Am I correctly creating and passing this C array to Objective-C method and referencing it with a property?

Question

I created a C array like this:

unsigned char colorComps[] = {2, 3, 22,   55, 9, 1};

which I want to pass to an initializer of an Objective-C object.

So I think I have to put the array on the heap:

size_t arrayByteSize = numColorCompVals * sizeof(unsigned char);
unsigned char *colorCompsHeap = (unsigned char*)malloc(arrayByteSize);

Then I have to write my first "stack memory array" to the heap array in for loop:

for (int i = 0; i < numColorCompVals; i++) {
   colorCompsHeap[i] = colorComps[i];
}

Side question: Is there a more elegant solution to avoid the for-loop step?

And then I pass it to the method:

defined as

- (id)initWithColorCompsC:(unsigned char *)colorCompsHeap;

TheObject *obj = [[TheObject alloc] initWithColorCompsC:colorCompsHeap];

TheObject has a property to hold the C-array:

@property (nonatomic, assign) unsigned char *colorComps;

And in -dealloc I free it:

free(_colorComps);

This is in theory. I use ARC for Objective-C. Am I doing this correct or is there a better way?

Answer 1

If TheObject is going to free the array, then its init method should be the one to make the copy, NOT the caller. That way each instance of TheObject make its own copy and frees its own copy, it owns that copy.

Also, then it doesn't matter where the parameter to the init comes from, stack or heap. It won't matter if the init method makes a copy of it.

Use memcpy to make the copy, with sizeof the destination array, like this for the .m file:

@interface PTTEST ()
@property (nonatomic, assign) unsigned char *colorComps;
@end

@implementation PTTEST

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

- (id)initWithColorCompsC:(unsigned char *)colorComps
       numberOfColorComps:(unsigned)numberOfColorComps
{
    self = [super init];
    if (self) {
        // compute size based on sizeof the first element (in case
        // the element type get changed later, this still works right)
        size_t arraySize = sizeof(colorComps[0]) * numberOfColorComps;

        _colorComps = malloc(arraySize);

        memcpy(_colorComps, colorComps, arraySize);
    }
    return self;
}

@end

Answer 2

This seems fine, however...

1. Don't cast the return value of malloc();
2. and don't reinvent memcpy(). Instead of your for loop, write memcpy(dest, src, size).

Answer 3

Doesn't seems correct to me, you are filling colorCompsHeap(an array of unsigned chars) with the values of colorComps[] (an array of doubles)

unsigned char u = 0.2;

printf("%f\n", (double)u);

Outputs 0.000000