Re: Strings and NSRanges

[Richard Frith-Macdonald]

On 19 Nov 2006, at 15:45, Graham J Lee wrote:

> Hi,
>
> I'm trying to implement NSNumberFormatter's -stringForObjectValue:  
> as I said on bug-.  However, I'm having trouble with parsing the  
> format string and I expect it has more to do with my confusion than  
> it does with any other problems ;-).
>
> The Cocoa behaviour for the formatter is that if the character '.'  
> appears anywhere in the format string, this "turns on" displaying  
> the fractional part of the number (unless allowsFloats is false)  
> with as many decimal places as the are placeholder characters (# or  
> a number) to the *immediate* right of the *rightmost* '.' in the  
> format string.  So I'm doing this:
>
> >   if ([self allowsFloats] && (NSNotFound != [useFormat  
> > rangeOfString:@"." ].location))
> >     {
> >       decimalPlaceRange = [useFormat rangeOfString: @"." options:  
> > NSBackwardsSearch];
> >       while ([placeHolders characterIsMember: [useFormat  
> > characterAtIndex: NSMaxRange(decimalPlaceRange)]])
> >         {
> >           decimalPlaceRange.length++;
> >           if (NSMaxRange(decimalPlaceRange) == [useFormat length])
> >             break;
> >         }
> >       decimalPlaces=decimalPlaceRange.length;
> >       if (0 != decimalPlaces)
> >         displayFractionalPart = YES;
> >     }
>
> which doesn't work...decimalPlaces ends up being some garbage value  
> such as 2412439 when the length of the string is only, say, 7.  For  
> that to happen then the length of the *range* must have always been  
> greater than the length of the string (so that the if(NSMaxRange 
> (...)...) line never breaks the loop) and the loops *starts* by  
> reading nonsense memory outside the string.  Why would that happen?

It's hard to tell without a complete test program ... but I can make  
a couple of observations.

1. the loop should not be able to start by 'reading nonsense memory  
outside the string' because the -characterAtIndex: method should  
raise an exception if asked to refer to an index outside the string.
2. if useFormat is nil then the code is totally broken since sending  
a message to a nil object is defined to return nil ... but where the  
method is expected to return a range this means that the resulting  
behavior is undefined.  On sparc it will cause an immediate crash,  
but on intel I think it probably just means the returned range  
contains garbage ... which would explain your symptoms.