Wt

I found out that type support may be extended for my project just like this

namespace Wt { namespace Dbo {

    template<typename T>
    typename std::enable_if< !std::is_enum<T>::value, void>::type
    JsonSerializer::act(FieldRef<T> field){
        writeFieldName(field.name());
        WString v = field.value().toString();
        JsonSerializer::act( FieldRef<WString>(v, field.name(), field.size(), field.flags()));
    }

    template<>
    void JsonSerializer::act(FieldRef<WString> field){
        std::string v = field.value().toUTF8();
        JsonSerializer::act( FieldRef<std::string>(v, field.name(), field.size(), field.flags()));
    }

}}

thanks to the template declered in the header file