boost::archive::xml_iarchive

Given:

#include <iostream>
#include <fstream>
#include <vector>
#include <string>
# include <sstream>
  
// Boost
#include <boost/scoped_ptr.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/archive/xml_oarchive.hpp>
#include <boost/archive/xml_iarchive.hpp>
#include <boost/serialization/string.hpp>
#include <boost/serialization/vector.hpp>
#include <boost/serialization/scoped_ptr.hpp>
#include <boost/serialization/shared_ptr.hpp>
#include <boost/serialization/export.hpp>
#include <boost/serialization/nvp.hpp>

int main(int argc, char* argv[]) {
 try {

    std::ifstream file("test.xml" );
    if ( !file ) {
      return EXIT_FAILURE ;
    }
    std::string str ( ( std::istreambuf_iterator<char> ( file ) ),
                        std::istreambuf_iterator<char>() );
    std::cout << str << std::endl;
    std::istringstream iss ( str ) ;
    boost::archive::xml_iarchive ia( iss );
  } catch ( const boost::archive::archive_exception& e ) {
    std::cout << e.what() << std::endl;
  }
}

The contents of test.xml is as follows:

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<boost_serialization signature="serialization::archive" version="3">
<s class_id="0" tracking_level="0" version="1">
        <attrs class_id="1" tracking_level="0" version="0">
                <count>2</count>
                <item class_id="2" tracking_level="0" version="0">
                        <first>a</first>
                        <second>abcde</second>
                </item>
                <item>
                        <first>b</first>
                        <second>badfaf</second>
                </item>
        </attrs>
</s>
</boost_serialization>

When I run the code I get 'unrecognized XML syntax' which is puzzling given the XML is well-formed at least according to XML spy.

  boost::archive::xml_iarchive ia( file);

Produce the same result. It's not clear to me what the issue ('unrecognized XML syntax doesn't tell me a whole lot) or fix is.