Question

I cannot initialize a non-const reference to type T1 from a convertible type T2. However, I can with a const reference.

long l;
const long long &const_ref = l; // fine
long long &ref = l;             // error: invalid initialization of reference of
                                // type 'long long int&' from expression of type
                                // 'long int'

Most problems I encountered were related to r-values that cannot be assigned to a non-const reference. This is not the case here — can someone explain? Thanks.

Answer 1

An integer promotion results in an rvalue. long can be promoted to a long long, and then it gets bound to a const reference. Just as if you had done:

typedef long long type;
const type& x = type(l); // temporary!

Contrarily an rvalue, as you know, cannot be bound to a non-const reference. (After all, there is no actual long long to refer to.)