Error: unable to open primary document entity - c++11

Question

I'm trying to parse an xml file(I've got the schema definition in xsd as well) in my c++ program using xerces library. To get things started I've written a small program, where I just initialise the std::unique_pointer with the xml file. I get the following error if I use an std::string object containing the xml file while initialising whereas the program runs fine if I use the xml file directly for initialisation.
The main program is as follows:
#include <stdio.h>
#include <iostream>
#include "ShDataTypeRel15.hxx"
#include<fstream>
#include<string>
using namespace std;
int main (int argc, char* argv[])
{
try
{
fstream t("/home/vishal/UDA_XML/ShDataTypeRel15.xml", ios::in);
stringstream buffer;
buffer << t.rdbuf();
std::string xml_file = buffer.str();
std::unique_ptr<tSh_Data> Shdata(Sh_Data(xml_file));
}
catch (const xml_schema::exception& e)
{
cout <<"Exception caught"<<std::endl;
std::cerr << e << std::endl;
return 1;
}
return 0;
}
When I replace std::unique_ptr<tSh_Data> Shdata(Sh_Data(xml_file)); with std::unique_ptr<tSh_Data> Shdata(Sh_Data(argv[1])); then the program runs fine(I provide the path to the xml file as command line input.)
I get the following error:
Exception caught
:0:0 error: unable to open primary document entity '/home/vishal/UDA_XML/<?xml version="1.0"?>
The above error statement is followed by the xml file.

Answer 1

Problem was solved after I stored the location of my XML file in the string object instead of the whole content of that XML file in it.
i.e.
Now my std::string xml_file = path_to_xml_file;