Working with Objects
The majority of work in an Objective-C application happens as a result of messages being sent back and forth across an ecosystem of objects. Some of these objects are instances of classes provided by Cocoa or Cocoa Touch, some are instances of your own classes.
The previous chapter described the syntax to define the interface and implementation for a class, including the syntax to implement methods containing the code to be executed in response to a message. This chapter explains how to send such a message to an object, and includes coverage of some of Objective-C’s dynamic features, including dynamic typing and the ability to determine which method should be invoked at runtime.
Before an object can be used, it must be created properly using a combination of memory allocation for its properties and any necessary initialization of its internal values. This chapter describes how to nest the method calls to allocate and initialize an object in order to ensure that it is configured correctly.
Objects Send and Receive Messages
Although there are several different ways to send messages between objects in Objective-C, by far the most common is the basic syntax that uses square brackets, like this:
[someObject doSomething]; |
The reference on the left, someObject
in this case, is the receiver of the message. The message on the right, doSomething
, is the name of the method to call on that receiver. In other words, when the above line of code is executed, someObject
will be sent the doSomething
message.
The previous chapter described how to create the interface for a class, like this:
@interface XYZPerson : NSObject |
- (void)sayHello; |
@end |
and how to create the implementation of that class, like this:
@implementation XYZPerson |
- (void)sayHello { |
NSLog(@"Hello, world!"); |
} |
@end |
Assuming you’ve got hold of an XYZPerson
object, you could send it the sayHello
message like this:
[somePerson sayHello]; |
Sending an Objective-C message is conceptually very much like calling a C function. Figure 2-1 shows the effective program flow for the sayHello
message.
In order to specify the receiver of a message, it’s important to understand how pointers are used to refer to objects in Objective-C.
Use Pointers to Keep Track of Objects
C and Objective-C use variables to keep track of values, just like most other programming languages.
There are a number of basic scalar variable types defined in standard C, including integers, floating-point numbers and characters, which are declared and assigned values like this:
int someInteger = 42; |
float someFloatingPointNumber = 3.14f; |
Local variables, which are variables declared within a method or function, like this:
- (void)myMethod { |
int someInteger = 42; |
} |
are limited in scope to the method in which they are defined.
In this example, someInteger
is declared as a local variable inside myMethod
; once execution reaches the closing brace of the method, someInteger
will no longer be accessible. When a local scalar variable (like an int
or a float
) goes away, the value disappears too.
Objective-C objects, by contrast, are allocated slightly differently. Objects normally have a longer life than the simple scope of a method call. In particular, an object often needs to stay alive longer than the original variable that was created to keep track of it, so an object’s memory is allocated and deallocated dynamically.
This requires you to use C pointers (which hold memory addresses) to keep track of their location in memory, like this:
- (void)myMethod { |
NSString *myString = // get a string from somewhere... |
[...] |
} |
Although the scope of the pointer variable myString
(the asterisk indicates it’s a pointer) is limited to the scope of myMethod
, the actual string object that it points to in memory may have a longer life outside that scope. It might already exist, or you might need to pass the object around in additional method calls, for example.
You Can Pass Objects for Method Parameters
If you need to pass along an object when sending a message, you supply an object pointer for one of the method parameters. The previous chapter described the syntax to declare a method with a single parameter:
- (void)someMethodWithValue:(SomeType)value; |
The syntax to declare a method that takes a string object, therefore, looks like this:
- (void)saySomething:(NSString *)greeting; |
You might implement the saySomething:
method like this:
- (void)saySomething:(NSString *)greeting { |
NSLog(@"%@", greeting); |
} |
The greeting
pointer behaves like a local variable and is limited in scope just to the saySomething:
method, even though the actual string object that it points to existed prior to the method being called, and will continue to exist after the method completes.
Methods Can Return Values
As well as passing values through method parameters, it’s possible for a method to return a value. Each method shown in this chapter so far has a return type of void
. The C void
keyword means a method doesn’t return anything.
Specifying a return type of int
means that the method returns a scalar integer value:
- (int)magicNumber; |
The implementation of the method uses a C return
statement to indicate the value that should be passed back after the method has finished executing, like this:
- (int)magicNumber { |
return 42; |
} |
It’s perfectly acceptable to ignore the fact that a method returns a value. In this case the magicNumber
method doesn’t do anything useful other than return a value, but there’s nothing wrong with calling the method like this:
[someObject magicNumber]; |
If you do need to keep track of the returned value, you can declare a variable and assign it to the result of the method call, like this:
int interestingNumber = [someObject magicNumber]; |
You can return objects from methods in just the same way. The NSString
class, for example, offers an uppercaseString
method:
- (NSString *)uppercaseString; |
It’s used in the same way as a method returning a scalar value, although you need to use a pointer to keep track of the result:
NSString *testString = @"Hello, world!"; |
NSString *revisedString = [testString uppercaseString]; |
When this method call returns, revisedString
will point to an NSString
object representing the characters HELLO WORLD!
.
Remember that when implementing a method to return an object, like this:
- (NSString *)magicString { |
NSString *stringToReturn = // create an interesting string... |
return stringToReturn; |
} |
the string object continues to exist when it is passed as a return value even though the stringToReturn
pointer goes out of scope.
There are some memory management considerations in this situation: a returned object (created on the heap) needs to exist long enough for it to be used by the original caller of the method, but not in perpetuity because that would create a memory leak. For the most part, the Automatic Reference Counting (ARC) feature of the Objective-C compiler takes care of these considerations for you.
Objects Can Send Messages to Themselves
Whenever you’re writing a method implementation, you have access to an important hidden value, self
. Conceptually, self
is a way to refer to “the object that’s received this message.” It’s a pointer, just like the greeting
value above, and can be used to call a method on the current receiving object.
You might decide to refactor the XYZPerson
implementation by modifying the sayHello
method to use the saySomething:
method shown above, thereby moving the NSLog()
call to a separate method. This would mean you could add further methods, like sayGoodbye
, that would each call through to the saySomething:
method to handle the actual greeting process. If you later wanted to display each greeting in a text field in the user interface, you’d only need to modify the saySomething:
method rather than having to go through and adjust each greeting method individually.
The new implementation using self
to call a message on the current object would look like this:
@implementation XYZPerson |
- (void)sayHello { |
[self saySomething:@"Hello, world!"]; |
} |
- (void)saySomething:(NSString *)greeting { |
NSLog(@"%@", greeting); |
} |
@end |
If you sent an XYZPerson
object the sayHello
message for this updated implementation, the effective program flow would be as shown in Figure 2-2.
Objects Can Call Methods Implemented by Their Superclasses
There’s another important keyword available to you in Objective-C, called super
. Sending a message to super
is a way to call through to a method implementation defined by a superclass further up the inheritance chain. The most common use of super
is when overriding a method.
Let’s say you want to create a new type of person class, a “shouting person” class, where every greeting is displayed using capital letters. You could duplicate the entire XYZPerson
class and modify each string in each method to be uppercase, but the simplest way would be to create a new class that inherits from XYZPerson
, and just override the saySomething:
method so that it displays the greeting in uppercase, like this:
@interface XYZShoutingPerson : XYZPerson |
@end |
@implementation XYZShoutingPerson |
- (void)saySomething:(NSString *)greeting { |
NSString *uppercaseGreeting = [greeting uppercaseString]; |
NSLog(@"%@", uppercaseGreeting); |
} |
@end |
This example declares an extra string pointer, uppercaseGreeting
and assigns it the value returned from sending the original greeting
object the uppercaseString
message. As you saw earlier, this will be a new string object built by converting each character in the original string to uppercase.
Because sayHello
is implemented by XYZPerson
, and XYZShoutingPerson
is set to inherit from XYZPerson
, you can call sayHello
on an XYZShoutingPerson
object as well. When you call sayHello
on an XYZShoutingPerson
, the call to [self saySomething:...]
will use the overridden implementation and display the greeting as uppercase, resulting in the effective program flow shown in Figure 2-3.
The new implementation isn’t ideal, however, because if you did decide later to modify the XYZPerson
implementation of saySomething:
to display the greeting in a user interface element rather than through NSLog()
, you’d need to modify the XYZShoutingPerson
implementation as well.
A better idea would be to change the XYZShoutingPerson
version of saySomething:
to call through to the superclass (XYZPerson
) implementation to handle the actual greeting:
@implementation XYZShoutingPerson |
- (void)saySomething:(NSString *)greeting { |
NSString *uppercaseGreeting = [greeting uppercaseString]; |
[super saySomething:uppercaseGreeting]; |
} |
@end |
The effective program flow that now results from sending an XYZShoutingPerson
object the sayHello
message is shown in Figure 2-4.
Objects Are Created Dynamically
As described earlier in this chapter, memory is allocated dynamically for an Objective-C object. The first step in creating an object is to make sure enough memory is allocated not only for the properties defined by an object’s class, but also the properties defined on each of the superclasses in its inheritance chain.
The NSObject
root class provides a class method, alloc
, which handles this process for you:
+ (id)alloc; |
Notice that the return type of this method is id
. This is a special keyword used in Objective-C to mean “some kind of object.” It is a pointer to an object, like (NSObject *)
, but is special in that it doesn’t use an asterisk. It’s described in more detail later in this chapter, in Objective-C Is a Dynamic Language.
The alloc
method has one other important task, which is to clear out the memory allocated for the object’s properties by setting them to zero. This avoids the usual problem of memory containing garbage from whatever was stored before, but is not enough to initialize an object completely.
You need to combine a call to alloc
with a call to init
, another NSObject
method:
- (id)init; |
The init
method is used by a class to make sure its properties have suitable initial values at creation, and is covered in more detail in the next chapter.
Note that init
also returns an id
.
If one method returns an object pointer, it’s possible to nest the call to that method as the receiver in a call to another method, thereby combining multiple message calls in one statement. The correct way to allocate and initialize an object is to nest the alloc
call inside the call to init
, like this:
NSObject *newObject = [[NSObject alloc] init]; |
This example sets the newObject
variable to point to a newly created NSObject
instance.
The innermost call is carried out first, so the NSObject
class is sent the alloc
method, which returns a newly allocated NSObject
instance. This returned object is then used as the receiver of the init
message, which itself returns the object back to be assigned to the newObject
pointer, as shown in Figure 2-5.
Initializer Methods Can Take Arguments
Some objects need to be initialized with required values. An NSNumber
object, for example, must be created with the numeric value it needs to represent.
The NSNumber
class defines several initializers, including:
- (id)initWithBool:(BOOL)value; |
- (id)initWithFloat:(float)value; |
- (id)initWithInt:(int)value; |
- (id)initWithLong:(long)value; |
Initialization methods with arguments are called in just the same way as plain init
methods—an NSNumber
object is allocated and initialized like this:
NSNumber *magicNumber = [[NSNumber alloc] initWithInt:42]; |
Class Factory Methods Are an Alternative to Allocation and Initialization
As mentioned in the previous chapter, a class can also define factory methods. Factory methods offer an alternative to the traditional alloc] init]
process, without the need to nest two methods.
The NSNumber
class defines several class factory methods to match its initializers, including:
+ (NSNumber *)numberWithBool:(BOOL)value; |
+ (NSNumber *)numberWithFloat:(float)value; |
+ (NSNumber *)numberWithInt:(int)value; |
+ (NSNumber *)numberWithLong:(long)value; |
A factory method is used like this:
NSNumber *magicNumber = [NSNumber numberWithInt:42]; |
This is effectively the same as the previous example using alloc] initWithInt:]
. Class factory methods usually just call straight through to alloc
and the relevant init
method, and are provided for convenience.
Use new to Create an Object If No Arguments Are Needed for Initialization
It’s also possible to create an instance of a class using the new
class method. This method is provided by NSObject
and doesn’t need to be overridden in your own subclasses.
It’s effectively the same as calling alloc
and init
with no arguments:
XYZObject *object = [XYZObject new]; |
// is effectively the same as: |
XYZObject *object = [[XYZObject alloc] init]; |
Literals Offer a Concise Object-Creation Syntax
Some classes allow you to use a more concise, literal syntax to create instances.
You can create an NSString
instance, for example, using a special literal notation, like this:
NSString *someString = @"Hello, World!"; |
This is effectively the same as allocating and initializing an NSString
or using one of its class factory methods:
NSString *someString = [NSString stringWithCString:"Hello, World!" |
encoding:NSUTF8StringEncoding]; |
The NSNumber
class also allows a variety of literals:
NSNumber *myBOOL = @YES; |
NSNumber *myFloat = @3.14f; |
NSNumber *myInt = @42; |
NSNumber *myLong = @42L; |
Again, each of these examples is effectively the same as using the relevant initializer or a class factory method.
You can also create an NSNumber
using a boxed expression, like this:
NSNumber *myInt = @(84 / 2); |
In this case, the expression is evaluated, and an NSNumber
instance created with the result.
Objective-C also supports literals to create immutable NSArray
and NSDictionary
objects; these are discussed further in Values and Collections.
Objective-C Is a Dynamic Language
As mentioned earlier, you need to use a pointer to keep track of an object in memory. Because of Objective-C’s dynamic nature, it doesn’t matter what specific class type you use for that pointer—the correct method will always be called on the relevant object when you send it a message.
The id
type defines a generic object pointer. It’s possible to use id
when declaring a variable, but you lose compile-time information about the object.
Consider the following code:
id someObject = @"Hello, World!"; |
[someObject removeAllObjects]; |
In this case, someObject
will point to an NSString
instance, but the compiler knows nothing about that instance beyond the fact that it’s some kind of object. The removeAllObjects
message is defined by some Cocoa or Cocoa Touch objects (such as NSMutableArray
) so the compiler doesn’t complain, even though this code would generate an exception at runtime because an NSString
object can’t respond to removeAllObjects
.
Rewriting the code to use a static type:
NSString *someObject = @"Hello, World!"; |
[someObject removeAllObjects]; |
means that the compiler will now generate an error because removeAllObjects
is not declared in any public NSString
interface that it knows about.
Because the class of an object is determined at runtime, it makes no difference what type you assign a variable when creating or working with an instance. To use the XYZPerson
and XYZShoutingPerson
classes described earlier in this chapter, you might use the following code:
XYZPerson *firstPerson = [[XYZPerson alloc] init]; |
XYZPerson *secondPerson = [[XYZShoutingPerson alloc] init]; |
[firstPerson sayHello]; |
[secondPerson sayHello]; |
Although both firstPerson
and secondPerson
are statically typed as XYZPerson
objects, secondPerson
will point, at runtime, to an XYZShoutingPerson
object. When the sayHello
method is called on each object, the correct implementations will be used; for secondPerson
, this means the XYZShoutingPerson
version.
Determining Equality of Objects
If you need to determine whether one object is the same as another object, it’s important to remember that you’re working with pointers.
The standard C equality operator ==
is used to test equality between the values of two variables, like this:
if (someInteger == 42) { |
// someInteger has the value 42 |
} |
When dealing with objects, the == operator is used to test whether two separate pointers are pointing to the same object:
if (firstPerson == secondPerson) { |
// firstPerson is the same object as secondPerson |
} |
If you need to test whether two objects represent the same data, you need to call a method like isEqual:
, available from NSObject
:
if ([firstPerson isEqual:secondPerson]) { |
// firstPerson is identical to secondPerson |
} |
If you need to compare whether one object represents a greater or lesser value than another object, you can’t use the standard C comparison operators >
and <
. Instead, the basic Foundation types, like NSNumber
, NSString
and NSDate
, provide a compare:
method:
if ([someDate compare:anotherDate] == NSOrderedAscending) { |
// someDate is earlier than anotherDate |
} |
Working with nil
It’s always a good idea to initialize scalar variables at the time you declare them, otherwise their initial values will contain garbage from the previous stack contents:
BOOL success = NO; |
int magicNumber = 42; |
This isn’t necessary for object pointers, because the compiler will automatically set the variable to nil
if you don’t specify any other initial value:
XYZPerson *somePerson; |
// somePerson is automatically set to nil |
A nil
value is the safest way to initialize an object pointer if you don’t have another value to use, because it’s perfectly acceptable in Objective-C to send a message to nil
. If you do send a message to nil
, obviously nothing happens.
If you need to check to make sure an object is not nil
(that a variable points to an object in memory), you can either use the standard C inequality operator:
if (somePerson != nil) { |
// somePerson points to an object |
} |
or simply supply the variable:
if (somePerson) { |
// somePerson points to an object |
} |
If the somePerson
variable is nil
, its logical value is 0
(false). If it has an address, it’s not zero, so evaluates as true.
Similarly, if you need to check for a nil
variable, you can either use the equality operator:
if (somePerson == nil) { |
// somePerson does not point to an object |
} |
or just use the C logical negation operator:
if (!somePerson) { |
// somePerson does not point to an object |
} |
Exercises
Open the
main.m
file in your project from the exercises at the end of the last chapter and find themain()
function. As with any executable written in C, this function represents the starting point for your application.Create a new
XYZPerson
instance usingalloc
andinit
, and then call thesayHello
method.Implement the
saySomething:
method shown earlier in this chapter, and rewrite thesayHello
method to use it. Add a variety of other greetings and call each of them on the instance you created above.Create new class files for the
XYZShoutingPerson
class, set to inherit fromXYZPerson
.Override the
saySomething:
method to display the uppercase greeting, and test the behavior on anXYZShoutingPerson
instance.Implement the
XYZPerson
classperson
factory method you declared in the previous chapter, to return a correctly allocated and initialized instance of theXYZPerson
class, then use the method inmain()
instead of your nestedalloc
andinit
.Create a new local
XYZPerson
pointer, but don’t include any value assignment.Use a branch (
if
statement) to check whether the variable is automatically assigned asnil
.
Copyright © 2014 Apple Inc. All Rights Reserved. Terms of Use | Privacy Policy | Updated: 2014-09-17