After switching to Java, Python and eventually Ruby, I honestly, honestly thought I’d never see another pointer again in my life. I’m still traumatized by the 4000 lines of X/Motif code I wrote as part of my undergraduate thesis many years ago. Nonetheless, it appears that he who wants to code on the IPhone must take a refresher, so here it is.

Pointers

Variable, meet pointer, who is nothing more than a memory address with your address in it.

 
	#import <Foundation/Foundation.h>

	int main (int argc, const char * argv[]) {
	    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

	    int count = 1;
		int *ptr;

		// ptr is equal to the address of count
		ptr = &count;
		NSLog(@"c = %i, p = %i", count, *ptr);

		// contents of ptr is equal to the contents of count
		count = 2;
		*ptr = count;
		NSLog(@"c = %i, p = %i", count, *ptr);

		// ptr is actually a memory address
		NSLog(@"p == %i", ptr);

	    [pool drain];
	    return 0;
	}

Gives us:

07:07:26.919 Pointers[1958:903] c = 1, p = 1
07:07:26.922 Pointers[1958:903] c = 2, p = 2
07:07:26.923 Pointers[1958:903] p == 1606414028

Structs

Pointer, meet struct. You two shall become very familiar. In fact, theres even some funky shorthand you can use together.

	int main (int argc, const char * argv[]) {
	    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

		struct date 
		{
			int month;
			int day;
			int year;
		};

		struct date today;
		struct date *ptr;

		today.month = 2;
		today.day = 19;
		today.year = 2010;

		// ptr points to today
		ptr = &today;

		// ptr is actually a memory address
		NSLog(@"t == %i", ptr);

		// we can still access the elements of thr struct
		NSLog(@"t == %i", (*ptr).month);

		// but we also get this cool shorthand
		NSLog(@"t == %i", ptr->year);	

	    [pool drain];
	    return 0;
	}

The product of your union (fnar, fnar):

07:16:58.706 Pointers[2360:903] t = 1606413984
07:16:58.709 Pointers[2360:903] t = 2
07:16:58.709 Pointers[2360:903] t = 2010

Methods and Functions

We have two options here – passing the address of a variable (&) is the same as declaring and passing a pointer.

	#import <Foundation/Foundation.h>

	// we're accepting pointers here
	void exchange(int *p1, int *p2)
	{
		int temp;

		temp = *p1;
		*p1 = *p2;
		*p2 = temp;
	}

	int main (int argc, const char * argv[]) {
	    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

		int x = 1, y = 2;

		// before exchange
		NSLog(@"x, y == %i, %i", x, y);

		// we can pass in the address of x, y
		exchange(&x, &y);
		NSLog(@"x, y == %i, %i", x, y);

		// or we can declare a pointer and pass that in instead
		int *p1, *p2;
		p1 = &x;
		p2 = &y;

		exchange(p1, p2);
		NSLog(@"x, y == %i, %i", *p1, *p2);

	    [pool drain];
	    return 0;
	}	

Mon cheri:

07:44:36.137 Pointers[2892:903] x, y == 1, 2
07:44:36.141 Pointers[2892:903] x, y == 2, 1
07:44:36.141 Pointers[2892:903] x, y == 1, 2

Arrays

And this, at last, is where the fun begins.

	#import <Foundation/Foundation.h>

	int main (int argc, const char * argv[]) {
	    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

		int *ptr;
		int myArray[] = {1, 2, 3};

		// we can point at the start of the array like this
		ptr = &myArray[0];
		NSLog(@"ptr == %i", *ptr);

		// or use the shorter version, the compiler treats the 
		// occurence of an array name with subscript as pointer
		// to first element of the array
		ptr = myArray;
		NSLog(@"ptr == %i", *ptr);

		// then we can access directly
		ptr[1] = 5;
		NSLog(@"ptr == %i", myArray[1]);

		// or this way. they key point here is that the compiler
		// will look after the size of the inc/decrement depending
		// on what the array objects are
		*(ptr + 1) = 6;
		NSLog(@"ptr == %i", myArray[1]);

		// or increment/decrement
		*(++ptr) = 7;
		NSLog(@"ptr == %i", myArray[1]);

	    [pool drain];
	    return 0;
	}

Gives us:

08:10:13.356 Pointers[3588:903] ptr == 1
08:10:13.359 Pointers[3588:903] ptr == 1
08:10:13.359 Pointers[3588:903] ptr == 5
08:10:13.360 Pointers[3588:903] ptr == 6
08:10:13.360 Pointers[3588:903] ptr == 7

Functions

Finally, its worth noting that you can declare a pointer to a function.

	#import <Foundation/Foundation.h>

	void myfunc(int *a, int *b)
	{
		int temp;
		temp = *a;
		*a = *b;
		*b = temp;
	}

	int main (int argc, const char * argv[]) {
	    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

		int a = 1, b = 2;

		// before function
		NSLog(@"a, b == %i, %i", a, b);

		// vanilla
		myfunc(&a, &b);
		NSLog(@"a, b == %i, %i", a, b);

		// we can declare a pointer to a function
		// note the parentheses around the function name
		void (*ptr) (int *a, int *b);
		ptr = myfunc;

		// using the ptr
		ptr(&a, &b);
		NSLog(@"a, b == %i, %i", a, b);

	    [pool drain];
	    return 0;
	}

Point to my function baby.

08:12:16.534 Pointers[3608:903] a, b == 1, 2
08:12:16.536 Pointers[3608:903] a, b == 2, 1
08:12:16.537 Pointers[3608:903] a, b == 1, 2