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Classes

As in many other object-oriented programming language, Objective-C classes provide the blueprint for creating objects. First, you define a reusable set of properties and behaviors inside of a class. Then, you instantiate objects from that class to interact with those properties and behaviors.

Objective-C is similar to C++ in that it abstracts a class’s interface from its implementation. An interface declares the public properties and methods of a class, and the corresponding implementation defines the code that actually makes these properties and methods work. This is the same separation of concerns that we saw with functions.

In this module, we’ll explore the basic syntax for class interfaces, implementations, properties, and methods, as well as the canonical way to instantiate objects. We’ll also introduce some of Objective-C’s introspection and reflection capabilities.

Creating Classes

We’ll be working with a class called Car whose interface resides in a file named Car.h (also called a “header”) and whose implementation resides in Car.m. These are the standard file extensions for Objective-C classes. The class’s header file is what other classes use when they need to interact with it, and its implementation file is used only by the compiler.

Xcode provides a convenient template for creating new classes. To create our Car class, navigate to File > New > File… or use the Cmd+N shortcut. For the template, choose Objective-C class under the iOS > Cocoa Touch category. After that, you’ll be presented with some configuration options:

Use Car for the Class field and NSObject for the Subclass of field. NSObject is Objective-C’s top-level class from which almost all others inherit. Clicking Next will prompt you to select a location for the file. Save it in the top-level of the project directory. At the bottom of that dialog, make sure that your project is checked in the Targets section. This is what actually adds the class to the list of compiled sources.

After clicking Next, you should see new Car.h and Car.m files in Xcode’s Project Navigator. If you select the project name in the navigator, you’ll also find Car.m in the Build Phases tab under the Compile Sources section. Any files that you want the compiler to see must be in this list (if you didn’t create your source files through Xcode, this is where you can manually add them).

Interfaces

Car.h contains some template code, but let’s go ahead and change it to the following. This declares a property called model and a method called drive.

// Car.h
#import <Foundation/Foundation.h>

@interface Car : NSObject {
    // Protected instance variables (not recommended)
}

@property (copy) NSString *model;

- (void)drive;

@end

An interface is created with the @interface directive, after which come the class and the superclass name, separated by a colon. Protected variables can be defined inside of the curly braces, but most developers treat instance variables as implementation details and prefer to store them in the .m file instead of the interface.

The @property directive declares a public property, and the (copy) attribute defines its memory management behavior. In this case, the value assigned to model will be stored as a copy instead of a direct pointer. The Properties module discusses this in more detail. Next come the property’s data type and name, just like a normal variable declaration.

The -(void)drive line declares a method called drive that takes no parameters, and the (void) portion defines its return type. The minus sign prepended to the method marks it as an instance method (opposed to a class method).

Implementations

The first thing any class implementation needs to do is import its corresponding interface. The @implementation directive is similar to @interface, except you don’t need to include the super class. Private instance variables can be stored between curly braces after the class name:

// Car.m
#import "Car.h"

@implementation Car {
    // Private instance variables
    double _odometer;
}

@synthesize model = _model;    // Optional for Xcode 4.4+

- (void)drive {
    NSLog(@"Driving a %@. Vrooooom!", self.model);
}

@end

@synthesize is a convenience directive that automatically generates accessor methods for the property. By default, the getter is simply the property name (model), and the setter is the capitalized name with the set prefix (setModel). This is much easier than manually creating accessors for every property. The _model portion of the synthesize statement defines the private instance variable name to use for the property.

As of Xcode 4.4, properties declared with @property will be automatically synthesized, so it’s safe to omit the @synthesize line if you’re ok with the default instance variable naming conventions.

The drive implementation has the same signature as the interface, but it’s followed by whatever code should be executed when the method is called. Note how we accessed the value via self.model instead of the _model instance variable. This is a best practice step because it utilizes the property’s accessor methods. Typically, the only place you’ll need to directly access instance variables is in init methods and the dealloc method.

The self keyword refers to the instance calling the method (like this in C++ and Java). In addition to accessing properties, this can be used to call other methods defined on the same class (e.g., [self anotherMethod]). We’ll see many examples of this throughout the tutorial.

Instantiation and Usage

Any files that need access to a class must import its header file (Car.h)—they should never, ever try to access the implementation file directly. That would defeat the goal of separating the public API from its underlying implementation. So, to see our Car class in action, change main.m to the following.

// main.m
#import <Foundation/Foundation.h>
#import "Car.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Car *toyota = [[Car alloc] init];
        
        [toyota setModel:@"Toyota Corolla"];
        NSLog(@"Created a %@", [toyota model]);
        
        toyota.model = @"Toyota Camry";
        NSLog(@"Changed the car to a %@", toyota.model);
        
        [toyota drive];
        
    }
    return 0;
}

After the interface has been imported with the #import directive, you can instantiate objects with the alloc/init pattern shown above. As you can see, instantiation is a two-step process: first you must allocate some memory for the object by calling the alloc method, then you need to initialize it so it’s ready to use. You should never use an uninitialized object.

It’s worth repeating that all objects must be stored as pointers. This is why we used Car *toyota instead of Car toyota to declare the variable.

To call a method on an Objective-C object, you place the instance and the method in square brackets, separated by a space. Arguments are passed after the method name, preceded by a colon. So, if you’re coming from a C++, Java, or Python background, the [toyota setModel:@"Toyota Corolla"] call would translate to:

toyota.setModel("Toyota Corolla");

This square-bracket syntax can be unsettling for newcomers to the language, but rest assured, you’ll be more than comfortable with Objective-C’s method conventions after reading through the Methods module.

This example also shows you both ways to work with an object’s properties. You can either use the synthesized model and setModel accessor methods, or you can use the convenient dot-syntax, which should be more familiar to developers who have been using Simula-style languages.

Class Methods and Variables

The above snippets define instance-level properties and methods, but it’s also possible to define class-level ones. These are commonly called “static” methods/properties in other programming languages (not to be confused with the static keyword).

Class method declarations look just like instance methods, except they are prefixed with a plus sign instead of a minus sign. For example, let’s add the following class-level method to Car.h:

// Car.h
+ (void)setDefaultModel:(NSString *)aModel;

Similarly, a class method implementation is also preceded by a plus sign. While there is technically no such thing as a class-level variable in Objective-C, you can emulate one by declaring a static variable before defining the implementation:

// Car.m
#import "Car.h"

static NSString *_defaultModel;

@implementation Car {
...

+ (void)setDefaultModel:(NSString *)aModel {
    _defaultModel = [aModel copy];
}

@end

The [aModel copy] call creates a copy of the parameter instead of assigning it directly. This is what’s going on under the hood when we used the (copy) attribute on the model property.

Class methods use the same square-bracket syntax as instance methods, but they must be called directly on the class, as shown below. They cannot be called on an instance of that class ([toyota setDefaultModel:@"Model T"] will throw an error).

// main.m
[Car setDefaultModel:@"Nissan Versa"];

“Constructor” Methods

There are no constructor methods in Objective-C. Instead, an object is initialized by calling the init method immediately after it’s allocated. This is why instantiation is always a two-step process: allocate, then initialize. There is also a class-level initialization method that will be discussed in a moment.

init is the default initialization method, but you can also define your own versions to accept configuration parameters. There’s nothing special about custom initialization methods—they’re just normal instance methods, except the method name should always begin with init. An exemplary “constructor” method is shown below.

// Car.h
- (id)initWithModel:(NSString *)aModel;

To implement this method, you should follow the canonical initialization pattern shown in initWithModel: below. The super keyword refers to the parent class, and again, the self keyword refers to the instance calling the method. Go ahead and add the following methods to Car.m.

// Car.m
- (id)initWithModel:(NSString *)aModel {
    self = [super init];
    if (self) {
        // Any custom setup work goes here
        _model = [aModel copy];
        _odometer = 0;
    }
    return self;
}

- (id)init {
    // Forward to the "designated" initialization method
    return [self initWithModel:_defaultModel];
}

Initialization methods should always return a reference to the object itself, and if it cannot be initialized, it should return nil. This is why we need to check if self exists before trying to use it. There should typically only be one initialization method that needs to do this, and the rest should forward calls to this designated initializer. This eliminates boilerplate code when you have several custom init methods.

Also notice how we directly assigned values to the _model and _odometer instance variables in initWithModel:. Remember that this is one of the only places you should do this—in the rest of your methods you should be using self.model and self.odometer.

Class-Level Initialization

The initialize method is the class-level equivalent of init. It gives you a chance to set up the class before anyone uses it. For example, we can use this to populate the _defaultModel variable with a valid value, like so:

// Car.m
+ (void)initialize {
    if (self == [Car class]) {
        // Makes sure this isn't executed more than once
        _defaultModel = @"Nissan Versa";
    }
}

The initialize class method is called once for every class before the class is used. This includes all subclasses of Car, which means that Car will get two initialize calls if one of its subclasses didn’t re-implement it. As a result, it’s good practice to use the self == [Car class] conditional to ensure that the initialization code is only run once. Also note that in class methods, the self keyword refers to the class itself, not an instance.

Objective-C doesn’t force you to mark methods as overrides. Even though init and initialize are both defined by its superclass, NSObject, the compiler won’t complain when you redefine them in Car.m.

The next iteration of main.m shows our custom initialization methods in action. Before the first time the class is used, [Car initialize] is called automatically, setting _defaultModel to @"Nissan Versa". This can be seen in the first NSLog(). You can also see the result of the custom initialization method (initWithModel:) in the second log output.

// main.m
#import <Foundation/Foundation.h>
#import "Car.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        // Instantiating objects
        Car *nissan = [[Car alloc] init];
        NSLog(@"Created a %@", [nissan model]);
        
        Car *chevy = [[Car alloc] initWithModel:@"Chevy Corvette"];
        NSLog(@"Created a %@, too.", chevy.model);
        
    }
    return 0;
}

Dynamic Typing

Classes themselves are represented as objects, which makes it possible to query their properties (introspection), or even change their behavior on-the-fly (reflection). These are very powerful dynamic typing capabilities, as they let you call methods and set properties on objects even when you don’t know what type of object they are.

The easiest way to get a class object is via the class class-level method (apologies for the redundant terminology). For example, [Car class] returns an object representing the Car class. You can pass this object around to methods like isMemberOfClass: and isKindOfClass: to get information about other instances. A comprehensive example is included below.

// main.m
#import <Foundation/Foundation.h>
#import "Car.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Car *delorean = [[Car alloc] initWithModel:@"DeLorean"];
        
        // Get the class of an object
        NSLog(@"%@ is an instance of the %@ class",
              [delorean model], [delorean class]);
        
        // Check an object against a class and all subclasses
        if ([delorean isKindOfClass:[NSObject class]]) {
            NSLog(@"%@ is an instance of NSObject or one "
                  "of its subclasses",
                  [delorean model]);
        } else {
            NSLog(@"%@ is not an instance of NSObject or "
                  "one of its subclasses",
                  [delorean model]);
        }
        
        // Check an object against a class, but not its subclasses
        if ([delorean isMemberOfClass:[NSObject class]]) {
            NSLog(@"%@ is a instance of NSObject",
                  [delorean model]);
        } else {
            NSLog(@"%@ is not an instance of NSObject",
                  [delorean model]);
        }
        
        // Convert between strings and classes
        if (NSClassFromString(@"Car") == [Car class]) {
            NSLog(@"I can convert between strings and classes!");
        }
    }
    return 0;
}

The NSClassFromString() function is an alternative way to get your hands on a class object. This is very flexible, as it lets you dynamically request class objects at runtime; however, it’s also rather inefficient. For this reason, you should opt for the class method whenever possible.

If you’re interested in dynamic typing, be sure to check out Selectors and The id Type.

Summary

In this module, we learned how to create classes, instantiate objects, define initialization methods, and work with class-level methods and variables. We also took a brief look at dynamic typing.

The previous module mentioned that Objective-C doesn’t support namespaces, which is why the Cocoa functions require prefixes like NS, CA, AV, etc to avoid naming collisions. This applies to classes, too. The recommended convention is to use a three-letter prefix for your application-specific classes (e.g., XYZCar).

While this is pretty much everything you need to know to start writing your own classes, we did skim over some important details, so don’t worry if you’re feeling not entirely comfortable with properties or methods. The next module will begin filling in these holes with a closer look at the @property directive and all of the attributes that affect its behavior.

Be sure to check out Ry’s Cocoa Tutorial. This brand new guide is a complete walkthrough of Mac App development, and it leverages all of the Objective-C skills that we just discussed. Learn more ›