main.cpp

#include <iostream>
#include <string>
 
int x = 3;
 
const int& GetX() {
    return x;
}
 
void DoStuff() {
    x = 5;
}
 
int main() {
  const auto& myConstX = GetX();
  std::cout << "x: " << myConstX << "\n"; // prints 3
  DoStuff();
  std::cout << "x: " << myConstX << "\n"; // prints 5
}

main.cpp

#include <iostream>
#include <string>
 
void what_happens(int & i, int const& j) {
    i += 5;
    if(i == j) std::cout << "Giggity.\n";
}
 
int main() {
    int val = 17;
    what_happens(val, val);
    return 0;
}

https://isocpp.org/wiki/faq/const-correctness