C++ Primer, Fifth Edition

2.4.1. References to const

As with any other object, we can bind a reference to an object of a const type. To do so we use a reference to const, which is a reference that refers to a const type. Unlike an ordinary reference, a reference to const cannot be used to change the object to which the reference is bound:

const int ci = 1024;const int &r1 = ci;   // ok: both reference and underlying object are constr1 = 42;              // error: r1 is a reference to constint &r2 = ci;         // error: non const reference to a const object

Because we cannot assign directly to ci, we also should not be able to use a reference to change ci. Therefore, ...