Programming in Objective-C, Sixth Edition

Operations on Pointers

As you have seen in this chapter, you can add or subtract integer values from pointers. Furthermore, you can compare two pointers to see whether they are equal or whether one pointer is less than or greater than another pointer. The only other operation permitted on pointers is the subtraction of two pointers of the same type. The result of subtracting two pointers in Objective-C is the number of elements contained between the two pointers. Thus, if a points to an array of elements of any type and b points to another element somewhere further along in the same array, the expression b - a represents the number of elements between these two pointers. For example, if p points to some element in an array x, the statement