You shouldn't ever be returning the operators though, only the result of operations or the number value in the node.

In the first branch, after you apply the operator to the two children, you...

A lot of important questions here, so I will try and answer them the best I can.

First, the main difference between passing by value and passing by reference.. When we pass by value, the receiving...

This is a prime example of how I see some students graduate in computer science and don't understand basic object oriented programming concepts. You need to figure this out yourself. Of course if you...

To be a bit clearer, you can use the stringstream class like so:

#include <iostream>
#include <sstream>
using namespace std;

int main() {
stringstream ss; // our stringstream object
...