Managing Memory with ARC - iOS Programming: The Big Nerd Ranch Guide - page 98

• A variable that points to the object is set to nil .
• A variable that points to the object is itself destroyed.
Let's take a look at each of these situations.
Why might a pointer change the object it points to? Imagine a BNRItem . The
NSString that its itemName instance variable points to reads “Rusty Spork.” If we
polished the rust off of that spork, it would become a shiny spork, and we'd want to
change the itemName to point at a different NSString ( Figure 3.5 ) .
Figure 3.5 Changing a pointer
When the value of itemName changes from the address of the “Rusty Spork” string to
the address of the “Shiny Spork” string, the “Rusty Spork” string loses an owner.
Why would you set a pointer to nil ? Remember that setting a pointer to nil represents
the absence of an object. For example, say you have a BNRItem that represents a televi-
sion. Then, someone scratches off the television's serial number. You would then set its
serialNumber instance variable to nil . The NSString that serialNumber used
to point to loses an owner.
When a pointer variable itself is destroyed, the object that the variable was pointing at
loses an owner. At what point a pointer variable will get destroyed depends on whether it
is a local variable or an instance variable.