Objective C Runtime's class_addIvar adding a new instance variable to a class

HCClassAddIvar.m

/**
 * class_addIvar
 * Adds a new instance variable to a class.
 * 
 * BOOL class_addIvar(Class cls, const char *name, size_t size, uint8_t alignment, const char *types)
 * 
 * Return Value
 * YES if the instance variable was added successfully, otherwise NO.
**/

- (BOOL)addInstanceVariable
{
  return class_addIvar(subClass, "varName", sizeof(id), rint(log2(sizeof(id))), @encode(id));;
}