NSString stringWithFormat called XX times causes memory warning

Question

I started to monitor app memory usage using technique described in here: Programmatically retrieve memory usage on iPhone

I wrote 3 tests to try it out and this is what I found:

- (void)test1 {
for (int i = 0; i < 1000; i++) {
    NSMutableString *str = [NSMutableString stringWithString:@""];
    for (int j = 0; j < 1000; j++) {
        [str appendString:@"some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string "];
    }
}    

- (void)test2 {
for (int i = 0; i < 100000; i++) {
    @autoreleasepool {
        NSString *stri = @"";
        stri = [NSString stringWithFormat:@"%d some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really", i];
    }
}


- (void)test3 {
NSString *str = @"";
for (int i = 0; i < 500; i++) {
    str = [str stringByAppendingFormat:@"%d some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really", i];
}

When I call test1 or test3 memory gets allocated and deallocated properly - I can see it using the report_memory function described in the link above. But when I call test2 memory does not get deallocated - report_memory goes up and up. If I call test2 several times my app receives memory warning and gets terminated.

I'm using ARC. Can anyone explain what's going in here?

where are you using result of stringWithFormat... like NSString *str =[NSString stringWithFormat:@""]; — Rajneesh071, Sep 24 '12 at 09:38
so why are you writing stringWithFormat when you are not using it further — Rajneesh071, Sep 24 '12 at 10:15
apple has plenty of doc explaining how to drain the autorelease pool if you are creating many of them in a tight loop; — deleted_user, Sep 24 '12 at 10:20
Found it!!! I enabled Zombie Objects and forgot about them thus any allocated objects were not getting released even when I was using the @autoreleasepool block. — Dmitry Labetsky, Sep 24 '12 at 11:07

score 1 · Answer 1 · answered Sep 24 '12 at 09:41

1

stringWithFormat returns a NSString object who is autoreleased. Thus, the object will be released when autorelease pool is released at the end of this thread.

answered Sep 24 '12 at 09:41

Kyokook Hwang

2,682
21
36

1. I update memory usage using NSTimer so memory usage gets update in a different event loop. 2. I call test2 several times using UIButton - different event loop here too. – Dmitry Labetsky Sep 24 '12 at 09:45

score 1 · Accepted Answer · answered Sep 26 '12 at 11:25

1

Nailed it!!! I enabled Zombie Objects and forgot about them thus any allocated objects were not getting deallocated even when I was using the @autoreleasepool block.

answered Sep 26 '12 at 11:25

Dmitry Labetsky

31
5

score 0 · Answer 3 · answered Sep 24 '12 at 09:40

This is because you are using separate auto released instances on each iteration. The first case you have a single object called str and you are appending to the same instance. That is why the memory is not leaking there.

Try change the call like this and check,

- (void)test2 
{
    NSString *str = @"";
    for (int i = 0; i < 100000; i++) 
    {
       str = [str stringByAppendingFormat:@"%d some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really long string some really", i];
    }
}

Thank you for your suggestion - this does work for i < 500. I updated my question and it still remains open. — Dmitry Labetsky, Sep 24 '12 at 10:15

NSString stringWithFormat called XX times causes memory warning

3 Answers3