Thread: input/output file stream variables as function parameters?

This is my main function for a program with two subfunctions (one to get user input of numbers that need to be averaged and another that averages the numbers):

Code:

#include <iostream>
#include <iomanip>
#include <string>
#include <fstream>


using namespace std;


void getNumbers (double num, int& count, char yn, ofstream outData);
double averageNumbers (double num1, double sum, double average, int counter, ifstream userData);


int main ()
{
    string sentinel;
    double number, sum, average;
    int counter;
    char yn;
    ofstream outData;
    ifstream userData;


    cout << "To continue with the program enter 'continue'. Otherwise enter 'STOP'";
    cin >> sentinel;


    if (sentinel != "STOP")
        {
        getNumbers (number, counter, yn, outData);
        
        averageNumbers (number, sum, average, counter, userData);
        cout << "The numbers you entered are: " << userData <<endl;
        cout << "The average of the numbers is: " << average <<endl;


        cout << "To continue with the program enter 'continue'. Otherwise enter 'STOP': ";
        cin >> sentinel;
    }


    return 0;
}

The problem is, my compiler is not letting me use input/output file stream variables as parameters for my functions. It is not 100% necessary that I use functions in this program but I want to impress the teacher and having all these operations enclosed in the function main is like just meeting expectations..not exceeding them like I want to. Is there a way to get around this?