Parse XML with boost property tree

Question

I have the following XML file and I want to store it using the below structures.

the data structs:

    struct transitions
    {
     string oldstate;
     string event;
     string newstate;
    };

    struct XML_Diagram
    {
     string diag_name;
     string diag_defaultstate;
     list<string> diag_states;
     list<string> diag_events;
     list<transitions> diag_transitions;
    };

the xml file:

    <diagram>
      <diagname>DiagaX</diagname>
      <states>
         <state>A</state>
         .............       
      </states>
      <events>
          <event>ev1</event>
          .................
      </events>
      <defaultstate>A</defaultstate>
      <transitions>
          <transition>
              <oldstate>A</oldstate>
              <event>ev1</event>
              <newstate>B</newstate>
          </transition>
          <transition>
              <oldstate>B</oldstate>
              <event>ev2</event>
              <newstate>C</newstate>
          </transition>
          .........................
      </transitions>
    </diagram>

It is clear to me how can I to access diagram.states . I can do it with the folowing code:

    using boost::property_tree::ptree;
    ptree pt;

    // Get diagram states
    BOOST_FOREACH(ptree::value_type &v, pt.get_child("diagram.states"))
    {
       diag_states.push_back(v.second.data());
    }

What is not clear to me is how can I access the data from at the level diagram.transitions.transition ?

My problem is that I could not find any examples in the documentation on how to parse more complex xml files with several levels.

Answer 1

This useful utility function traverses and pretty-prints an entire property tree:

using boost::property_tree::ptree;

std::string q(const std::string& s) { return "\"" + s + "\""; }

void print_tree(const ptree& pt, int level)
{
    const std::string sep(2 * level, ' ');
    BOOST_FOREACH(const ptree::value_type &v, pt) {
        std::cout
            << sep
            << q(v.first) << " : " << q(v.second.data()) << "\n";
        print_tree(v.second, level + 1);
    }
}

void print_tree(const ptree& pt) { print_tree(pt, 0); }

The v.second values are trees themselves that can be accessed with the usual get methods. The transitions can for example be accessed and printed like this:

using std::string;

void print_transitions(const ptree& pt)
{
    BOOST_FOREACH(
        const ptree::value_type &v,
        pt.get_child("diagram.transitions"))
    {
        const ptree& child = v.second;
        std::cout
            << "Event "
            << child.get<string>("event")
            << " in state "
            << child.get<string>("oldstate")
            << " leads to state "
            << child.get<string>("newstate")
            << "\n";
    }
}

Answer 2

Here is another example of how to print ptree with attributes:

namespace pt = boost::property_tree;

// worker
typedef std::pair<const pt::ptree&, unsigned> tree_printer;

// empty ptree helper
const pt::ptree& empty_ptree()
{
    static pt::ptree t;
    return t;
}


std::ostream& operator <<(std::ostream& os, const tree_printer& p)
{
    const pt::ptree& tree = p.first;

    if(tree.empty()) return os;

    const std::string indent(p.second, ' ');

    BOOST_FOREACH(const pt::ptree::value_type& v, tree)
    {
        const std::string& nodeName = v.first;

        if(nodeName == "<xmlattr>") continue;

        os << indent << nodeName;
        const pt::ptree& attributes =
            v.second.get_child("<xmlattr>", empty_ptree());

        if(!attributes.empty())
        {
            os << " [ ";
            BOOST_FOREACH(const pt::ptree::value_type& attr, attributes)
            {
                const std::string& attrName = attr.first;
                const std::string& attrVal = attr.second.data();

                os << attrName << " = '" << attrVal << "'; ";
            }
            os << "]";
        }
        os << "\n";

        const pt::ptree& childNode = v.second;
        os << tree_printer(childNode, p.second + 1);
    }

    return os;
}


std::ostream& operator <<(std::ostream& os, const pt::ptree& tree)
{
    os << tree_printer(tree, 0);

    return os;
}

Hope this helps.