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Defining a Class

Much of object-oriented programming consists of writing the code for new objects—defining new classes. In Objective-C, classes are defined in two parts:

These are typically split between two files, sometimes however a class definition may span several files through the use of a feature called a “category.” Categories can compartmentalize a class definition or extend an existing one. Categories are described in “Categories and Extensions.”

In this section:

Source Files
Class Interface
Class Implementation
Messages to self and super


Source Files

Although the compiler doesn’t require it, the interface and implementation are usually separated into two different files. The interface file must be made available to anyone who uses the class.

A single file can declare or implement more than one class. Nevertheless, it’s customary to have a separate interface file for each class, if not also a separate implementation file. Keeping class interfaces separate better reflects their status as independent entities.

Interface and implementation files typically are named after the class. The name of the implementation file has the .m extension, indicating that it contains Objective-C source code. The interface file can be assigned any other extension. Because it’s included in other source files, the name of the interface file usually has the .h extension typical of header files. For example, the Rectangle class would be declared in Rectangle.h and defined in Rectangle.m.

Separating an object’s interface from its implementation fits well with the design of object-oriented programs. An object is a self-contained entity that can be viewed from the outside almost as a “black box.” Once you’ve determined how an object interacts with other elements in your program—that is, once you’ve declared its interface—you can freely alter its implementation without affecting any other part of the application.

Class Interface

The declaration of a class interface begins with the compiler directive @interface and ends with the directive @end. (All Objective-C directives to the compiler begin with “@”.)

@interface ClassName : ItsSuperclass
{
    instance variable declarations
}
method declarations
@end

The first line of the declaration presents the new class name and links it to its superclass. The superclass defines the position of the new class in the inheritance hierarchy, as discussed under “Inheritance.” If the colon and superclass name are omitted, the new class is declared as a root class, a rival to the NSObject class.

Following the first part of the class declaration, braces enclose declarations of instance variables, the data structures that are part of each instance of the class. Here’s a partial list of instance variables that might be declared in the Rectangle class:

float width;
float height;
BOOL filled;
NSColor *fillColor;

Methods for the class are declared next, after the braces enclosing instance variables and before the end of the class declaration. The names of methods that can be used by class objects, class methods, are preceded by a plus sign:

+ alloc;

The methods that instances of a class can use, instance methods, are marked with a minus sign:

- (void)display;

Although it’s not a common practice, you can define a class method and an instance method with the same name. A method can also have the same name as an instance variable. This is more common, especially if the method returns the value in the variable. For example, Circle has a radius method that could match a radius instance variable.

Method return types are declared using the standard C syntax for casting one type to another:

- (float)radius;

Argument types are declared in the same way:

- (void)setRadius:(float)aRadius;

If a return or argument type isn’t explicitly declared, it’s assumed to be the default type for methods and messages—an id. The alloc method illustrated earlier returns id.

When there’s more than one argument, the arguments are declared within the method name after the colons. Arguments break the name apart in the declaration, just as in a message. For example:

- (void)setWidth:(float)width height:(float)height;

Methods that take a variable number of arguments declare them using a comma and ellipsis points, just as a function would:

- makeGroup:group, ...;

Importing the Interface

The interface file must be included in any source module that depends on the class interface—that includes any module that creates an instance of the class, sends a message to invoke a method declared for the class, or mentions an instance variable declared in the class. The interface is usually included with the #import directive:

#import "Rectangle.h"

This directive is identical to #include, except that it makes sure that the same file is never included more than once. It’s therefore preferred and is used in place of #include in code examples throughout Objective-C–based documentation.

To reflect the fact that a class definition builds on the definitions of inherited classes, an interface file begins by importing the interface for its superclass:

#import "ItsSuperclass.h"
 
@interface ClassName : ItsSuperclass
{
    instance variable declarations
}
method declarations
@end

This convention means that every interface file includes, indirectly, the interface files for all inherited classes. When a source module imports a class interface, it gets interfaces for the entire inheritance hierarchy that the class is built upon.

Note that if there is a precomp—a precompiled header—that supports the superclass, you may prefer to import the precomp instead.

Referring to Other Classes

An interface file declares a class and, by importing its superclass, implicitly contains declarations for all inherited classes, from NSObject on down through its superclass. If the interface mentions classes not in this hierarchy, it must import them explicitly or declare them with the @class directive:

@class Rectangle, Circle;

This directive simply informs the compiler that “Rectangle” and “Circle” are class names. It doesn’t import their interface files.

An interface file mentions class names when it statically types instance variables, return values, and arguments. For example, this declaration

- (void)setPrimaryColor:(NSColor *)aColor;

mentions the NSColor class.

Since declarations like this simply use the class name as a type and don’t depend on any details of the class interface (its methods and instance variables), the @class directive gives the compiler sufficient forewarning of what to expect. However, where the interface to a class is actually used (instances created, messages sent), the class interface must be imported. Typically, an interface file uses @class to declare classes, and the corresponding implementation file imports their interfaces (since it will need to create instances of those classes or send them messages).

The @class directive minimizes the amount of code seen by the compiler and linker, and is therefore the simplest way to give a forward declaration of a class name. Being simple, it avoids potential problems that may come with importing files that import still other files. For example, if one class declares a statically typed instance variable of another class, and their two interface files import each other, neither class may compile correctly.

The Role of the Interface

The purpose of the interface file is to declare the new class to other source modules (and to other programmers). It contains all the information they need to work with the class (programmers might also appreciate a little documentation).

Class Implementation

The definition of a class is structured very much like its declaration. It begins with the @implementation directive and ends with the @end directive:

@implementation ClassName : ItsSuperclass
{
    instance variable declarations
}
method definitions
@end

However, every implementation file must import its own interface. For example, Rectangle.m imports Rectangle.h. Because the implementation doesn’t need to repeat any of the declarations it imports, it can safely omit:

This simplifies the implementation and makes it mainly devoted to method definitions:

#import "ClassName.h"
 
@implementation ClassName
method definitions
@end

Methods for a class are defined, like C functions, within a pair of braces. Before the braces, they’re declared in the same manner as in the interface file, but without the semicolon. For example:

+ (id)alloc
{
    ...
}
 
- (BOOL)isfilled
{
    ...
}
 
- (void)setFilled:(BOOL)flag
{
    ...
}

Methods that take a variable number of arguments handle them just as a function would:

#import <stdarg.h>
 
 ...
 
- getGroup:group, ...
{
    va_list ap;
    va_start(ap, group);
    ...
}

Referring to Instance Variables

By default, the definition of an instance method has all the instance variables of the object within its scope. It can refer to them simply by name. Although the compiler creates the equivalent of C structures to store instance variables, the exact nature of the structure is hidden. You don’t need either of the structure operators (. or ->) to refer to an object’s data. For example, the following method definition refers to the receiver’s filled instance variable:

- (void)setFilled:(BOOL)flag
{
    filled = flag;
    ...
}

Neither the receiving object nor its filled instance variable is declared as an argument to this method, yet the instance variable falls within its scope. This simplification of method syntax is a significant shorthand in the writing of Objective-C code.

When the instance variable belongs to an object that’s not the receiver, the object’s type must be made explicit to the compiler through static typing. In referring to the instance variable of a statically typed object, the structure pointer operator (->) is used.

Suppose, for example, that the Sibling class declares a statically typed object, twin, as an instance variable:

@interface Sibling : NSObject
{
    Sibling *twin;
    int gender;
    struct features *appearance;
}

As long as the instance variables of the statically typed object are within the scope of the class (as they are here because twin is typed to the same class), a Sibling method can set them directly:

- makeIdenticalTwin
{
    if ( !twin ) {
        twin = [[Sibling alloc] init];
        twin->gender = gender;
        twin->appearance = appearance;
    }
    return twin;
}

The Scope of Instance Variables

Although they’re declared in the class interface, instance variables are more a matter of the way a class is implemented than of the way it’s used. An object’s interface lies in its methods, not in its internal data structures.

Often there’s a one-to-one correspondence between a method and an instance variable, as in the following example:

- (BOOL)isFilled
{
    return filled;
}

But this need not be the case. Some methods might return information not stored in instance variables, and some instance variables might store information that an object is unwilling to reveal.

As a class is revised from time to time, the choice of instance variables may change, even though the methods it declares remain the same. As long as messages are the vehicle for interacting with instances of the class, these changes won’t really affect its interface.

To enforce the ability of an object to hide its data, the compiler limits the scope of instance variables—that is, limits their visibility within the program. But to provide flexibility, it also lets you explicitly set the scope at three different levels. Each level is marked by a compiler directive:

Directive

Meaning

@private

The instance variable is accessible only within the class that declares it.

@protected

The instance variable is accessible within the class that declares it and within classes that inherit it.

@public

The instance variable is accessible everywhere.

@package

On 64-bit, an @package instance variable acts like @public inside the image that implements the class, but @private outside.

This is analogous to private_extern for variables and functions. Any code outside the class implementation’s image that tries to use the instance variable will get a link error. This is most useful for instance variables in framework classes, where @private may be too restrictive but @protected or @public too permissive.

This is illustrated in Figure 2-1.


Figure 2-1  The scope of instance variables


A directive applies to all the instance variables listed after it, up to the next directive or the end of the list. In the following example, the age and evaluation instance variables are private, name, job, and wage are protected, and boss is public.

@interface Worker : NSObject
{
    char *name;
@private
    int age;
    char *evaluation;
@protected
    id job;
    float wage;
@public
    id boss;
}

By default, all unmarked instance variables (like name above) are @protected.

All instance variables that a class declares, no matter how they’re marked, are within the scope of the class definition. For example, a class that declares a job instance variable, such as the Worker class shown above, can refer to it in a method definition:

- promoteTo:newPosition
{
    id old = job;
    job = newPosition;
    return old;
}

Obviously, if a class couldn’t access its own instance variables, the instance variables would be of no use whatsoever.

Normally, a class also has access to the instance variables it inherits. The ability to refer to an instance variable is usually inherited along with the variable. It makes sense for classes to have their entire data structures within their scope, especially if you think of a class definition as merely an elaboration of the classes it inherits from. The promoteTo: method illustrated earlier could just as well have been defined in any class that inherits the job instance variable from the Worker class.

However, there are reasons why you might want to restrict inheriting classes from directly accessing an instance variable:

To limit an instance variable’s scope to just the class that declares it, you must mark it @private. Instance variables marked @private are only available to subclasses by calling public accessor methods, if they exist.

At the other extreme, marking a variable @public makes it generally available, even outside of class definitions that inherit or declare the variable. Normally, to get information stored in an instance variable, other objects must send a message requesting it. However, a public instance variable can be accessed anywhere as if it were a field in a C structure. For example:

Worker *ceo = [[Worker alloc] init];
ceo->boss = nil;

Note that the object must be statically typed.

Marking instance variables @public defeats the ability of an object to hide its data. It runs counter to a fundamental principle of object-oriented programming—the encapsulation of data within objects where it’s protected from view and inadvertent error. Public instance variables should therefore be avoided except in extraordinary cases.

Messages to self and super

Objective-C provides two terms that can be used within a method definition to refer to the object that performs the method—self and super.

Suppose, for example, that you define a reposition method that needs to change the coordinates of whatever object it acts on. It can invoke the setOrigin:: method to make the change. All it needs to do is send a setOrigin:: message to the same object that the reposition message itself was sent to. When you’re writing the reposition code, you can refer to that object as either self or super. The reposition method could read either:

- reposition
{
    ...
    [self setOrigin:someX :someY];
    ...
}

or:

- reposition
{
    ...
    [super setOrigin:someX :someY];
    ...
}

Here, self and super both refer to the object receiving a reposition message, whatever object that may happen to be. The two terms are quite different, however. self is one of the hidden arguments that the messaging routine passes to every method; it’s a local variable that can be used freely within a method implementation, just as the names of instance variables can be. super is a term that substitutes for self only as the receiver in a message expression. As receivers, the two terms differ principally in how they affect the messaging process:

Wherever super receives a message, the compiler substitutes another messaging routine for the objc_msgSend function. The substitute routine looks directly to the superclass of the defining class—that is, to the superclass of the class sending the message to super—rather than to the class of the object receiving the message.

An Example

The difference between self and super becomes clear in a hierarchy of three classes. Suppose, for example, that we create an object belonging to a class called Low. Low’s superclass is Mid; Mid’s superclass is High. All three classes define a method called negotiate, which they use for a variety of purposes. In addition, Mid defines an ambitious method called makeLastingPeace, which also has need of the negotiate method. This is illustrated in Figure 2-2:


Figure 2-2  High, Mid, Low


We now send a message to our Low object to perform the makeLastingPeace method, and makeLastingPeace, in turn, sends a negotiate message to the same Low object. If source code calls this object self,

- makeLastingPeace
{
    [self negotiate];
    ...
}

the messaging routine finds the version of negotiate defined in Low, self’s class. However, if Mid’s source code calls this object super,

- makeLastingPeace
{
    [super negotiate];
    ...
}

the messaging routine will find the version of negotiate defined in High. It ignores the receiving object’s class (Low) and skips to the superclass of Mid, since Mid is where makeLastingPeace is defined. Neither message finds Mid’s version of negotiate.

As this example illustrates, super provides a way to bypass a method that overrides another method. Here it enabled makeLastingPeace to avoid the Mid version of negotiate that redefined the original High version.

Not being able to reach Mid’s version of negotiate may seem like a flaw, but, under the circumstances, it’s right to avoid it:

Mid’s version of negotiate could still be used, but it would take a direct message to a Mid instance to do it.

Using super

Messages to super allow method implementations to be distributed over more than one class. You can override an existing method to modify or add to it, and still incorporate the original method in the modification:

- negotiate
{
    ...
    return [super negotiate];
}

For some tasks, each class in the inheritance hierarchy can implement a method that does part of the job and passes the message on to super for the rest. The init method, which initializes a newly allocated instance, is designed to work like this. Each init method has responsibility for initializing the instance variables defined in its class. But before doing so, it sends an init message to super to have the classes it inherits from initialize their instance variables. Each version of init follows this procedure, so classes initialize their instance variables in the order of inheritance:

- (id)init
{
    if (self = [super init]) {
        ...
    }
}

Initializer methods have some additional constraints, and are described in more detail in “Allocating and Initializing Objects.”

It’s also possible to concentrate core functionality in one method defined in a superclass, and have subclasses incorporate the method through messages to super. For example, every class method that creates an instance must allocate storage for the new object and initialize its isa variable to the class structure. This is typically left to the alloc and allocWithZone: methods defined in the NSObject class. If another class overrides these methods (a rare case), it can still get the basic functionality by sending a message to super.

Redefining self

super is simply a flag to the compiler telling it where to begin searching for the method to perform; it’s used only as the receiver of a message. But self is a variable name that can be used in any number of ways, even assigned a new value.

There’s a tendency to do just that in definitions of class methods. Class methods are often concerned not with the class object, but with instances of the class. For example, many class methods combine allocation and initialization of an instance, often setting up instance variable values at the same time. In such a method, it might be tempting to send messages to the newly allocated instance and to call the instance self, just as in an instance method. But that would be an error. self and super both refer to the receiving object—the object that gets a message telling it to perform the method. Inside an instance method, self refers to the instance; but inside a class method, self refers to the class object. This is an example of what not to do:

+ (Rectangle *)rectangleOfColor:(NSColor *) color
{
    self = [[Rectangle alloc] init]; // BAD
    [self setColor:color];
    return [self autorelease];
}

To avoid confusion, it’s usually better to use a variable other than self to refer to an instance inside a class method:

+ (id)rectangleOfColor:(NSColor *)color
{
    id newInstance = [[Rectangle alloc] init]; // GOOD
    [newInstance setColor:color];
    return [newInstance autorelease];
}

In fact, rather than sending the alloc message to the class in a class method, it’s often better to send alloc to self. This way, if the class is subclassed, and the rectangleOfColor: message is received by a subclass, the instance returned will be the same type as the subclass (for example, the array method of NSArray is inherited by NSMutableArray).

+ (id)rectangleOfColor:(NSColor *)color
{
    id newInstance = [[self alloc] init]; // EXCELLENT
    [newInstance setColor:color];
    return [newInstance autorelease];
}

See “Allocating and Initializing Objects” for more information about object allocation.



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© 2009 Apple Inc. All Rights Reserved. (Last updated: 2009-05-06)

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