Objective-C Fundamentals

62 CHAPTER 3 An introduction to objects
Indicating the absence of a value
Sometimes you want to detect if a pointer variable is currently pointing at anything of
relevance. For this purpose, you’ll most likely initialize the pointer to one of the special
constants NULL or nil:
int *x = NULL;
NSString *y = nil;
Both constants are equivalent to the value 0 and are used to indicate that the pointer
isn’t currently pointing to anything. The Objective-C convention is to use nil when
referring to an object and relegating NULL for use with older C-style data types.
Initializing the pointer to one of these special values enables an if (y != nil)
check to determine if the pointer is currently pointing to anything. Because nil is
equivalent to the value 0, you may also see this condition written as if (!y).
Also be careful not to dereference a NULL pointer. Trying to read or write from a
pointer that points to nothing causes an access violation error, which immediately
exits your application.
3.4 Communicating with objects
In most C-inspired languages such as C++, Java, and C#, developers call a method
implemented by an object. Developers using Objective-C, on the other hand, don’t
“call” a method directly; instead, they “send” a message to an object. The object
“receives” the message and decides if it wants to process it, usually by invoking a
method with the same name. This is a fundamentally different approach and is one of
the many features that make Objective-C a more dynamic language because it enables
the object to have finer-grained control over how method dispatch occurs.
3.4.1 Sending a message to an object
Figure 3.3 outlines the basic Objective-C syntax for sending a message to an object. In
source code, a message send is represented by a set of square brackets; immediately
after the opening bracket is the target, followed by the name of the message itself. The
target can be any expression that evaluates to an object that should receive the message.
You can consider a basic message send such as the one
shown in figure 3.3 as analogous to addressing an envelope
with a person’s name and address. It sets up a container
that can then be delivered to its intended recipient.
Sometimes, for an object to make sense of a message,
additional information must be sent along with it, similar
to placing a letter or invoice in an envelope. Additional
information provided in a message send is represented by
one or more values called arguments. When sending a
message, arguments are provided by placing a colon character
after the method name, as shown in figure 3.4.
Figure 3.3 The basic syntax
for sending a message to an
object in Objective-C. In a pair
of square brackets, the target
(or object that receives the
message) is specified,
followed by the name of the
message itself.