Hmm, I don't believe that algorithm will work. Go with the post above by LethalEncounter. The field is a bitfield, and you can use bitmask operators to determine the classes that can use it.
I don't feel like writing a novel to explain bitfield operators, so hopefully sample code will do. Bitfield operators differ from language to language, but here they are in c++:
Code:
// Class number from classes.h
#define ROGUE 9
// Create a number with only the rogue bit turned on
uint32 rogue_mask = (1 << (ROGUE-1));
// Apply mask to 'classes' field to see if rogue bit is turned on
uint32 val = (rogue_mask & MyClassesValue);
if (val == rogue_mask)
// .. item can be used by rogues