boost property_tree adding nodes in a loop

Question

How do you add nodes to a property tree while looping a array?

This is probably a simple thing but I just don't understand how to work with boost_property_tree.

I have an array of values that I want to add to a property tree and output it as xml. If I hardcode the nodes and add them by hand to root node it works but if I try to do it from inside a loop the xml is invalid, contains only one node from the array and none of the xml nodes are terminate.

It's a variable length array of data so hardcoding the nodes are not possible. Doing something like this simply doesn't work. And I don't understand why?

ptree listnode;
std::vector<data> dataarray= ...
for(auto data : dataarray)
{
    ptree node;
    ...

    listnode.add_child("value", node)
}
...

What I want is something like this:

<list>
    <value active="true">12</value>
    <value active="true">44</value>
    <value active="true">23</value>
</list>

But the resulting xml looks like this:

<list>
    <value active="true">

It just stops after the first value node. No exceptions or anything so I'm assuming that either my property_tree is invalid or its the xml_parser::write_xml that fails to create the xml.

Most likely I have misunderstood how to use boost::property_tree. But really, outputting a list must be a pretty simple thing?

Answer 1

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

using namespace boost::property_tree;

int main() {
    ptree pt;

    auto& list = pt.add_child("list", ptree{});

    for (auto data : { 12, 44, 23 })
        list.add("value", data)
            .add("<xmlattr>.active", true);

    xml_parser::write_xml(std::cout, pt);
}

Resulting XML:

<?xml version="1.0" encoding="utf-8"?>
<list>
    <value active="true">12</value>
    <value active="true">44</value>
    <value active="true">23</value>
</list>