#include <iostream>
#include <fstream>
#include <sstream>

using namespace std ;

void foo (ostream &os = cout) {
  os << "Hello World\n" ;
}

int  main() {
  ofstream ofile ;
  ostringstream oss ;

  cout << "Default argument\n" ;
  foo() ;

  ofile.open("test.txt") ;
  ofile << "File output\n" ;
  foo(ofile) ;
  ofile.close() ;

  cout << "String stream\n" ;
  foo(oss) ;
  cout << oss.str() ;

}