#include <iostream>

int main() {

// int a = 1;

// int b = a;

// std::cout << "before a: " << a << std::endl;

// std::cout << "before b: " << b << std::endl;

// b = 5;

// std::cout << "after a: " << a << std::endl;

// std::cout << "after b: " << b << std::endl;

// std::cout << "addr a: " << &a << std::endl;

// std::cout << "addr b: " << &b << std::endl;

// `b` is an alias to `a`

int a = 1;

int& b = a;

b = 5;

// std::cout << "after a: " << a << std::endl;

// std::cout << "after b: " << b << std::endl;

// std::cout << "addr a: " << &a << std::endl;

// std::cout << "addr b: " << &b << std::endl;

// Compiler error: Must be init immediately.

// Reference is an alias, so it must bind to valid object.

// C++ language does not have concept of empty reference.

// int a = 1;

// int& b; // cannot be null

// b = a;

// `c` will always be an alias to `b`, value inside of it can change

// int a = 1;

// int b = 2;

// int& c = b;

// c = a; // c still refers to b

return 0;

}