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autorelease: how does this work!? (if at all)
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autorelease: how does this work!? (if at all)


  • Subject: autorelease: how does this work!? (if at all)
  • From: Jonny Taylor <email@hidden>
  • Date: Fri, 18 Jun 2010 16:00:40 +0100

I've just been looking back at some code that has been working fine for me for a while, and as far as I can see it shouldn't actually work! I'd be interested for peoples' comments. The code is as follows:

dispatch_async(queue1,
^{
	NSImage *theImage = [frame GetNSImage];
	NSData *tiffRep = [theImage TIFFRepresentation];
	dispatch_async(queue2,
	^{
		[tiffRep writeToFile:[NSString stringWithFormat:@"%@%d.tif",
					[[frame Camera] ExportFilePrefix], [frame FrameNumber]]
				atomically:YES];
	});
	[theImage release];
});

Work running on serial queue "queue1" calculates a TIFF representation for an image, and then schedules work on serial queue "queue2" to write that data to disk. What I can't work out is why tiffrep isn't autoreleased as soon as the outer block completes. Is the compiler/runtime being clever enough to retain it because it is going to be needed in the inner block (if so: very clever!)? If not, am I just getting lucky here with exactly when/how grand central does its autorelease cleanup? Or maybe the TIFF representation and/or my frame data is still being retained elsewhere for a while (possible, depending on how the thread timings work out...).

I'm pretty new to Cocoa so I'm keen to understand this properly - I'd be interested to hear peoples thoughts on this: is what I am doing ok, or do I need to add some explicit retain/releasing of tiffRep?

Cheers
Jonny_______________________________________________

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