When you call the function calculation , you will have to pass the variable names that you want to get the output from.

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int main(void)
{
    int num;
    int first, last, total;
    while (num != 0)
    { 
        cout << "Enter an integer (0 to stop): ";
        cin >> num;
        
        if ( num != 0)
        {               
            calculation(num, last, first, total);
            cout << "Last digit:   " << last << endl;
            cout << "First digit:  " << first << endl;
            cout << "Total digits: " << total  << endl;
        }
    }
    
    cout << "End of task.\n\n";
    system("pause");
    
    return 0;   
    }

int main(void) { int num; int first, last, total; while (num != 0) { cout << "Enter an integer (0 to stop): "; cin >> num; if ( num != 0) { calculation(num, last, first, total); cout << "Last digit: " << last << endl; cout << "First digit: " << first << endl; cout << "Total digits: " << total << endl; } } cout << "End of task.\n\n"; system("pause"); return 0; }

You do not want to pass t1 as an input for calculation, so change it like this. You do not need t1 at all.

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void calculation (int x, int &last, int &first, int &total)
{
    last = (x % 10);
    while (x>9) 
        first = x / 10;

    while (x>0)
    { 
          total++;
          x /=10;
    }
}

void calculation (int x, int &last, int &first, int &total) { last = (x % 10); while (x>9) first = x / 10; while (x>0) { total++; x /=10; } }

congratulations! It works, albiet a bit inefficient.
As I said before, the only reference needed was x
and you pulled that one in as a value!
All you are doing by taking in all those other unneccessary references is to bring the variable scopes INSIDE the function. It works just as well if the variables were set as global. Aguments pass things in, not out!

I get the feeling that you have missed this point.
good luck.

No he has not missed the point. You have. Arguments can be used to pass things both in and out. Passing by reference can be used to get things out. It is also memory efficient because another copy of the variable is not created in the stack like it is done when you use passing by value.