Re: [OT] Retain count riddle
Re: [OT] Retain count riddle
- Subject: Re: [OT] Retain count riddle
- From: Bob Smith <email@hidden>
- Date: Wed, 31 Mar 2004 13:46:22 -0800
This discussion raises a question I'd like to pose. (I'm relatively
new to Cocoa and Objective-C, so apologies if this is a dumb question
or one already answered a hundred times.) In this example the macro
does not work because [path lastPathComponent] returns a newly created
and autoreleased object, so each time it is invoked you get a different
object. However, if that method returned a weak reference to an
object, such as methods like [mySubView window] do, the macro would
have worked fine because the returned object would always be the same
one. How do I know which sort of object reference a particular method
is going to return? Likewise, in designing my own objects, are there
any rules I should follow about this sort of behavior?
Bob S.
On Mar 30, 2004, at 12:23 PM, Christopher Behm wrote:
Well, sub in your arguments to the macro:
[[path lastPathComponent] retain];
[m_Path release];
m_Path = [path lastPathComponent];
If lastPathComponent returns a new object at each call (and it
probably does, likely an autoreleased string), then you'd have your
problem. Even if it didn't return a new object, it'd be bad practice
to do it that way. I'm not macro guru, so I don't know if there's a
way to get what you want either.
Chris
On Tuesday, March 30, 2004, at 02:49PM, Sailesh Agrawal <email@hidden>
wrote:
So I had this bug the other day that I thought would make an
interesting
riddle.
The code looked like this:
#define SAFE_SET(old, new) [new retain]; [old release]; old = new;
...
- (void) foo : (NSString*)path {
SAFE_SET(m_Path, [path lastPathComponent]);
}
Now the problem was any time I accessed m_Path my app would crash.
This
isn't cocoa specific, I'm just dumb for using macros where I should
have
been using a function. Can anyone guess what the problem is ?
good luck !
Sailesh
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