````cpp
int main() {
// reference is alias to variable
int num {100};
int &ref{num};
cout << num << endl; // 100
cout << ref << endl; // 100
// can change variable
num = 200;
cout << num << endl; // 200
cout << ref << endl; // 200
// or reference
ref = 300;
cout << num << endl; // 300
cout << ref << endl; // 300
}
```