C++/RapidXML: Edit node and write to a new XML file doesn't have the updated nodes

I'm parsing a XML file from a string. My node Id is bar, and I want to change it to foo and then write to file.

After writing to file, the file still have the bar, and not the foo.

#include "rapidxml.hpp"
#include "rapidxml_print.hpp"
void main()
{
    std::string newXml = "<?xml version=\"1.0\" encoding=\"UTF - 8\"?><Parent><FileId>fileID</FileId><IniVersion>2.0.0</IniVersion><Child><Id>bar</Id></Child></Parent>";

    xml_document<> doc;
    xml_node<> * root_node;

    std::string str = newXml;
    std::vector<char> buffer(str.begin(), str.end());
    buffer.push_back('\0');

    doc.parse<0>(&buffer[0]);

    root_node = doc.first_node("Parent");

    xml_node<> * node = root_node->first_node("Child");
    xml_node<> * xml = node->first_node("Id");
    xml->value("foo"); // I want to change my id from bar to foo!!!!

    std::ofstream outFile("output.xml");
    outFile << doc; // after I write to file, I still see the ID as bar
}

What am I missing here?

The issue is in the layout of data. Under node_element node xml there is yet another node_data node that contains "bar". Your posted code also does not compile. Here I made your code to compile and did show how to fix it:

#include <vector>
#include <iostream>
#include "rapidxml.hpp"
#include "rapidxml_print.hpp"

int main()
{
    std::string newXml = "<?xml version=\"1.0\" encoding=\"UTF - 8\"?><Parent><FileId>fileID</FileId><IniVersion>2.0.0</IniVersion><Child><Id>bar</Id></Child></Parent>";

    rapidxml::xml_document<> doc;

    std::string str = newXml;
    std::vector<char> buffer(str.begin(), str.end());
    buffer.push_back('\0');

    doc.parse<0>(&buffer[0]);

    rapidxml::xml_node<>* root_node = doc.first_node("Parent");

    rapidxml::xml_node<>* node = root_node->first_node("Child");
    rapidxml::xml_node<>* xml = node->first_node("Id");
    // xml->value("foo"); // does change something that isn't output!!!!

    rapidxml::xml_node<> *real_thing = xml->first_node();
    if (real_thing != nullptr                         // these checks just demonstrate that
       &&  real_thing->next_sibling() == nullptr      // it is there and how it is located
       && real_thing->type() == rapidxml::node_data)  // when element does contain text data 
    {
        real_thing->value("yuck");  // now that should work
    }

    std::cout << doc; // lets see it
}

And so it outputs:

<Parent>
    <FileId>fileID</FileId>
    <IniVersion>2.0.0</IniVersion>
    <Child>
        <Id>yuck</Id>
    </Child>
</Parent>

See? Note that how data is laid out during parse depends on flags that you give to parse. For example if you first put doc.parse<rapidxml::parse_fastest> then parser will not create such node_data nodes and then changing node_element data (like you first tried) will work (and what I did above will not). Read the details from manual.