Why can const int& bind to an int?

Question

In the C++ primer, I found that const int & can bind with a int object.I don\'t understand that,because I think const int & should bind with a

Answer 1

const in this example only guarantee that a can not be changed where r is used. Usually this is used for functions that do not change input parameters like:

int doNotModifyFoo(const int &foo);