iOS Freeing up objects

[pinsrw]

Hi all,

Can someone clarify what the deal is with freeing up objects. I am wondering, is this valid:

NSString *foo = [[NSString alloc] initWithCString: myStr];
[my_label setText: foo];
[foo release];

This type of usage seems to cause an error. But I'm wondering, what about this?

NSString *foo = [[NSString alloc] initWithCString: myStr];
NSMutableString *moo = [[NSMutableString alloc] initWithString: @"moo"];
[moo appendString: foo];
[foo release];

When I've searched for tutorials on this I don't find much via google except spam pages.

Thanks.

 

[Menge]

The first usage is perfectly valid because the convention is that setText will either [copy] or [retain] the object. It should only throw an error if "myStr" is invalid.

The second usage has a memory leak in that it leaks "moo".

In Objective-C, if you allocate it via [[Class alloc] init(...)], then you must either [release] or [autorelease].

 

[pinsrw]

In Objective-C, if you allocate it via [[Class alloc] init(...)], then you must either [release] or [autorelease].

As opposed to using [Class new] ?

 

[chown33]

As opposed to using [Class new] ?

Consult the Memory Management Rules:
http://developer.apple.com/library/ios/#documentation/cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html

The basic rule is simple enough you can memorize it. If you come across a situation where you're unsure, consult the rules.

 

[pinsrw]

Hi again folks,

Something that makes little sense. If I try to free up the NSIndexPath below, my program crashes. Is there some special rule about using this class with UITableView?

int total = myFunc();
NSIndexPath *ip = [NSIndexPath indexPathForRow: total-5
                    inSection: 0];
[table scrollToRowAtIndexPath: ip
       atScrollPosition: UITableViewScrollPositionTop
        animated: NO];

Thanks.

 

[chown33]

Reread the Memory Management Guide (see above post), especially the Rules section. Start with the fundamental rule, and ask yourself this question: "Do I own the NSIndexPath when I'm telling it to release?". If you don't know what ownership is, you haven't read the guide.

 

[robbieduncan]

Hi again folks,

Something that makes little sense. If I try to free up the NSIndexPath below, my program crashes. Is there some special rule about using this class with UITableView?

int total = myFunc();
NSIndexPath *ip = [NSIndexPath indexPathForRow: total-5
                    inSection: 0];
[table scrollToRowAtIndexPath: ip
       atScrollPosition: UITableViewScrollPositionTop
        animated: NO];

Thanks.

What if total is less than 5? I'm pretty sure scrolling to a negative index is not possible

 

[pinsrw]

What if total is less than 5? I'm pretty sure scrolling to a negative index is not possible

Yes, I checked for that earlier. Thanks for your concern.

 

[hayesk]

Hi again folks,

Something that makes little sense. If I try to free up the NSIndexPath below, my program crashes. Is there some special rule about using this class with UITableView?

int total = myFunc();
NSIndexPath *ip = [NSIndexPath indexPathForRow: total-5
                    inSection: 0];
[table scrollToRowAtIndexPath: ip
       atScrollPosition: UITableViewScrollPositionTop
        animated: NO];

Thanks.

Basically,

Classname *myThing = [Classname thingFromParameter:myParameter];

is autoreleased already. You don't need to free it because you don't own it.

If you had

Classname *myThing = [[Classname alloc] initWithMyParameter:myParameter]

then it would be retained since you allocated it yourself.