using an NSString in its customized setter causes infinite threads

Question

Firstly, I wrote my customized setter for an NSString* like this:

- (void)setDateString:(NSString *)newDateString {
    self.dateString = newDateString;
    NSInteger dateNumber = [dateString integerValue];
        // this line causes crash

    // do something here..blah blah
}

then the program stops due to infinitely many threads which does [XXX setDateString:].

After several useless tries I found this question/answer which tells me

do not use self. inside of custom accessors. access the variable directly,

so I made my code into

- (void)setDateString:(NSString *)newDateString {
    //self.dateString = newDateString;
    dateString = newDateString;
    NSInteger dateNumber = [dateString integerValue];

    // do something here..blah blah
}

then everything works like a charm!!

I am a junior developer of some objective languages, and a newbie to Objective-C.

I want to learn in details for this issue, instead of solving problems without understanding the reason.

So please provide me with some materials/website to understand more about this.

BTW, I use ARC.

Thank you all. :)

Note that those aren't threads. Those are stack frames, which are quite different from threads. — Catfish_Man, Jun 14 '12 at 19:04
You are right!! I was wondering why it does setter in several threads. Yes, they are stacks. — tddian, Jun 14 '12 at 19:17
like [this](https://lh5.googleusercontent.com/-XI195OdmaBk/T9o5yyOrYDI/AAAAAAAAL00/o3EHym1nPWU/s800/Screen%2520Shot%25202012-06-15%2520at%25203.21.21%2520AM.png) — tddian, Jun 14 '12 at 19:23

CodaFi · Answer 1 · 2012-06-14T18:54:01.410

3

The problem isn't actually this line:

NSInteger dateNumber = [dateString integerValue];

It is this line:

self.dateString = newDateString;

You see, that equals sign is akin to literally calling [self setDateString:newDateString] (in fact, that is what the compiler reinterprets it as) which creates an infinite loop. Naturally, commenting out that line would never have created that loop in the first place.

edited Jun 14 '12 at 18:54

answered Jun 14 '12 at 18:51

CodaFi

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score 3 · Accepted Answer · edited May 23 '17 at 10:24

3

self.dateString = newDateString;

is equivalent to

[self setDateString:newDateString];

so this will cause infinite recursion because you are calling it inside -setDateString:.

See also Difference between self.ivar and ivar?.

edited May 23 '17 at 10:24

Community

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answered Jun 14 '12 at 18:51

kennytm

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Thanks all! I got better understanding. – tddian Jun 14 '12 at 19:12

score 2 · Answer 3 · answered Jun 14 '12 at 18:52

What happend is that you kept calling the function again and again causing a calling functions to fall in stak over flow

Whats wrong is int this line

self.dateString = newDateString;

This line is a objective c property, it has a getter and setters method, these method are automatically generated when you @synthesize it the setter and getters method for dateString are:

 - (void)setDateString:(NSString *)newDateString;

and

 - (NSString*)dateString;

So when you call self.dateString = newDateString; This line will call this function again

- (void)setDateString:(NSString *)newDateString {

And this function again contains the self.dateString = newDateString;, your call stack will get deeper and deeper till you are out of stack storage

using an NSString in its customized setter causes infinite threads

3 Answers3