On Tue, Mar 25, 2008 at 12:19 AM, Samvel <[EMAIL PROTECTED]> wrote:
>
> Lots of classes define static methods that create objects for instance:
>
> Class NSString:
> +(id) stringWithString: (NSString *) string;
>
> or Class NSArray:
> +(id) arrayWithObjects: (id) firstObj, ...;
>
> I am really confused about next situation and memory management of
> objects created via those methods. Please, explain.
These methods don't have +alloc, +new, or -copy in their names, so it's your
responsibility to retain the returned object if you're going to need it
later.
> Imagine class with
>
> NSString *string;
>
> instance variable. I create it and initiate with method above in:
> -(id) init;
>
> of my class. Now, want to use this string in another method sometime
> later, say in:
> -(void) plug;
>
> Should I retain string in init?
> e.g.:
>
> -(id) init
> {
> // ...
> string = [NSString stringWithString: @"hello"];
> [string retain];
> // ...
> }
>
> -(void) dealloc
> {
> // ...
> [string dealloc];
> // ...
> }
Retaining "string" is required here - you created it with a method named
+stringWithString:, and you need to use it later.
But, I wouldn't assign directly to the variable, nor would I place the call
to -retain in your -init method. That's fragile - think about what happens
if you want to assign a new value to string from elsewhere in your code.
You'd have to remember how your -init method first created it, release the
old value, and also remember to retain the new value, because -dealloc will
eventually release it. That breaks encapsulation; why should you have to be
thinking about what's inside your -init method, when you're writing a -foo
method? And you'd have to repeat this every time you assigned a new value,
leaving retains and releases scattered through your code - forget just one
of them, and you'll have a difficult bug to find.
Instead, I would create a setter method:
- (void)setString:(NSString *)value {
if (string != value) {
[string release];
string = [value copy];
}
}
And, in your -init, and anywhere else you need to assign a new value to
string, just do this: [self setString:[NSString
stringWithString:@"string"]].
Now, your retain and release calls are grouped in one place, making it very
simple to verify that they're balanced properly.
sherm--
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