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?

You'd probably be better off allocating and copying the array in the constructor. — Kevin
– Kevin, Commented Aug 7, 2013 at 14:29

Community · Accepted Answer · 2017-05-23 12:05:17Z

2

This seems fine, however...

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

edited May 23, 2017 at 12:05

CommunityBot

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answered Aug 7, 2013 at 14:02

user529758

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8 Comments

Richard J. Ross III Over a year ago

Even better - use memcpy with a C99 compound literal array. Much more consise.

user529758 Over a year ago

@RichardJ.RossIII Yeah, I am aware of them (I just didn't think this was the question here.)

Proud Member Over a year ago

Richard can you explain what you mean with a C99 compound literal array?

Proud Member Over a year ago

According to the documentation memcpy is for chars. What about other data types? tutorialspoint.com/c_standard_library/c_function_memcpy.htm

Stephen Canon Over a year ago

@ProudMember: memcpy copies data. It is not picky about data type. The documentation you link to clearly specifies that it takes void * arguments, not char or char *.

|

progrmr · Accepted Answer · 2013-08-07 21:50:55Z

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

edited Aug 7, 2013 at 21:50

answered Aug 7, 2013 at 15:39

progrmr

77.5k16 gold badges116 silver badges148 bronze badges

6 Comments

Proud Member Over a year ago

One more question: Why wouldn't size_t arraySize = sizeof(colorComps) work?

Richard J. Ross III Over a year ago

@ProudMember because that returns the size of the pointer, not the array itself.

Proud Member Over a year ago

And how about sizeof(&colorComps)?

Proud Member Over a year ago

@H2CO3 Care to explain?

user529758 Over a year ago

@ProudMember Explain what?

|

David Ranieri · Accepted Answer · 2013-08-07 14:10:47Z

0

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

answered Aug 7, 2013 at 14:10

David Ranieri

41.2k8 gold badges58 silver badges103 bronze badges

1 Comment

user529758 Over a year ago

@DavidRF Then it was someone else, and he still didn't leave any explanation.

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