Objective-C - iOS Programming: The Big Nerd Ranch Guide - page 46

Destroying objects
To destroy an object, you set the variable that points at it to nil .
partyInstance = nil;
This line of code destroys the object pointed to by the partyInstance variable and
sets the value of the partyInstance variable to nil . (It's actually a bit more complic-
ated than that, and you'll learn about the details of memory management in the next
chapter.)
The value nil is the zero pointer. (C programmers know it as NULL . Java programmers
know it as null .) A pointer that has a value of nil is typically used to represent the ab-
sence of an object. For example, a party could have a venue. While the organizer of the
party is still determining where to host the party, venue would point to nil . This allows
us to do things like so:
if (venue == nil) {
[organizer remindToFindVenueForParty];
}
Objective-C programmers typically use the shorthand form of determining if a pointer is
nil :
if (!venue) {
[organizer remindToFindVenueForParty];
}
Since the ! operator means “not,” this reads as “if there is not a venue” and will evaluate
to true if venue is nil .
If you send a message to a variable that is nil , nothing happens. In other languages,
sending a message to the zero pointer is illegal, so you see this sort of thing a lot:
// Is venue non-nil?
if (venue) {
[venue sendConfirmation];
}
In Objective-C, this check is unnecessary because a message sent to nil is ignored.
Therefore, you can simply send a message without a nil -check:
[venue sendConfirmation];