is iPhone autorelease working for c arrays?

Question

will autorelease release my non-object c array? I am wondering, because perhaps only objects know their reference count? here's my code:

-(int *)getCombination{
    int xIndex = arc4random() % [self._num1 count] + 1;
    int yIndex = arc4random() % [self._num2 count] + 1;
    int *combination;
    combination[0] = [[self._num1 objectAtIndex:xIndex]intValue];
    combination[1] = [[self._num2 objectAtIndex:yIndex]intValue];
    return combination;
}

and this is my main() function:

int main(int argc, char *argv[])
{
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([YYAAppDelegate class]));
    }
}

So is the autorelease working for objects only or will it release my c array from getCombination?

Edit: Since the answer is no, autorelease doesn't work for c arrays/pointers I used the following code which uses NSArrays instead:

#import <Foundation/Foundation.h>

@interface Multiplication : NSObject

@property (strong, nonatomic) NSMutableArray *_combinations;

-(id)initArrays;
-(NSArray *)getCombination;

@end

#import "Multiplication.h"

@implementation Multiplication
@synthesize _combinations;

-(void)initializeArray{
    self._combinations = [[NSMutableArray alloc]init];
    for (int i = 1; i <= 10; i++) {
        for (int j = 1; j <= 10; j++) {
            NSNumber *x = [NSNumber numberWithInt:i];
            NSNumber *y = [NSNumber numberWithInt:j];
            [self._combinations addObject:[NSArray arrayWithObjects:x, y, [NSNumber numberWithInt:([x intValue] * [y intValue])], nil]];
        }
    }
}

-(NSArray *)getCombination{
    if ([self._combinations count] == 0) {
        [self initializeArray];
    }

    int index = arc4random() % [self._combinations count];
    NSArray *arr = [self._combinations objectAtIndex:index];
    [self._combinations removeObjectAtIndex:index];
    return arr;
}

-(id)initArrays{
    self = [super init];
    if (self) {
        [self initializeArray];
    }
    return self;
}

@end

BTW this function is supposed to provide a method for randomly displaying every combination in the multiplication table of 10X10 and restart when all combinations have been displayed and equal number of times.

Answer 1

autorelease is a message that can be sent to Objective C objects. Before ARC, you needed to do it explicitly, like this:

MyObject *obj = [[[MyObject alloc] init] autorelease];

Under ARC, the compiler figures out the autorelease part for you, but the message is stil sent to the object; that object gets added to the autorelease pool as a result. When autorelease pool gets drained, all objects inside it are sent a release message. If there are no other references to the object, its reference count drops to zero, and the object get cleaned up.

C arrays do not respond to autorelease or release message: they are not Objective C entities, so they do not respond to messages at all. Therefore, you must deal with the memory that you allocate for these arrays manually, by calling malloc¹ and free.

Of course you can place autoreleased objects inside a C array, and these objects will be cleaned up in the regular course of action. For example

NSNumber **numbers = malloc(2, sizeof(NSNumber*));
numbers[0] = [NSNumber numberWithInt:123];
numbers[1] = [NSNumber numberWithInt:456];
return numbers;

If the caller does not retain NSNumber objects inside the numbers array, these objects will be autoreleased². The numbers object, however, needs to be cleaned up separately:

free(numbers);

---

¹ You did not call malloc in your code, so accessing combinatopn[...] is undefined behavior.

² Causing hanging references in the process.

Answer 2

Autorelease is for Objective-C objects. Also, you aren't creating a C array, just a pointer variable. Your assignments to combination[n] are writing to unallocated memory.