Brief Introduction to Objective-C

Objective-C is a superset of the C programming language that adds object-oriented constructs. Unlike C++, which was influenced by Simula, Objective-C was influenced by Smalltalk, something we can definitely see in its braces syntax.

Just like C, Objective-C keeps the tradition of separating the source code in two files: a header that defines an interface (.h) and a source file that provides the implementation of said interfaces (.m).

Contrary to popular belief, you do not need a computer running Mac OS X to develop Objective-C programs. The GNUstep project is an open source implementation of the OpenStep reference that provides a basic runtime environment and an implementation of part of the library, including Strings, Arrays and Dictionaries. GNUstep is available for Linux, other UNIX-based OSs and even Windows.

Here’s a short example that defines a 2D point structure with two operations: initialize from a given pair of (x,y) coordinates and printing to stdout.

#import <Foundation/Foundation.h>

@interface Point2D : NSObject
{
	float x;
	float y;
}

- (id) initWithX:(float)xval andY:(float)yval;
- (void) print;

@end

Now, we implement this interface in its own file:

#import "point2d.h"
#include <stdio.h>

@implementation Point2D

- (id) initWithX:(float)xval andY:(float)yval;
{
	self = [super init];
	if (self)
	{
		x = xval;
		y = yval;
	}
	return self;
}

- (void) print
{
	printf("(%.2f, %.2f)\n", x, y);
}

@end

Finally, let me show you how to instantiate and send messages to a Point2D object.

#import "point2d.h"
int main()
{
	NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];

	Point2D* p = [[Point2D alloc] initWithX:1.0f andY:2.0f];

	[p print];
	[p release];

	[pool release];
	return 0;
}

Although the Autorelease pool was not really necessary, it is a good practice to have one in place.

If you are trying this example on a Mac, then it’s relatively simple to get it to compile and run. Using GNUstep it gets a little harder however. Here’s a simple Makefile for building this application:

CC=gcc -x objective-c

CFLAGS=-I$(GNUSTEP_HOME)/Local/Library/Headers/ -Wall
LDFLAGS=-L$(GNUSTEP_HOME)/Local/Library/Libraries/ -lgnustep-base

all:
	$(CC) $(CFLAGS) $(LDFLAGS) main.m