' 'const std::vector<_ty> &' 1> 1> [ 1> t=int, 1> _ty=int 1> ] 1> , 1> [ 1> _ty=int 1> ] 1> reason: cannot convert 't1' 'const std::vector<_ty>' 1> 1> [ 1> _ty=int 1> ] 1> no user-defined-conversion operator available can perform conversion, or operator cannot called 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(145) : see reference function template instantiation 'void boost::detail::variant::invoke_visitor<visitor>::internal_visit<t>(t &,int)' being compiled 1> 1> [ 1> visitor=const record_value_visitor<int>, 1> t=t1 1> ] 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(173) : see reference function template instantiation 'void boost::detail::variant::visitation_impl_invoke_impl<visitor,voidptrcv,t>(int,visitor &,voidptrcv,t *,boost::mpl::false_)' being compiled 1> 1> [ 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<int>>, 1> voidptrcv=void *, 1> t=t1 1> ] 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(256) : see reference function template instantiation 'void boost::detail::variant::visitation_impl_invoke<visitor,voidptrcv,t1,nobackupflag>(int,visitor &,voidptrcv,t *,nobackupflag,int)' being compiled 1> 1> [ 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<int>>, 1> voidptrcv=void *, 1> nobackupflag=boost::variant<std::vector<int>,std::vector<double>,std::vector<std::string>>::has_fallback_type_, 1> t=t1 1> ] 1> c:\boost_1_46_1\boost\variant\variant.hpp(1776) : see reference function template instantiation 'void 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> 1> [ 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<int>>, 1> voidptrcv=void *, 1> t0_=std::vector<int>, 1> t1=std::vector<double>, 1> t2=std::vector<std::string>, 1> nobackupflag=boost::variant<std::vector<int>,std::vector<double>,std::vector<std::string>>::has_fallback_type_, 1> which=first_which, 1> step0=first_step 1> ] 1> c:\boost_1_46_1\boost\variant\variant.hpp(1787) : see reference function template instantiation 'void boost::variant<t0_,t1,t2>::internal_apply_visitor_impl<visitor,void*>(int,int,visitor &,voidptrcv)' being compiled 1> 1> [ 1> t0_=std::vector<int>, 1> t1=std::vector<double>, 1> t2=std::vector<std::string>, 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<int>>, 1> voidptrcv=void * 1> ] 1> c:\boost_1_46_1\boost\variant\variant.hpp(1810) : see reference function template instantiation 'void boost::variant<t0_,t1,t2>::internal_apply_visitor<boost::detail::variant::invoke_visitor<visitor>>(boost::detail::variant::invoke_visitor<visitor> &)' being compiled 1> 1> [ 1> t0_=std::vector<int>, 1> t1=std::vector<double>, 1> t2=std::vector<std::string>, 1> visitor=const record_value_visitor<int> 1> ] 1> c:\boost_1_46_1\boost\variant\detail\apply_visitor_unary.hpp(76) : see reference function template instantiation 'void boost::variant<t0_,t1,t2>::apply_visitor<const visitor>(visitor &)' being compiled 1> 1> [ 1> t0_=std::vector<int>, 1> t1=std::vector<double>, 1> t2=std::vector<std::string>, 1> visitor=record_value_visitor<int> 1> ] 1> c:\documents , settings\g88791\my documents\visual studio 2010\projects\test_boost\test_boost.cpp(58) : see reference function template instantiation 'void boost::apply_visitor<record_value_visitor<t>,boost::variant<t0_,t1,t2>>(const visitor &,visitable &)' being compiled 1> 1> [ 1> t=int, 1> t0_=std::vector<int>, 1> t1=std::vector<double>, 1> t2=std::vector<std::string>, 1> visitor=record_value_visitor<int>, 1> visitable=boost::variant<std::vector<int>,std::vector<double>,std::vector<std::string>> 1> ] 1>c:\boost_1_46_1\boost\variant\variant.hpp(832): error c2664: 'void record_value_visitor<t>::operator ()(const std::vector<_ty> &) const' : cannot convert parameter 1 't2' 'const std::vector<_ty> &' 1> 1> [ 1> t=int, 1> _ty=int 1> ] 1> , 1> [ 1> _ty=int 1> ] 1> reason: cannot convert 't2' 'const std::vector<_ty>' 1> 1> [ 1> _ty=int 1> ] 1> no user-defined-conversion operator available can perform conversion, or operator cannot called 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(145) : see reference function template instantiation 'void boost::detail::variant::invoke_visitor<visitor>::internal_visit<t>(t &,int)' being compiled 1> 1> [ 1> visitor=const record_value_visitor<int>, 1> t=t2 1> ] 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(173) : see reference function template instantiation 'void boost::detail::variant::visitation_impl_invoke_impl<visitor,voidptrcv,t>(int,visitor &,voidptrcv,t *,boost::mpl::false_)' being compiled 1> 1> [ 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<int>>, 1> voidptrcv=void *, 1> t=t2 1> ] 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(256) : see reference function template instantiation 'void boost::detail::variant::visitation_impl_invoke<visitor,voidptrcv,t2,nobackupflag>(int,visitor &,voidptrcv,t *,nobackupflag,int)' being compiled 1> 1> [ 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<int>>, 1> voidptrcv=void *, 1> nobackupflag=boost::variant<std::vector<int>,std::vector<double>,std::vector<std::string>>::has_fallback_type_, 1> t=t2 1> ] 1>c:\boost_1_46_1\boost\variant\variant.hpp(832): error c2664: 'void record_value_visitor<t>::operator ()(const std::vector<_ty> &) const' : cannot convert parameter 1 't0' 'const std::vector<_ty> &' 1> 1> [ 1> t=std::string, 1> _ty=std::string 1> ] 1> , 1> [ 1> _ty=std::string 1> ] 1> reason: cannot convert 't0' 'const std::vector<_ty>' 1> 1> [ 1> _ty=std::string 1> ] 1> no user-defined-conversion operator available can perform conversion, or operator cannot called 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(145) : see reference function template instantiation 'void boost::detail::variant::invoke_visitor<visitor>::internal_visit<t>(t &,int)' being compiled 1> 1> [ 1> visitor=const record_value_visitor<std::string>, 1> t=t0 1> ] 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(173) : see reference function template instantiation 'void boost::detail::variant::visitation_impl_invoke_impl<visitor,voidptrcv,t>(int,visitor &,voidptrcv,t *,boost::mpl::false_)' being compiled 1> 1> [ 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<std::string>>, 1> voidptrcv=void *, 1> t=t0 1> ] 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(256) : see reference function template instantiation 'void boost::detail::variant::visitation_impl_invoke<visitor,voidptrcv,t0,nobackupflag>(int,visitor &,voidptrcv,t *,nobackupflag,int)' being compiled 1> 1> [ 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<std::string>>, 1> voidptrcv=void *, 1> nobackupflag=boost::variant<std::vector<int>,std::vector<double>,std::vector<std::string>>::has_fallback_type_, 1> t=t0 1> ] 1> c:\boost_1_46_1\boost\variant\variant.hpp(1776) : see reference function template instantiation 'void 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> 1> [ 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<std::string>>, 1> voidptrcv=void *, 1> t0_=std::vector<int>, 1> t1=std::vector<double>, 1> t2=std::vector<std::string>, 1> nobackupflag=boost::variant<std::vector<int>,std::vector<double>,std::vector<std::string>>::has_fallback_type_, 1> which=first_which, 1> step0=first_step 1> ] 1> c:\boost_1_46_1\boost\variant\variant.hpp(1787) : see reference function template instantiation 'void boost::variant<t0_,t1,t2>::internal_apply_visitor_impl<visitor,void*>(int,int,visitor &,voidptrcv)' being compiled 1> 1> [ 1> t0_=std::vector<int>, 1> t1=std::vector<double>, 1> t2=std::vector<std::string>, 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<std::string>>, 1> voidptrcv=void * 1> ] 1> c:\boost_1_46_1\boost\variant\variant.hpp(1810) : see reference function template instantiation 'void boost::variant<t0_,t1,t2>::internal_apply_visitor<boost::detail::variant::invoke_visitor<visitor>>(boost::detail::variant::invoke_visitor<visitor> &)' being compiled 1> 1> [ 1> t0_=std::vector<int>, 1> t1=std::vector<double>, 1> t2=std::vector<std::string>, 1> visitor=const record_value_visitor<std::string> 1> ] 1> c:\boost_1_46_1\boost\variant\detail\apply_visitor_unary.hpp(76) : see reference function template instantiation 'void boost::variant<t0_,t1,t2>::apply_visitor<const visitor>(visitor &)' being compiled 1> 1> [ 1> t0_=std::vector<int>, 1> t1=std::vector<double>, 1> t2=std::vector<std::string>, 1> visitor=record_value_visitor<std::string> 1> ] 1> c:\documents , settings\g88791\my documents\visual studio 2010\projects\test_boost\test_boost.cpp(63) : see reference function template instantiation 'void boost::apply_visitor<record_value_visitor<t>,boost::variant<t0_,t1,t2>>(const visitor &,visitable &)' being compiled 1> 1> [ 1> t=std::string, 1> t0_=std::vector<int>, 1> t1=std::vector<double>, 1> t2=std::vector<std::string>, 1> visitor=record_value_visitor<std::string>, 1> visitable=boost::variant<std::vector<int>,std::vector<double>,std::vector<std::string>> 1> ] 1>c:\boost_1_46_1\boost\variant\variant.hpp(832): error c2664: 'void record_value_visitor<t>::operator ()(const std::vector<_ty> &) const' : cannot convert parameter 1 't1' 'const std::vector<_ty> &' 1> 1> [ 1> t=std::string, 1> _ty=std::string 1> ] 1> , 1> [ 1> _ty=std::string 1> ] 1> reason: cannot convert 't1' 'const std::vector<_ty>' 1> 1> [ 1> _ty=std::string 1> ] 1> no user-defined-conversion operator available can perform conversion, or operator cannot called 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(145) : see reference function template instantiation 'void boost::detail::variant::invoke_visitor<visitor>::internal_visit<t>(t &,int)' being compiled 1> 1> [ 1> visitor=const record_value_visitor<std::string>, 1> t=t1 1> ] 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(173) : see reference function template instantiation 'void boost::detail::variant::visitation_impl_invoke_impl<visitor,voidptrcv,t>(int,visitor &,voidptrcv,t *,boost::mpl::false_)' being compiled 1> 1> [ 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<std::string>>, 1> voidptrcv=void *, 1> t=t1 1> ] 1> c:\boost_1_46_1\boost\variant\detail\visitation_impl.hpp(256) : see reference function template instantiation 'void boost::detail::variant::visitation_impl_invoke<visitor,voidptrcv,t1,nobackupflag>(int,visitor &,voidptrcv,t *,nobackupflag,int)' being compiled 1> 1> [ 1> visitor=boost::detail::variant::invoke_visitor<const record_value_visitor<std::string>>, 1> voidptrcv=void *, 1> nobackupflag=boost::variant<std::vector<int>,std::vector<double>,std::vector<std::string>>::has_fallback_type_, 1> t=t1 1> ] ========== build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
the following compiles , runs expected:
template<typename t> struct record_value_visitor : boost::static_visitor<> { explicit record_value_visitor(t const& val) : val_(val) { } void operator ()(std::vector<t>& vec) const { vec.push_back(this->val_); } template<typename u> void operator ()(std::vector<u> const&) const { /* noop */ } private: t val_; }; template<typename t> record_value_visitor<t> make_record_value_visitor(t const& val) { return record_value_visitor<t>(val); } struct monitor { explicit monitor(int const type) : type_(type) { switch (type) { case 1: this->values_ = std::vector<int>(); break; case 2: this->values_ = std::vector<std::string>(); break; default: throw std::exception("invalid type"); } } void add_value(int const val) { if (this->type_ != 1) throw std::runtime_error("invalid type add_value()"); boost::apply_visitor(make_record_value_visitor(val), this->values_); } void add_value(std::string const& val) { if (this->type_ != 2) throw std::runtime_error("invalid type add_value()"); boost::apply_visitor(make_record_value_visitor(val), this->values_); } private: int type_; boost::variant< std::vector<int>, std::vector<double>, std::vector<std::string> > values_; }; int main() { monitor int_mon(1); int_mon.add_value(4); monitor str_mon(2); str_mon.add_value("foo"); }
the visitor's operator()
needs callable every possible type in variant – templated record_value_visitor::operator()
key here.
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