Re: What Is The Difference Between i++ and ++i?

From:
"Alan Carre" <alan@twilightgames.com>
Newsgroups:
microsoft.public.vc.mfc
Date:
Sat, 13 Dec 2008 07:21:45 +0700
Message-ID:
<O3bk$iLXJHA.2016@TK2MSFTNGP03.phx.gbl>
"Giovanni Dicanio" <giovanniDOTdicanio@REMOVEMEgmail.com> wrote in message
news:eNW7HjDXJHA.5764@TK2MSFTNGP04.phx.gbl...

Landon wrote:

I use MFC. I would like to know the difference of ++ if it is used
preceeding the variable name and if it is used after the variable name?


To add to what Alec S. wrote, there is a difference in performance when
'i' is not a simple integer, but an STL iterator.

For example, considering this code to iterate in a STL container (like
std::vector, or std::list...):

   typedef std::vector< CString > Strings;
   Strings strings;
   Strings::iterator i;
   for ( i = strings.begin(); i != strings.end(); i++)
   {
       ...
   }

It is considered better practice to use the prefix form ++i instead of the
postfix form i++:

   for ( i = strings.begin(); i != strings.end(); ++i)
   {
       ...
   }


It's not a minor difference either. The speed difference between the two
loops is several orders of magnitude (base 10).

I remember porting some MFC list stuff over to STL and I innocently wrote
down itr++ for a bunch of loops (a game's sprite list, just a few hundred
objects tops at 125 updates / second). Well I ran it and the thing was so
sluggish I had to use taskman to kill it. Couldn't even get the cursor over
to the exit button! I then switch everything to ++itr and the speed was
identical to the MFC version. That is, no delay, everything going as fast as
possible (Update time was barely measurable in nanoseconds - compared with
"deci-seconds" for the post increment).

Of course this is probably all to do with the safety/security enhancements
to the STL which might be great for catching bugs, but unfortunately creeps
in when a temporary is created as a return value (and it doesn't get
optimized away as far as I can tell).

- Alan Carre

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