Boost::property_tree: Using std::vector<> in XML parser to store multiple values in one key

Question

My question is related to this question: Boost property_tree: multiple values per key and to a question following that question: Boost property_tree: multiple values per key, on a template class.

I am trying to parse an XML file in which multiple values are listed at a single key value using std::vector<>. The following code is what I have implemented so far:

#include <boost/optional.hpp>
#include <boost/property_tree/xml_parser.hpp>

namespace boost { namespace property_tree
{

template<typename type>
struct vector_xml_translator
{
    boost::optional<std::vector<type> > get_value(const std::string& str)
    {
        if (!str.empty())
        {
            std::vector<type> values;
            std::stringstream ss(str);

            while (ss)
            {
                type temp_value;
                ss >> temp_value;
                values.push_back(temp_value);
            }

            return boost::optional<std::vector<type> >(values);
        }
        else
        {
            return boost::optional<std::vector<type> >(boost::none);
        }
    }

    boost::optional<std::string> put_value(const std::vector<type>& b)
    {
        std::stringstream ss;
        for (unsigned int i = 0; i < b.size(); i++)
        {
            ss << b[i];
            if (i != b.size()-1)
            {
                ss << " ";
            }
        }
        return boost::optional<std::string>(ss.str());
    }
};

template<typename ch, typename traits, typename alloc, typename data_type>
struct translator_between<std::basic_string<ch, traits, alloc>, std::vector<data_type> >
{
    typedef vector_xml_translator<data_type> type;
};

} // namespace property_tree
} // namespace boost

A minimal example to test this code is as follows:

#include <fstream>
#include <iostream>
#include <boost/property_tree/ptree.hpp>
#include <XML_Vector_Translator.hpp>


int main()
{
    using boost::property_tree::ptree;

    std::vector<double> test_vector;

    test_vector.push_back(1);
    test_vector.push_back(6);
    test_vector.push_back(3);


    ptree pt;
    pt.add("base", test_vector);

    std::ofstream os("test_file.xml");

    write_xml(os, pt, boost::property_tree::xml_writer_settings<std::string>(' ', 2));

    std::ifstream is("test_file.xml");

    ptree pt_2;
    read_xml(is, pt_2);

    std::vector<int> test_vector_2;

    test_vector_2 = pt_2.get<std::vector<int> >("base");

    for (unsigned int i = 0; i < test_vector_2.size(); i++)
    {
        std::cout << test_vector_2[i] << std::endl;
    }

    return 0;
}

When I run this code I get a number of errors, which lead me to believe that the registration of the translator structure is not right. Does anybody have an idea of how to solve this issue and/or improve this code?

Answer 1

1. As the older answer also points out¹ you must satisfy the requirements for boost::property_tree::detail::is_translator, so you need the internal_type / external_type typedefs.

   typedef T internal_type;
   typedef T external_type;
   

2. Next up, the loop is wrong, you need check the result of the value extraction:

   while (ss >> temp_value)
       values.push_back(temp_value);
   

3. It's bad practice to put your own types inside the boost namespace. Only the specialization of translator_between<> needs to be there.

4. You can simplify and generalize a lot of the code

All in a working demo:

Live On Coliru

#include <boost/optional.hpp>
#include <boost/property_tree/ptree.hpp>
#include <vector>
#include <list>

namespace mylib { namespace xml_translators {

    template<typename T> struct container
    {
        // types
        typedef T internal_type;
        typedef T external_type;

        boost::optional<T> get_value(const std::string& str) const
        {
            if (str.empty())
                return boost::none;

            T values;
            std::stringstream ss(str);

            typename T::value_type temp_value;
            while (ss >> temp_value)
                values.insert(values.end(), temp_value);

            return boost::make_optional(values);
        }

        boost::optional<std::string> put_value(const T& b) {
            std::stringstream ss;
            size_t i = 0;
            for (auto v : b)
                ss << (i++?" ":"") << v;
            return ss.str();
        }
    };

} }

namespace boost { namespace property_tree {
    template<typename ch, typename traits, typename alloc, typename T>
        struct translator_between<std::basic_string<ch, traits, alloc>, std::vector<T> > {
            typedef mylib::xml_translators::container<std::vector<T> > type;
        };

    template<typename ch, typename traits, typename alloc, typename T>
        struct translator_between<std::basic_string<ch, traits, alloc>, std::list<T> > {
            typedef mylib::xml_translators::container<std::list<T> > type;
        };
} }

#include <sstream>
#include <iostream>
#include <boost/property_tree/xml_parser.hpp>

int main()
{
    std::stringstream ss;
    using boost::property_tree::ptree;

    {
        ptree pt;
        pt.add("base", std::vector<double> { 1, 6, 3 });

        write_xml(ss, pt, boost::property_tree::xml_writer_settings<std::string>(' ', 2));
    }

    {
        ptree pt;
        read_xml(ss, pt);

        std::cout << "As string: '" << pt.get("base", "") << "'\n";
        auto roundtrip = pt.get<std::list<int> >("base");
        for (auto i : roundtrip)
            std::cout << i << std::endl;
    }
}

Prints

As string: '1 6 3'
1
6
3

¹ Boost property_tree: multiple values per key, see also the identity translator http://www.boost.org/doc/libs/1_64_0/doc/html/boost/property_tree/id_translator.html