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List:       kde-commits
Subject:    Re: playground/pim/krss/resource
From:       Marc Mutz <marc () kdab ! net>
Date:       2008-05-06 13:44:22
Message-ID: 200805061544.24830.marc () kdab ! net
[Download RAW message or body]

On Monday May 5 2008 15:19, Dmitry Ivanov wrote:
> On 5/5/08, Frank Osterfeld <frank@kdab.net> wrote:
> > On Sunday 04 May 2008 21:50:15 Dmitry Ivanov wrote:
> > > SVN commit 804009 by divanov:
> > >
> > > Get sourceFeedId from the collection being retrieved
> > >
> > >
> > >  M  +2 -3      rssresource.cpp
> > >  M  +1 -0      rssresource.h
> > >
> > >
> > >
> > > --- trunk/playground/pim/krss/resource/rssresource.h #804008:804009
> > > @@ -48,6 +48,7 @@
> > >
> > >  private:
> > >
> > > +        KFeed::Item convertToKFeedItem(const Syndication::ItemPtr
> > > syndItem);
> >
> > Better pass it per const reference:
> >
> > const Syndication::ItemPtr & syndItem
>
> That would be a double indirection to the data (i.e. the Item) because
> ItemPtr is just a wrapper containing the pointer to the Item.
> So if syndItem is passed as a reference it would be basically a reference
> to a pointer.
>
> I learnt this trick from Thiago's blog [1] though I'm not completely sure
> about this very case.
>
> [1] http://labs.trolltech.com/blogs/2008/04/28/string-theory/
<snip>

This has premature pessimisation written all over it. This is a slow function, 
you can't compare it to the well-researched case of QLatin1String, which is 
used 1000x more frequently than this function here.

Whether or not this is an optimization at all depends on the actual 
implementation of the smart pointer. E.g. sizeof(std::tr1::shared_ptr<>) can 
be 2*sizeof(void*), or 1*sizeof(void*), and only the second implementation 
would benefit from passing in registers, the first one would be too large. On 
top of that, shared pointers need to update the reference count on copying, 
which has a much bigger performance impact than pass-in-registers.

Please follow standard advice (as found e.g. in "C++ Coding Standards" by 
Sutter/Alexandrescu) in C++, and leave optimizations for when they're needed.

Thanks,
Marc

-- 
Marc Mutz - marc@kdab.com, mutz@kde.org - Klarälvdalens Datakonsult AB
Platform-independent software solutions - www.kdab.com info@kdab.com

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