Allocating/Freeing a Pointer to an Array from a Class Method

Question

I have a custom object in which I am trying to write a convenience class method that returns an array of CGPoint structures. The reason for this is there are several methods such as CGPathAddLines which take that as a parameter, and a class method would save me from converting my custom objects to an array of CGPoint structures every time. Unfortunately, I am not sure of the best way to achieve this, especially in regards to memory management.

Here is my implementation thus far:

+ (CGPoint*)CGPointsFromData:(NSArray*)data
{
    CGPoint *points = (CGPoint*)malloc(data.count * sizeof(CGPoint));
    DataPoint *dataPoint;
    int i = 0;
    for (id object in data) {
        if ([object isKindOfClass:[DataPoint class]]) {
            dataPoint = (DataPoint*)object;
            points[i] = dataPoint.point;
            i++;
        }
    }
    return points;
}

Edit: In case it isn't clear, the DataPoint class has a property CGPoint point.

You are using _points and points as though they are @propertys of an instance, but you are in a class method. Can you show where _points (and points for that matter) are declared/initialized? — Chris Trahey
– Chris Trahey, Commented Jul 17, 2012 at 16:52
Sorry I corrected it. I have been messing around with my code for a while so I forgot to change some stuff back. — Alec
– Alec, Commented Jul 17, 2012 at 16:54

Chris Trahey · Accepted Answer · 2012-07-17 17:01:40Z

1

If you want this to be a class method, place the word 'alloc' or something like it in the method name and place the obligation of free-ing the memory on the caller of your method, then simply return the pointer you allocated (no need to involve variables from any other scope inside this method). Then whomever calls this method can store the pointer in an instance variable and free it whenever appropriate.

+ (CGPoint*)allocCGPointsArrayWithData:(NSArray*)data
{
    CGPoint *thesePoints = (CGPoint*)malloc(data.count * sizeof(CGFloat));
    DataPoint *dataPoint;
    int i = 0;
    for (id object in data) {
        if ([object isKindOfClass:[DataPoint class]]) {
            dataPoint = (DataPoint*)object;
            thesePoints[i] = dataPoint.point;
            i++;
        }
    }
    return thesePoints;
}

and then to consume this (from the same class in this case)

// @interface
@interface FooViewController:UIViewController {
    CGPoint *_myPointsCArray;
}

// @implementation    
-(void) viewWillAppear {
    _myPointsCArray = [[self class] allocCGPointsArrayWithData:self.dataArray];
    [super viewWillAppear];
}
-(void) viewWillDisappear {
    free(_myPointsCArray);
    [super viewWillDisappear];
}

answered Jul 17, 2012 at 17:01

Chris Trahey

18.3k1 gold badge44 silver badges55 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Alec Over a year ago

This is along the lines of what I am looking for, but I have a few follow up questions. My use for this method is not in a UIViewController but instead is in a UIView which, as far as I know, doesn't have a built in deallocation method like viewDidUnload. So I don't know where I would free the memory in UIView. Perhaps setting autorelease in the class method? Is that what I am looking for?

Chris Trahey Over a year ago

Traditionally, this is what you implement -(void) dealloc for. However, if this is a shared data structure perhaps it is worth considering not having a UIView "own" it, and instead just having a "weak" pointer to it (i.e. it has a pointer to it... nothing special here) but that view's ViewController actually "owns" it and can dealloc appropriately (thus also better enabling the VC to share the data amongst views).

justin · Accepted Answer · 2012-07-17 17:30:42Z

0

If you're using ObjC only, you might instead consider a class (e.g. MONCGPointArray) which holds a NSData or NSMutableData instance. The NSData would take care of the allocation and reallocation, and the class interface could be used to introduce type safety.

To illustrate:

@interface MONCGPointArray : NSObject
{
@private
    NSMutableData * cgPoints;
}

- (NSUInteger)count;
- (CGPoint)CGPointAtIndex:(NSUInteger)idx;
- (const CGPoint*)CGPoints;

- (void)appendCGPoint:(CGPoint)point;

// ...

@end

Personally, I just use types such as C++'s std::vector<CGPoint> for such collections.

edited Jul 17, 2012 at 17:30

answered Jul 17, 2012 at 17:22

justin

105k14 gold badges185 silver badges227 bronze badges

1 Comment

Alec Over a year ago

Changing my object structure to incorporate this isn't really an option in my case. And the class method itself needs to return an array of CGPoint structures.

Collectives™ on Stack Overflow

Allocating/Freeing a Pointer to an Array from a Class Method

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related