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!