Question

I recently rewrote some code in one of my classes, which gave me an error with an NSString. Here is what I have now:

My class header:

@interface MyViewController : UITableViewController {
   NSString *myString;
}

@property (nonatomic, retain) NSString *myString; // Or copy instead of retain.

@end

And implemented some methods:

- (void)viewDidLoad {
    myString = @"This is";

    if (something) {
        myString = [NSString stringWithFormat:@"%@ a string.", myString]; // *1
    }

    [myString retain]; // <-- Why do I have to retain/copy here?
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    if (indexPath.row == 0) {
        /* Some code for creating a UITextView called myTextView */
        // ..and then setting the text property:
        myTextView.text = myString; // <-- Crashes here if I don't retain/copy in viewDidLoad.
    }
}

After some debugging I figured I had to retain/copy the NSString.

Why do I have to retain/copy the NSString in viewDidLoad if I want to use it later on?

Also, I noticed that if I remove the line marked *1, I don’t have to retain/copy.

Answer 1

If you use the synthesized setter method to assign the variable, it will automatically retain the NSString for you:

if (something) {
    self.myString = [NSString stringWithFormat:@"%@ a string.", myString]; // *1
}