C++ function with 3 parameters for reading in generic binary input - c++

Question

I'm trying to create a function getIdFromFile in files.cpp that takes 3
parameters: (a C++ string representing a filename, an istream, and an ostream) and returns an int.
This is the starting code I have:
// files.cpp
#include "files.hpp"
#include <iostream>
#include <fstream>
using std::ofstream;
using std::ifstream;
using std::ostream;
#include <sstream>
using std::string;
using std::stringstream;
using std::istream;
using std::istringstream;
int getIdFromFile (std::string(fileName),std::istream &usersData, std::ostream theStream){
&usersData.open(fileName, std::ios::in|std::ios::binary);
}
I'm getting the error:
No member named 'open' in 'std::__1::basic_istream<char>'
Is there something I need to include or add from the standard library?
Thanks

Answer 1

You should consider either
int getIdFromFile (std::string fileName, std::ifstream &usersData, std::ostream &theStream)
or
int getIdFromStream (std::istream &usersData, std::ostream &theStream)
In the first case you'd expect a new or a closed file stream that you would open. You'd need to define all the parameters (e.g. mode). In principle you should foresee some error handling, for example if the file name is invalid.
In the second case, you'd open the file somewhere else and pass a stream. But you could as well pass a string stream if you'd want.