Storing a struct in an NSArray

Question

Back with my daily silly question. Today I'm trying to put a struct in an NSArray. Alright, that's easy, wrap it in a NSData. If you know me, you know you're about to see code. In this example, I'm loading a vertex line from a Wavefront OBJ into a struct and sticking it in an array.

        NSArray *verticeLineParts = [currentLine componentsSeparatedByCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
        Coordinate c = CoordinateMake([[verticeLineParts objectAtIndex:1] floatValue],
                                      [[verticeLineParts objectAtIndex:2] floatValue],
                                      [[verticeLineParts objectAtIndex:3] floatValue]);
        [vertices addObject:[[NSData alloc] initWithBytes:&c length:sizeof(c)]];

This seems to work fine. The first few objects in the output:

2010-10-18 14:18:08.237 ObjLoader[3778:207] (
    <5d1613bd 13f3da3f 8ac745bd>,
    <f04c28bd 13f3da3f d88048bd>,
    <649427bd d61ddb3f d88048bd>,
    <477625bd d845db3f d88048bd>,
    <4c1722bd 1668db3f d88048bd>,

When it comes to printing them out after pulling them from the array, not so much. The offending code:

for (int i = 0; i < [vertices count]; i++) {
    Coordinate c;
    fillCoordinateFromData(c, [vertices objectAtIndex:i]);
    NSLog(@"%@", CoordinateToNSString(c));
}

[snip]

void fillCoordinateFromData(Coordinate c, NSData *d) {
    void *bytes = malloc(12);
    [d getBytes:bytes];
    memcpy((void *)&c, &bytes, 12);
    free(bytes);
}

Of course, the offending output:

2010-10-18 14:22:00.692 ObjLoader[3825:207] 9.885923, -6.837428, 0.000000
2010-10-18 14:22:00.693 ObjLoader[3825:207] 9.885923, -6.837428, 0.000000
2010-10-18 14:22:00.694 ObjLoader[3825:207] 9.885923, -6.837428, 0.000000
2010-10-18 14:22:00.695 ObjLoader[3825:207] 9.885923, -6.837428, 0.000000
2010-10-18 14:22:00.695 ObjLoader[3825:207] 9.885923, -6.837428, 0.000000

I think this is caused by C wizardry attempted by someone who's not a C wizard, kind of like The Sorcerer's Apprentice. I think this should be obvious to a wizard of the 99th degree, or if someone can suggest a more modern way to do this, let me know.

Thanks, Will

Answer 1

void fillCoordinateFromData(Coordinate c, NSData *d) {

This passes c by value. The modification of c in this function won't be reflected back to the caller.

You need to pass by pointer (reference) instead.

fillCoordinateFromData(&c, [vertices objectAtIndex:i]);

void fillCoordinateFromData(Coordinate* pc, NSData *d) {
    void *bytes = malloc(12);
    [d getBytes:bytes];
    memcpy((void *)c, &bytes, 12);
    free(bytes);
}

but it is better to just return the Coordinate.

Coordinate c = getCoordinateFromData([vertices objectAtIndex:i]);

Coordinate getCoordinateFromData(NSData* d) {
  Coordinate c;
  [d getBytes:&c];   // and let's not waste time allocating 12 bytes.
  return c;
}

---

BTW, by convention structs are wrapped in NSValue, not NSData.

[vertices addObject:[NSValue valueWithBytes:&c withObjCType:@encode(Coordinate)]];

Coordinate getCoordinateFromData(NSValue* d) {
  Coordinate c;
  [d getValue:&c];
  return c;
}