How do I using NSCoding for a c-array of structs? (MKPolyline)

Question

I want to add NSCoding support to an c array of structs. Specifically this is for a subclass of MKPolyline, i.e. this is what I have to work with:

@property (nonatomic, readonly) MKMapPoint *points;
@property (nonatomic, readonly) NSUInteger pointCount;

+ (MKPolyline *)polylineWithPoints:(MKMapPoint *)points count:(NSUInteger)count;

I found a good answer on how to encode a individual struct. E.g.

NSValue* point = [NSValue value:&aPoint withObjCType:@encode(MKMapPoint)];
[aCoder encodeObject:point forKey:@"point"];

.... 

NSValue* point = [aDecoder decodeObjectForKey:@"point"];
[endCoordinateValue getValue:&aPoint];

Is there a nice way to apply this to a c Array - or will I simply have to iterate over the c-array?

How about [NSValue value:aPointArray withObjCType:@encode(MKMapPoint[12])] or similar? — user529758
– user529758, Commented Feb 16, 2013 at 18:49
@H2CO3 This method may be deprecated in a future release. You should use valueWithBytes:objCType: instead. — voromax
– voromax, Commented Feb 16, 2013 at 18:54
@voromax Didn't check the docs, just repeated what OP had, but true. — user529758
– user529758, Commented Feb 16, 2013 at 18:56

Robert · Accepted Answer · 2013-02-16 19:18:03Z

4

Note: This approach only works if the data isn't going between processors with different "endian-ness". It should be safe going from iOS to iOS, certainly if only used on a given device.

You should be able to load the memory for the C-array into an NSData object then encode the NSData object.

MKMapPoint *points = self.points;
NSData *pointData = [NSData dataWithBytes:points length:self.pointCount * sizeof(MKMapPoint)];
[aCoder encodeObject:pointData forKey:@"points"];

Update: to get the data back:

NSData *pointData = [aCode decodeObjectForKey:@"points"];
MKMapPoint *points = malloc(pointData.length);
memcpy([pointData bytes], points);
self.points = points;

edited Feb 16, 2013 at 19:18

Robert

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answered Feb 16, 2013 at 18:50

rmaddy

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5 Comments

user529758 Over a year ago

And this will nicely break portability (memdumping floats, ehh...)

rmaddy Over a year ago

Yes, if Apple starts using non-ARM chips there may be an issue going between big-endian and little-endian devices.

Robert Over a year ago

Should this portability issues be a concern, or is it unlikely to cause impact? Also how would you convert it back from NSData to a MKMapPoint array?

Robert Over a year ago

Im a little stuck again.... I made a new question. stackoverflow.com/q/14914265/296446

davvilla Over a year ago

This wasn't working for me for some reason and it also gave me a warning about sending const void* to void* parameter. I fixed it by using the following: [pointData getBytes:& points length:pointsSize];

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