Re: NSAttributeString source code....

[Richard Frith-Macdonald]

On 2 Jun 2006, at 10:24, Lloyd Dupont wrote:

> In NSAttributedString I found the following code (snippets)
> ======= NSAttributedString.m (snippets) ==============
>
> static SEL getSel;
>
> @implementation NSAttributedString
>
> + (void) initialize
> {
>  if (self == [NSAttributedString class])
>    {
>      getSel = @selector(attributesAtIndex:effectiveRange:);
>    }
> }
>
> - (NSDictionary*) attributesAtIndex: (unsigned int)index
>       longestEffectiveRange: (NSRange*)aRange
>       inRange: (NSRange)rangeLimit
> {
>  NSDictionary *attrDictionary;
>  IMP  getImp;
>
>  getImp = [self methodForSelector: getSel];
>  attrDictionary = (*getImp)(self, getSel, index, aRange);
>  return attrDictionary;
> }
> @end
>
> ===========================================
>
> Now I wonder.....
> Why in Hell write that?
>
> I mean what's wrong with:
>  attrDictionary = [self attributesAtIndex: index effectiveRange:  
> aRange]
> Why replace it by:
>  static SEL getSel;
>  getSel = @selector(attributesAtIndex:effectiveRange:);
>  getImp = [self methodForSelector: getSel];
>  attrDictionary = (*getImp)(self, getSel, index, aRange);
>
>
> Yes, why????
> why GNUstep code work so hard at being so obscure???

Well if the code was as listed above it would be really stupid ...  
but of course it's not.

In fact the call to (*getImp)(self, getSel, index, aRange) is done  
inside a loop and might get called a lot of times.
Where the attributed string is actually a large document that can  
make a significant difference to performance by removing method  
lookup overheads.

Most (maybe all) cases where the code caches method implementations  
are for situations where it's been tuned for performance because the  
method call overheads were becoming an issue.  By the nature of  
things this needs to be done mostly in heavily used parts of the base  
library ... and the fact that it's done there does *NOT* in any way  
imply that it's sensible to do that sort of thing routinely in  
applications (or other libraries).  That is to say, the base library  
source is a good example of how to write a base/foundation library,  
not a good example of how to write general code.  I almost never  
cache method implementations in my appllication code.