NSString length and retainCount. Clarification needed

Question

Based on the following code, please advise

NSString *str= [[NSString alloc] initWithString:@"Hello world"];   

NSLog(@"Length: %lu\n", [str length]);              // 11
NSLog(@"Retain count is %lu\n", [str retainCount]); //1152921504606846975

[str release];

NSLog(@"%lu\n", [str length]);                      // 11
NSLog(@"Retain count is %lu\n", [str retainCount]); //1152921504606846975

Originally I was wondering why the number was so large but then saw a a post explaining it. Let me ask this instead ... Why does this number change greatly whether i use %d vs %lu. Originally, i used %d, but got a warning saying that "Conversion specified type int but the argument has the type NSUInteger (aka unsigned long). A fix was to change %d to %lu"
Why doesn't the retain count decrement? Large number still shows up unchanged, after the str is sent release
Why am i still able to access the str, after it was sent release?

jtbandes · Accepted Answer · 2011-07-18T00:33:55.610

8

This may be a hard answer to accept, but it's what you should do:

Don't worry about it. (in terms of %d/%lu, those specifiers simply expect different data types, and %d (int) has a much smaller and different range from %lu (unsigned long))
Don't worry about it.
Don't do it, and especially don't rely on it.

It may be because you started with a constant string (@"Hello world") that the memory isn't being deallocated when you call release, and the retainCount is large. But if you have to care about the retainCount, you're doing it wrong.

You are releasing the string in the right place, and that's what matters — don't ever try to use it later.

edited Jul 18 '11 at 00:33

answered Jul 18 '11 at 00:28

jtbandes

115,675
35
233
266

everything I've read about retain counts in objective-c leads me to the same conclusion. Don't worry about it. – iandotkelly Jul 18 '11 at 00:31
Soon we'll have ARC, and then we'll see what the next level of difficulty is. – Abizern Jul 18 '11 at 00:39
The next level of difficulty will be mentally grasping object graphs :) – jtbandes Jul 18 '11 at 00:41
1

@jtbandes - In that case there will be a lot of questions asking about how to fix all the compiler errors Xcode and LLVM will throw up. – Abizern Jul 18 '11 at 00:51
@Abizern: yeah, like "why can't I call `-release`??!1!" – jtbandes Jul 18 '11 at 00:55

score 0 · Answer 2 · answered Jul 18 '11 at 03:52

ultimately, the behaviour is defined by the implementation of NSString -- just use retain/release properly in every case and you will be safe.

what's likely happening: NSString literals are immortal. you can write while (1) { [@"String" release]; } and there will be no issue.

the copy ctor you have called is likely returning a pointer to the string literal. in pseudocode:

- (id)initWithString:(NSString *)str
{
    self = [super init];
    if (self != nil) {
        if ([str _isStringLiteral] || [str _isImmutable]) {
            [self release];
            return [str retain];
        }
        ...
}

in this case, you are returned the pointer to the string literal, when passed through [[NSString alloc] initWithString:stringLiteral]. since sending release to the immortal string literal does nothing, you can see why it is still a valid object following what you could presume is a released object.

never rely on such details/optimizations, just use normal lifetimes and ref counting and you'll be fine.

NSString length and retainCount. Clarification needed

2 Answers2

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