Making an Integer Array in Objective-C

Question

I want to have an internal int array for my class, but I can't seem to get XCode to let me. The array size needs to be set on initialization so I can't put the size directly into the interface.

At the moment I've been trying:

@interface TestClass : NSObject {
  int test[];
}

But it tells me that I'm not allowed. How to I refer to it in my interface, and then how do I allocate it when I create the implementation?

Sorry for a somewhat standard sounding question, but I can't seem to find the answer I need from searching.

edit: I want to use an array because it's apparently much faster than using an NSArray

I'm trying to move my implementation away from NSArray at the moment. If it's something every class should have then it should be in the interface shouldn't it? I'm not too sure. — ahansen
– ahansen, Commented Aug 12, 2013 at 9:31
" I want to use an array because it's apparently much faster than using an NSArray" ... Honestly, the difference would be so imperceptible that it's not worth the hassle. — Frederic Adda
– Frederic Adda, Commented May 19, 2015 at 8:37

Jack Greenhill · Accepted Answer · 2013-08-12 10:17:29Z

2

You can use a number of methods to overcome this problem, but the easiest is to simply make the instance variable a pointer, like this:

@interface TestClass : NSObject {
    int *test;
}

@property int *test;

@end

Synthesizing the property will give it getter and setter methods which you can use to set its contents:

@implementation TestClass
@synthesize test;

//contents of class

@end

You can then use it like this:

TestClass *pointerTest = [[TestClass alloc] init];

int *array = (int *)malloc(sizeof(int) * count);
//set values

[pointerTest setTest:array];
[pointerTest doSomething];

However, using objects like NSNumber in an NSArray is a better way to go, perhaps you could do something like this:

@interface TestClass : NSObject {
    NSArray *objectArray;
}

@property (nonatomic, strong) NSArray *objectArray;

@end

@implementation TestClass
@synthesize objectArray;

//contents of class

@end

You can then set its contents with a pointer to an NSArray object:

NSArray *items = [NSArray arrayWithObjects:[NSNumber numberWithInt:1], [NSNumber numberWithInt:2], nil];

TestClass *arrayClass = [[TestClass alloc] init];
[arrayClass setItems:items];

[arrayClass doSomething];

When retaining objects upon setting them (like the previous example), always make sure you deallocate the object in the classes dealloc method.

edited Aug 12, 2013 at 10:17

answered Aug 12, 2013 at 9:34

Jack Greenhill

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3 Comments

Nikolai Ruhe Over a year ago

Nowadays (with ARC) there's no need to override dealloc to release any objects. This would only be needed for the free of the raw buffer. Yet in your proposed code above the class does not own the buffer. So no dealloc in any case.

ahansen Over a year ago

It may be easier to use an NSArray, but I was told int array was much faster which is something we are supposed to optimise for in this assignment. Are the gains in speed worth the effort?

Frederic Adda Over a year ago

"Premature optimization is the root of all evil"

Nikolai Ruhe · Accepted Answer · 2013-08-12 10:02:34Z

A C array is just a sufficiently sized raw memory buffer. Foundation has a nice wrapper around raw memory that frees you from all the manual memory management: NSMutableData

The following approach gives you automatic memory management plus proper encapsulation.

@interface TestClass : NSObject

@property (nonatomic, readonly) int *testArray;
@property (nonatomic, readonly) NSUInteger testArraySize;

@end


@implementation TestClass
{
    NSMutableData *_testData;
}

- (id)initWithSize:(NSUInteger)size
{
    self = [self init];
    if (self != nil) {
        _testData = [NSMutableData dataWithLength:size];
    }
}

- (int *)testArray
{
    return [_testData mutableBytes];
}

- (NSUInteger)testArraySize
{
    return [_testData length];
}

@end

As you see, the ivar does not have to be declared in the @interface.

Bo A · Accepted Answer · 2013-08-12 10:42:42Z

-1

Try something like this:

@interface TestClass : NSObject
{
    int *_test; 
}

@property (assign) int *test;

@end

@implementation TestClass

- (instancetype)init
{
    if (self = [super init])
    {
        _test = malloc(sizeof(int) * 20);
    }
    return self;
}

- (int *)test
{
    return _test;
}

- (void)setTest:(int*)test
{
    memcpy(&_test, &test, sizeof(_test));
}

- (void)dealloc
{
    free(_test);
}

@end

edited Aug 12, 2013 at 10:42

answered Aug 12, 2013 at 9:38

Bo A

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4 Comments

Nikolai Ruhe Over a year ago

"The array size needs to be set on initialization". You use a statically sized array.

Bo A Over a year ago

@NikolaiRuhe Check my edit. It's only an extra line of magic. I was only showing a guide but I've cleared it up for you.

Nikolai Ruhe Over a year ago

The problem now is that setTest: does not do what it promises: it does not change test. The property should rather be readonly and the setter something like setTestBytes:length:.

Bo A Over a year ago

@NikolaiRuhe I did make a typo in setTest: which I have now fixed but other than that, it works perfectly for me.

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