Stack topElem function question

I have a function to get the top element from a stack...but I'm having a problem calling it

Code:

---

void stack::topElem(el_t& elem)
{
        if(isEmpty())
        {
                stackError("stack is empty");
        }
        else
        {
                elem = el[top];
        }
       
}

---

problem is I didn't write this the professor did...I would've just returned something but I have no idea what to put in the parameters. I though possibly this:

i

Code:

---

f ((let != '*') && (let != '+') && (let != '-') && (let != '/'))
      {
                  thisStack.push(int(mystring[i]));
      }
          else
          {
                  thisStack.pop(thisStack[thisStack.topElem(?????)]);
          }

---

Those ??? symbolize my confusion. Any help would be appreciated....

As you can see by the "&" in the parameter declaration, the function can change the parameter passed in. So whatever you want the function to change, you pass in.