Declaring a const variable as reference

Question

At some places I have seen variables declared like this

const int &var1;
extern int & var2;

These variables are defined somewhere else. I always thought that when you declare a reference to a variable you bind the reference to that variable at the same place. I am missing something here. Also I have not seen references declared as above when there is no qualifier (e.g. const or extern).

Answer 1

const int &var1;

this is illegal if not part of a class or a struct. If it is, it must be initialized in the constructor initializer list.

The reason it's legal is because the variable doesn't exist until an object of that class is created. So there's no unbound reference, as there is no reference to speak of.