[haiku-gsoc] Re: locale kit patch : use haiku hash map instead of stl one.
- From: Stephan Assmus <superstippi@xxxxxx>
- To: haiku-gsoc@xxxxxxxxxxxxx
- Date: Tue, 21 Jul 2009 22:37:07 +0200
Hi,
On 2009-07-21 at 21:10:16 [+0200], Oliver Tappe <zooey@xxxxxxxxxxxxxxx>
wrote:
> On 2009-07-21 at 16:55:02 [+0200], Stephan Assmus <superstippi@xxxxxx>
> wrote:
> >
> [ ... ]
> >
> > This code works fine, but you can save some cycles if you change it
> > like this:
> >
> > - CatMap::const_iterator iter;
> > - for (iter = fCatMap.begin(); res == B_OK && iter != fCatMap.end();
> > ++iter) {
> > + CatMap::Iterator iter = fCatMap.GetIterator();
> > + while (res == B_OK && iter.HasNext()) {
> > archive.MakeEmpty();
> > - res = archive.AddString("c:ostr", iter->first.fString.String())
> > - || archive.AddString("c:ctxt",
> > iter->first.fContext.String())
> > - || archive.AddString("c:comt",
> > iter->first.fComment.String())
> > - || archive.AddInt32("c:hash", iter->first.fHashVal)
> > - || archive.AddString("c:tstr", iter->second.String());
> > + const CatMap::Entry& entry = iter.Next();
>
> This can't work since iter.Next() does return a temp value. So while in
> general the hint is good, of course, it can't be applied in this case.
Wow, sorry about that. That tip came from memory, and maybe HashMap should
be improved instead. I remember that it needs to return an
uninitialzed/invalid Entry when there is out of bounds access, but this
could as well be a reference to some kind of const "Null" entry.
> [ ... ]
> >
> > You declare the entry right where you need it, which is even something
> > in our coding style, and thereby save the operator=() invokation.
>
> Well, I hope that our coding style does not recommend declaring variables
> inside of loops unless necessary, as that should really waste a couple of
> cycles instead of saving them (because of the additional invocations of
> constructors and destructors on that variable).
>
> Or am I missing something?
The compiler should usually notice the possibility for optimization, no?
But when I am in doubt and I really want to save the cycles, I declare the
variable outside. However, in most situations, it would work as I proposed,
or it's a while loop that has the object declared outside anyway, like
BDirectory::GetNextEntry()...
Best regards,
-Stephan
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