Passing by value versus reference

pass_by.cpp

#include <iostream>

void passValue(int x)
{
    x += 2;
}

void passRef(int &x)
{
    x += 2;
}

int main()
{
    int x = 3;
    int &y = x;

    passValue(x);                // pass by value
    std::cout << x << std::endl; // x is still 3

    passRef(y);                  // pass by reference
    std::cout << x << std::endl; // x is now 5!
}