A reference variable is an object that is referring to another object by its address in memory. It is a kind of synonym, a nickname. We have to define its value immediately (it cannot be left just declared). A reference variable can't change its value (the association with a particular variable). A reference variable must be the same type as the variable assigned to it. We create reference variables by adding an ampersand before their name (like when getting to a variable's address).

int normal_var = 4;
cout << "normal_var equals " << normal_var << " and has the address " << &normal_var << endl;
int &reference_var = normal_var; 
cout << "reference_var equals " << reference_var << " and has the address " << &reference_var << endl; // the address is the same

reference_var = 3;
cout << "normal_var equals " << normal_var << " and has the address " << &normal_var << endl;
cout << "reference_var equals " << reference_var << " and has the address " << &reference_var << endl;

int normal_var_2 = normal_var; 
cout << "normal_var_2 equals " << normal_var_2 << " and has the address " << &normal_var_2 << endl; // the address is different

int x = 10;
const int &y = x;

/* y = 10; - this wouldn't work because "y" is a constant ("reference_var" also was a constant in a way
because we couldn't change its associated address, but now we also can't change the value under the address) */
x = 40; // we can change the value of the "y" constant by using the variable it is referring to ("x")
cout << "x equals " << x << " and has the address " << &x << endl;
cout << "y equals " << y << " and has the address " << &y << endl;
                                    

The code below wouldn't work without setting the arguments as reference variables because x and y are local variables, and we would have to return them. Here, we refer to the addresses of a and b and modify their values directly with x and y reference variables inside the function.

#include <iostream>
using namespace std;

void swap(int &x, int &y) {
    int t = x;
    x = y;
    y = t;
}

int main() {
    int a = 10;
    int b = 20;
    
    swap(a, b);
    
    cout << "a: " << a << endl;
    cout << "b: " << b << endl;
    
    return 0;
}