On 2011-06-29, at 18:09 , Matt Neuburg wrote:
> As I said then, "Ownership is *never* something you are magically given.
Actually I have to correct my previous statement, since it is incorrect. I said:
Following your reasoning, Apple has to re-design Cocoa and re-write all guides
and sample code, since no method should ever return a retain+autorelease
result, instead all code must always follow the (currently inexistent rule):
"An object returned by another object is only guaranteed to live as long as the
object lives that returned it. To extend its lifetime you always have to retain
the object".
However, this is far from true. Even if the object stays alive and even if you
don't call a setter for this property (e.g. it might even be a readonly
property), any other call to the parent might cause it to release the previous
returned object. So I always would have to retain any object I received through
any method call you can think of before using it. Even if I just want to pass
it on I have to retain it:
id value = [someObject value];
[value retain];
[someOtherObject setValue:value];
[value release];
Why? How can I know that someOtherObject won't have a reference to someObject
and that calling setValue of someOtherObject won't make a call to someObject
which causes some object to release the value it returned before, without
someOtherObject having a chance to retain it? Since someOtherObject cannot know
from where it received the value and it certainly won't expect this value to
"go away" just because it calls some method of someObject. Does that make any
sense? If not, maybe with some more code. Assume that my code is only like that:
[someOtherObject setValue:[someObject value]];
Pretty typical code you find in millions of source files. However, now consider
someOtherObject has a reference to someObject and part of their implementation
will look like this:
// someObject
- (id)value
{
return value;
}
- (void)doSomething
{
// ...
[value release];
value = ...;
/ /...
}
// someOtherObject
- (void)setValue:(id)aValue
{
[someObject doSomething];
if (value == aValue) return;
[value release];
value = [aValue retain]; // <--- BANG, CRASH
}
So I would have in fact to always retain any value before doing anything with
it, even when I just want to pass it on! Think about it.
Kind regards,
Markus_______________________________________________
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