Managing Memory with ARC - iOS Programming: The Big Nerd Ranch Guide - page 91

Notice in Figure 3.1 that the NSDate object does not live inside the BNRItem . Objects
never live inside one another; they exist separately on the heap. Instead, objects keep ref-
erences to other objects as needed. These references are the pointer instance variables of
an object. Thus, when a BNRItem 's dateCreated instance variable is set, the address
of the NSDate instance is stored in the BNRItem , not the NSDate itself. So, if the
NSDate was 10, 20, or even 1000 bytes, it wouldn't affect the size of the BNRItem .)