Objective-C / iPhone: changing TextView.text problem

Why 2 strings concat only when run for the 1st time?

- (void)viewDidLoad {
    NSInteger tmpCurrImg  = 1;
    NSString *tmp1 = [[[someclass instance] getSomeValAtPos:tmpCurrImg];    
    NSString *tmp2 = [[[someclass instance] getOtherValAtPos:tmpCurrImg];
    txt = [tmp1 stringByAppendingString:tmp2];
}

txt is NSString defined in header. Singleton works good, tmp1 and tmp2 point to exactly what I want. SegmentedControl starts this function:

- (IBAction)changeText {
    if (txtChooser.selectedSegmentIndex == 0) {
        txtView.text = txt;
    }
    if (txtChooser.selectedSegmentIndex == 1) {
        txtView.text = txt;
    }
}

When I start this View it displays 2 merged strings. When I switch back to t开发者_如何学Gohis view with SegmentedControl button the app crashes. If I use just tmp1 or tmp2 it works but when I try to call the merged string it doesn't work. Do I forget about something with pointers?

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You have to retain txt if you expect it to stay alive beyond the end of viewDidLoad.

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stringByAppendingString return string that was autoreleased, so you must retain it. All methods that names begin as class name, always return autorelease objects.