For the following short program, which function calls are errors and why? For the valid calls, what does the reference argument refer to?
#include
using namespace std;
double up(double x) { return 2.0* x;}
void r1(const double &rx) {cout << rx << endl;}
void r2(double &rx) {cout << rx << endl;}
void r3(double &&rx) {cout << rx << endl;}
int main()
{
double w = 10.0;
r1(w);
r1(w+1);
r1(up(w));

r2(w);
r2(w+1);
r2(up(w));
r3(w);
r3(w+1);
r3(up(w));
return 0;
}