When I left it as

Code:
 void getNumber(int &number)
I got a compile error:

prime.cpp: In function 'int isPrime(int)':
prime.cpp:44: warning: converting to 'int' from 'double'
Undefined first referenced
symbol in file
getNumber(int) /var/tmp//cc1BaH0f.o
ld: fatal: Symbol referencing errors. No output written to a.out
collect2: ld returned 1 exit status