From 8354885584193779288 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,2e93f66caf45e850 X-Google-Attributes: gidf78e5,public From: Valentin Bonnard Subject: Re: foo++ vs. ++foo (was Re: var::x) Date: 1998/04/29 Message-ID: <35473903.167EB0E7@pratique.fr>#1/1 X-Deja-AN: 348754149 Approved: Fergus Henderson Content-Transfer-Encoding: 7bit References: <6hac24$8lb$2@news2.isdnet.net> <6hd9vn$dau$1@camel15.mindspring.com> <354185A9.AB4@pratique.fr> <35461308.52A9@noSPAM.central.beasys.com> X-Original-Date: Wed, 29 Apr 1998 16:28:19 +0200 Content-Type: text/plain; charset=us-ascii Organization: Ecole Normale Superieure, Paris, France X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBEAgUANUc5xuEDnX0m9pzZAQHzbAF0CBTj8gOqsa4C3CY59q4mvOqo8MqxRDG7 OOKmjsX+Y8SNAsps6EBBKrwfyxKRj5k= =pfD/ Mime-Version: 1.0 Newsgroups: comp.std.c++ David R Tribble wrote: > > On 20 Apr 1998, Chuck McCorvey wrote: > >>> Also, notice that I used "++i" rather than "i++" in the macro. > >>> This can make a significant performance difference as the former > >>> avoids the necessity of creating a temporary. > > B. James Phillippe wrote: > >> Really? How so? I have never heard this before, and I generally > >> use foo++ over ++foo (in situations where it doesn't matter). > > Valentin Bonnard wrote: > > ++x increment x and is the x lvalue; x++ increment x and > > returns the rvalue of x before the increment; so a copy of > > x has to be made: > [bobbit] > > No, Phillippe is correct. The compiler can optimize almost anything, I have explainned in details how and when the compiler can optimise. > and 'x++' and '++x' will probably be translated into identical > code in most cases. For built-ins, yes (they are a particular case of PODs). > the argument that 'p' is more efficient than 'p != NULL' since > the latter requires a comparison; it's a bogus argument. ;-) I like this one. > > But in general, if the appropriate functions aren't inline, > > or the compiler doesn't optimise well, it's better to use > > prefix ++, even on an iterator. > > No. IT'S BETTER TO WRITE READABLE CODE. Who said the contrary ? Do you think that postfix increment in the middle of an expression enhance readabillity ? Are there any common cases where ++it isn't readable but it++ is ? > Leave the optimizing to the compiler, that's it's job, > and it's better at it than you. The compiler is (on average) stupid; programmers are (on average) intelligents. The compiler kmows the machine well; I don't know it. So I let the compiler optimise the low-level details for me, but I will help him (when it's possible). > I'll wager than 98% of your > increments account for less than 2% of the execution time of your > program, so making them "faster" is a waste of time and just adds > to the confusion of the poor guy who has to maintain your code. I would say that it can make a difference for programs using the STL a lot. -- Valentin Bonnard mailto:bonnardv@pratique.fr info about C++/a propos du C++: http://pages.pratique.fr/~bonnardv/ --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://reality.sgi.com/austern_mti/std-c++/faq.html ]