when should you use __bridge vs. CFBridgingRelease/CFBridgingRetain?

Question

I have this code that uses "__bridge" to cast the ids of colors:

  CGColorRef tabColor = (5 == 5
                         ? [UIColor blueColor].CGColor
                         : [UIColor greenColor].CGColor);

  CGColorRef startColor = [UIColor whiteColor].CGColor;
  CGColorRef endColor   = tabColor;
  NSArray    *colors    = [NSArray arrayWithObjects:(__bridge id)startColor, (__bridge id)endColor, nil];

  CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, (__bridge CFArrayRef)colors, locations);

but would:

  NSArray    *colors    = [NSArray arrayWithObjects:(id)CFBridgingRelease(startColor), (id)CFBridgingRelease(endColor), nil];

  CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, (CFArrayRef)CFBridgingRetain(colors), locations);

be a better solution?

Answer 1

You don't "own" the Core Foundation objects startColor, endColor because they were not returned by a function that has "Create" or "Copy" in its name (compare "The Create Rule" in the "Memory Management Programming Guide for Core Foundation". And because you don't own the objects, you must not "transfer the ownership" to ARC with CFBridgingRelease(). So

[NSArray arrayWithObjects:(__bridge id)startColor, (__bridge id)endColor, nil];

is correct. And

CGGradientCreateWithColors(colorSpace, (__bridge CFArrayRef)colors, locations);

is also correct because

CGGradientCreateWithColors(colorSpace, (CFArrayRef)CFBridgingRetain(colors), locations);

would pass a (+1) retained array to CGGradientCreateWithColors(). This would be a memory leak because that function does not release the colors argument.

Answer 2

With NSURL is the same problem

NSString *soundPath = [[NSBundle mainBundle] pathForResource:@"sound" ofType:@"wav"];
NSURL *soundURL = [NSURL fileURLWithPath:soundPath];
AudioServicesCreateSystemSoundID(CFBridgingRetain(soundURL), &soundEffect);