Memory management bug in formatter class

I have followed all the rules of memory management but can someone explain to me why this is causing a double free exception:

- (NSString *)formattedStringWithDecimal:(NSDecimalNumber *)decimalNumber
{
    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
    [formatter setMaximumFractionDigits:0]; //zero deimal spaces
    [formatter setRoundingMode: NSNumberFormatterRoundHalfUp]; //round up

    return [[formatter stringFromNumber:decimalNumber] autorelease];
}

This is how I am calling it:

//Set the label of the cell formatting the distance from the device 
cell.textLabel.text = [NSString stringWithFormat:@"%@ - %@ miles", 
                               selectedAirport.name, [self formattedStringWithDecimal:selectedAirport.milesFromDevice]];

I have subsequently changed it to this which now works - however I still don't know what I did wrong in the first piece of code.

- (NSString *)formattedStringWithDecimal:(NSDecimalNumber *)decimalNumber
{
    NSNumberFormatter *formatter = [[NSNumberFormatter al开发者_如何学JAVAloc] init];
    [formatter setMaximumFractionDigits:0]; //zero deimal spaces
    [formatter setRoundingMode: NSNumberFormatterRoundHalfUp]; //round up

    NSString *result = [NSString stringWithString:[formatter stringFromNumber:decimalNumber]];
    [formatter release];
    return result;
}

---

The method stringFromNumber: returns an autoreleased string. In the first method, that returned string is being autoreleased a second time. Also, in the first method, formatter was never being released. In short, the string was being released an extra time and the formatter was leaking.

The second method looks much better. Perhaps there was some confusion between releasing formatter and releasing the string returned by stringFromNumber:. The line:

return [[formatter stringFromNumber:decimalNumber] autorelease];

sends the autorelease message to the string produced by [formatter stringFromNumber:decimalNumber], not to the formatter itself.