Adventures in the transition from C to Cocoa.

Friday, June 15, 2007

Cocoa Objects

Objects in Cocoa are very similar to objects in other languages such as C++ or Java. Their structure and syntax is very different though. While C++ and Java objects are essentially extensions (in syntax and organization) of the struct in C, Cocoa's objects are implemented in a completely different way, with a completely different syntax.

In C++, you'd create a class using some code kind of like this:

class myClass
int anInteger;
float aFloat;
void someMethod(float f);
int someOtherMethod(int i, float f);

and you'd have a class called myClass with 2 private member variables and 2 public member functions, or methods.

If you wanted, you could define the member functions inside the class declaration itself, or you could define them outside the class like this:

void myClass::someMethod(float f)
aFloat = f;

This would define the someMethod method of myClass. Because you can have different return types and argument types, C++ allows you to have different methods with the same name, as long as their types are different. This is a convention that is not allowed in Objective-C, as we shall shortly see.

In Objective-C, objects come in two parts, an interface and an implementation. These look completely weird to run-of-the-mill C/C++ Programmers, so brace yourself. This is an equivalent object in Objective-C.

@interface myObject
int anInteger;
float aFloat;
-(void) someMethod:(float) f;
-(int) someOtherMethod:(int) i andFloat: (float) f;

Right. So, we covered a lot right there. An object's interface is defined between @interface and @end. The name of the object is immediately after the @interface. Member variables are can be marked @public, @protected, or @private, with @protected being the default protection.

After the member variables, we declare the methods. The syntax here is pretty weird too. First, the method is declared either instance-scoped or class-scoped. Class-scoped is like static in C++. To declare a method as instance-scoped, use a minus sign: -. To declare it as class-scoped, use a plus sign: +.

Following the scope of the method, the return type is specified in parenthesis. Next comes the method name. If the method takes no parameters, we tack on a semicolon, and we're done. If it does, we tack on a colon, then specify the data type in parenthesis, followed by the variable name. If we have multiple parameters, it gets even more interesting.

-(int) someOtherMethod:(int) i andFloat: (float) f; uses two parameters. Instead of just having a list of arguments, Objective-C names them so that the user can keep track of what's going on. The andFloat part is doing just that. We don't strictly need to specify this part. We could simply use -(int) someOtherMethod:(int) i: (float) f; to make it look more like C. However, the names are pretty helpful, so I'd recommend using them.

As you've probably guessed by now, the @interface is similar the the class declaration. Thus, we're on to the @implementation.

@implementation myObject
-(void) someMethod:(float) f
aFloat = f;
-(int) someOtherMethod:(int) i andFloat: (float) f
anInteger = i;
aFloat = f;

Not so surprisingly now, the @implementation defines the methods using almost the exact same syntax. It's pretty straightforward.

Typically the @interface goes in a .h header file. The @implementation goes in a .m source file. You can put them all in the same source file though if you want.

Inheritance is fairly common in Objective-C as well. In fact, it's practically essential for get to actually use our objects. Almost all the objects we create will extend NSObject. To do this, our interface line will look like this: @interface myObject: NSObject. Pretty close to C++'s method. It gets more complicated than this though, but that's for a future entry.

So now that we have basic objects at our disposal, it's time to talk about some rules regarding objects. First, objects cannot be statically allocated. If you try to, gcc will complain with an error like this: "error: statically allocated instance of Objective-C class." This is one of the reasons why we will almost always extend NSObject. The NSObject gives us a couple methods to dynamically create our objects. myObject *mo = [[myObject alloc] init]; is the typical way to allocate and initialize an object. The alloc method, as the name implies, allocates the necessary storage space, and init similarly initializes the object. This is in league with C++'s constructors.

Instead of delete, as we would use in C++ to free space used by an object, we use the release method, like this: [mo release];.

Before we continue, you've probably noticed that we call methods in a really different way. Instead of the expected myObject->method(args); syntax, we use [object method ... args]. To call a method with arguments from our object, it looks like this: [mo someMethod: 3.1415926];. You can probably guess how multiple parameters are handled, but here's another example just in case: [mo someOtherMethod: 42 andFloat: 1.234];.

So, to tie it all together into a working -- though useless -- example, here we go.

#import <Foundation/NSObject.h>
#import <stdio.h>

@interface myObject :NSObject
int anInteger;
float aFloat;
-(void) someMethod:(float) f;
-(int) someOtherMethod:(int) i andFloat: (float) f;

@implementation myObject
-(void) someMethod:(float) f
aFloat = f;
-(int) someOtherMethod:(int) i andFloat: (float) f
anInteger = i;
aFloat = f;

int main(void)
myObject *mo = [[myObject alloc] init];

[mo someMethod: 3.1415926];
[mo someOtherMethod: 42 andFloat: 1.234];

[mo release];
return 0;

When compiling, be sure to use -framework Cocoa or else you'll get errors that are hard to understand. Like the following:

/usr/bin/ld: Undefined symbols:
collect2: ld returned 1 exit status

As an added bonus, you've been exposed to #import. You can safely assume that it's basically the same as #include -- in fact, you can often use them interchangeably -- except that #import makes sure to only include the file once without the #ifndef HEADER_H / #define HEADER_H / #endif stuff that litters the top and bottom C header files.

There you have it, the basics of Objective-C objects, and simple usage of Cocoa's framework.


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