Re: NSBitMapImageRep - memory footprint

FROM : Uli Kusterer
DATE : Fri Sep 01 12:15:44 2006

Am 01.09.2006 um 05:43 schrieb Bruce Johnson:
> Is it safe to assume that the removed representation is forever gone
> and it's memory freed?


  Yes. Though I'm not sure why you don't just use [NSImage 
initWithSize: ...]. When you remove an image rep, NSImage will (auto)
release it, relinquishing its ownership. If nobody else retained the 
image rep, it'll be released in response to that.

> Or do I need to somehow release this reps memory footprint to avoid a
> memory leak?


  You don't release anything unless you retained or created it using 
one of the methods that return an object owned by you. See Apple's 
docs on memory management (mmalc posts a link to those in his "Cocoa 
and List resources" post every month or so) for the details of 
"return an object owned by you".

> And that by adding a rep to the NSImage it is retained, so I then must
> release it to avoid an extra increment in the retain count.


  Well, essentially true, but you're thinking of this in a wrong way: 
Don't think about "avoiding extra increments". There is nothing you 
can do to keep NSImage from retaining your object. Instead, what you 
should do is simply release or autorelease anything *owned by you* 
when you don't need it anymore. If you create an NSImageRep using 
alloc/init, you own it, so you release it. Had you used 
imageRepWithData, you wouldn't own the object (in this particular 
case, it would probably be autoreleased already, so you don't dispose 
of it. Simple as that.

  If you want to keep around an object that you don't own, retain it, 
in which case you later have to release it again.

  Don't look at the retain count, because for all you know the object 
itself may start a timer or a new thread which retains it, and then 
your retain count might magically be one more than you expected, but 
still you *mustn't* release it.

  The whole point of reference counting and release/retain is that 
you *don't* have to care who else uses an object. If you want to use 
an object, request ownership by retaining it, do your stuff, release 
it. If nobody else wanted the object, it will get disposed, if anyone 
else has been using it, it'll still be around to serve the other 
code's needs. If it didn't work that way, you'd have to take great 
care not to dispose an object for fear of disposing something under 
someone else's ass, causing a crash. Don't fight it, just go with the 
flow.

Cheers,
-- M. Uli Kusterer
http://www.zathras.de

PS - One nitpick: You can't release a memory footprint, only the 
memory causing it :-)