Question

Error!

$ cat double.cc
#include<iostream>

int main() {
    int i = 42;
    double &r = i;
}
$ g++ double.cc
double.cc: In function ‘int main()’:
double.cc:5: error: invalid initialization of reference of type ‘double&’
from expression of type ‘int’
$

Works!

$ cat double.cc
#include<iostream>

int main() {
    int i = 42;
    const double &r = i;
}
$ g++ double.cc
$

Why do we get the error in the first case? I know this is discussed here, but I am not sure I understand why this not allowed, while int to const double& is allowed?

Answer 1

When you write

int i = 42;
double const &r = i;

i is implicitly converted to a double, which results in a temporary value, or rvalue. A non-const reference is not allowed to bind to an rvalue, so r needs to be a double const& for the above to work.