Question

Both work, but which one would you use and why?

@implementation NSString (Extender)

    -(NSString *) stringByTrimmingPrefix:(NSString *)strPrefix 
    {
        while ([self hasPrefix:strPrefix]) 
        {
            self = [self substringFromIndex:strPrefix.length];
        }
        return self;
    }

@end

or

@implementation NSString (Extender)

    -(NSString *) stringByTrimmingPrefix:(NSString *)strPrefix 
    {
        NSString *returnValue = [NSString stringWithString:self];
        while ([returnValue hasPrefix:strPrefix]) 
        {
            returnValue = [returnValue substringFromIndex:strPrefix.length];
        }
        return returnValue;
    }

@end

Answer 1

Option #2.

NSString is intended to be an immutable object. All of the standard “stringBy” methods on NSString return new autoreleased NSStrings.

While #1 also ends up returning a new NSString, it is at best semantically incorrect and at worst altering the referenced object of what was supposed to be an immutable object.