specializing boost::lexical_cast for enums in boost::property_maps

Question

I am trying to use a boost::graph that has an enum inside its bundled vertex property. The problem starts when I try to use the bundled property for a boost::dynamic_property. Looks like I can't get the correct template specialization for boost::lexical_cast to recognize the type.

#include <string>
#include <iostream>
#include <boost/lexical_cast.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/property_map/dynamic_property_map.hpp>

enum MyEnum { A, B, C };

namespace boost{
    template<>
    std::string lexical_cast(const MyEnum & m)
    {
        return std::to_string(int(m));
    }
} /* boost */

struct EnumWrapper
{
    MyEnum e;

    EnumWrapper() = default;
    EnumWrapper(MyEnum e) : e{e}
    {}
};

int main()
{
    using namespace boost;

    // This works
    MyEnum e{MyEnum::B};
    std::cout << lexical_cast<std::string>(e) << std::endl;

    // This doesn't compile
    adjacency_list<vecS, vecS, directedS, EnumWrapper> graph;
    dynamic_properties dp;
    dp.property("test", get(&EnumWrapper::e, graph));

    return 0;
}

If I try writing the enum directly, everything works fine. When try to put it in the dynamic property, the following error appears:

In file included from /usr/include/boost/iterator/iterator_categories.hpp:22:0,
                 from /usr/include/boost/iterator/iterator_facade.hpp:14,
                 from /usr/include/boost/range/iterator_range_core.hpp:27,
                 from /usr/include/boost/lexical_cast.hpp:30,
                 from main.cpp:3:
/usr/include/boost/lexical_cast/detail/converter_lexical.hpp: In instantiation of ‘struct boost::detail::deduce_target_char_impl<boost::detail::deduce_character_type_later<MyEnum> >’:
/usr/include/boost/lexical_cast/detail/converter_lexical.hpp:270:89:   required from ‘struct boost::detail::deduce_target_char<MyEnum>’
/usr/include/boost/lexical_cast/detail/converter_lexical.hpp:407:92:   required from ‘struct boost::detail::lexical_cast_stream_traits<std::__cxx11::basic_string<char>, MyEnum>’
/usr/include/boost/lexical_cast/detail/converter_lexical.hpp:468:15:   required from ‘struct boost::detail::lexical_converter_impl<MyEnum, std::__cxx11::basic_string<char> >’
/usr/include/boost/lexical_cast/try_lexical_convert.hpp:181:44:   required from ‘bool boost::conversion::detail::try_lexical_convert(const Source&, Target&) [with Target = MyEnum; Source = std::__cxx11::basic_string<char>]’
/usr/include/boost/lexical_cast.hpp:41:60:   required from ‘Target boost::lexical_cast(const Source&) [with Target = MyEnum; Source = std::__cxx11::basic_string<char>]’
/usr/include/boost/property_map/dynamic_property_map.hpp:44:38:   required from ‘Value boost::detail::read_value(const string&) [with Value = MyEnum; std::__cxx11::string = std::__cxx11::basic_string<char>]’
/usr/include/boost/property_map/dynamic_property_map.hpp:158:64:   required from ‘void boost::detail::dynamic_property_map_adaptor<PropertyMap>::do_put(const boost::any&, const boost::any&, mpl_::bool_<true>) [with PropertyMap = boost::vec_adj_list_vertex_property_map<boost::adjacency_list<boost::vecS, boost::vecS, boost::directedS, EnumWrapper>, boost::adjacency_list<boost::vecS, boost::vecS, boost::directedS, EnumWrapper>*, MyEnum, MyEnum&, MyEnum EnumWrapper::*>]’
/usr/include/boost/property_map/dynamic_property_map.hpp:186:11:   required from ‘void boost::detail::dynamic_property_map_adaptor<PropertyMap>::put(const boost::any&, const boost::any&) [with PropertyMap = boost::vec_adj_list_vertex_property_map<boost::adjacency_list<boost::vecS, boost::vecS, boost::directedS, EnumWrapper>, boost::adjacency_list<boost::vecS, boost::vecS, boost::directedS, EnumWrapper>*, MyEnum, MyEnum&, MyEnum EnumWrapper::*>]’
main.cpp:40:1:   required from here
/usr/include/boost/lexical_cast/detail/converter_lexical.hpp:243:13: error: static assertion failed: Target type is neither std::istream`able nor std::wistream`able
             BOOST_STATIC_ASSERT_MSG((result_t::value || boost::has_right_shift<std::basic_istream<wchar_t>, T >::value),

The line Target type is neither std::istreamable nor std::wistreamable suggests to me, that the lexical_cast is not specialized correctly. However, using the lexical cast directly appears to work just fine.

Answer 1

Target = MyEnum; Source = std::__cxx11::basic_string<char>. Boost is trying to convert from string to MyEnum; you've only provided the reverse direction.

You need to provide a conversion from string to MyEnum:

namespace boost{
    template<>
    MyEnum lexical_cast(const std::string & s) {
        return MyEnum::A;    // for example
    }
} /* boost */

Example.