ptree_example.cpp

#include <iostream>
#include <stdexcept>
#include <map>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/xml_parser.hpp>

using namespace std;
using namespace boost::property_tree;

int main(int argc, char const *argv[])
try
{
    multimap<string, string>  records = {
        {"a", "a1"},
        {"a", "a2"},
        {"b", "b1"},
        {"a", "a3"},
        {"", "empty"},
        {"c", "c1"},
        {"", "empty"}
    };

    { // variant 1 
        ptree p;
        for(const auto& item: records)
        {
            const auto& key = item.first;
            const auto& value = item.second;
            if(!key.empty())
            {
                p.put(key+"."+value, "smth");
            }
        }
        xml_writer_settings<char> settings('\t', 1);
        xml_parser::write_xml(cout, p, settings);
    }

    { // variant 2
        ptree p;
        ptree subTree;
        string currentKey; 
        for(const auto& item: records)
        {
            const auto& key = item.first;
            const auto& value = item.second;

            if(key.empty()) continue;

            if(key!=currentKey)
            {
                if(!subTree.empty())
                    p.add_child(currentKey, subTree);
                subTree = ptree();
                currentKey = key;
            }
            subTree.put(value, "smth");
        }
        if(!subTree.empty())
            p.add_child(currentKey, subTree);
        xml_writer_settings<char> settings('\t', 1);
        xml_parser::write_xml(cout, p, settings);
    }



    return 0;
}
catch(const exception& ex) {
    cerr << "Exception: " << ex.what() << endl;
    return 1;
}