Managing Memory with ARC - iOS Programming: The Big Nerd Ranch Guide - page 90

The Heap
All Objective-C objects are stored in a part of memory called the heap . When we send an
alloc message to a class, a chunk of memory is allocated from the heap. This chunk in-
cludes space for the object's instance variables.
For example, consider an instance of NSDate , which represents a specific point in time.
An NSDate has two instance variables: a double that stores the number of seconds since
a fixed reference point in time and the isa pointer, which every object inherits from
NSObject . A double is eight bytes, and a pointer is 4 bytes, so each time alloc is sent
to the NSDate class, 12 bytes is allocated from the heap for a new NSDate object.
Consider another example: BNRItem . A BNRItem has five instance variables: four point-
ers ( isa , itemName , serialNumber , and dateCreated ) and an int
( valueInDollars ). The amount of memory needed for an int is four bytes, so the
total size of a BNRItem is 20 bytes ( Figure 3.1 ).
Figure 3.1 Byte count of BNRItem and NSDate instances