Using substringFromIndex on NSMutableString

Question

Hey all

Why is substringFromIndex not working for my NSMutableString?

Here code similar to what I have:

NSMutableString *myString = [[NSMutableString alloc] initWithString:@"This is my String"];

[myString substringFromIndex:5

NSLog(@"myString = %@", myString); //will output This is my String

If I use substringFromIndex on NSString it will work, for example like so:

NSString *tempStr = [[NSString alloc] init];
tempStr = [myString substringFromIndex:5]; 
NSLog(@"tempStr = %@", tempStr); //will output is my String 

Why does it not work in the first example, and I have one more question, if I do it using the second method, and then I set:

[myString setString:tempStr];
[tempStr release];

This will result in a crash, I thaught, since I used setString on NSMutableString, that I do not need the NSString and I release it, but apparently that is not the case, however if I use autorelease it will be OK

Answer 1

That method never alters the string you call it on. It returns a new string in both cases. So assign it to a new string variable and your good.

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It's crashing because you over releasing one object and and leaking another. You alloc the first string, then make a new autoreleased string from substringFromIndex:, then release it. You dont need to try this hard.

Simply assign the output of the substring method to a variable, and let it be autoreleased for you. No alloc, no release.

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A full proper example might look like this:

NSMutableString *myString = [[NSMutableString alloc] initWithString:@"This is my String"];
NSString *tmpString = [myString substringFromIndex:5];
NSLog(@"tempStr = %@", tempString);
[myString setString:tempStr];

// later do [myString release]

or even simpler:

NSString *myString = @"This is my String";
myString = [myString substringFromIndex:5];

Answer 2

[myString substringFromIndex:5] returns a new NSString that starts from the specified index. It does not modify myString.

try this code instead:

NSLog(@"myString = %@", [myString substringFromIndex:5]);