The reference used so far allows you to change the variable it is an alias for, therefore it has lvalue semantics. There are also const lvalue references, that is, a reference to an object that you can read, but not write to.
As with const pointers, you declare a const reference using the const keyword on a lvalue reference. This essentially makes the reference read-only: you can access the variable's data to read it, but not to change it.
int i = 42; const int& ri = i; ri = 99; // error!
