Programming in Objective-C

18. Copying Objects

This chapter discusses some of the subtleties involved in copying objects. We will introduce the concept of shallow versus deep copying and how to make copies under the Foundation framework.

Chapter 8, “Inheritance,” discussed what happens when you assign one object to another with a simple assignment statement, such as

origin = pt;

In that example, origin and pt are both Point objects, that we defined like this:

@interface Point: Object{    int x;    int y;};  ...@end

You will recall that the effect of the assignment is to simply copy the address of the object pt into origin. At the end of the assignment operation, both variables point to the same location in memory. Making changes to the instance variables with a message ...