C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Possible Defect report in various mutating algorithms          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Chris Jefferson Guest Posted: Sat Feb 26, 2005 3:41 pm    Post subject: Possible Defect report in various mutating algorithms I'm fairly sure this is a fairly simple defect report to various mutating algorithms, but I thought I would post before reporting it in an "offical" defect report for comments. Various algorithms (including remove and list::remove if passed an element of the list, and possibly others) take a parameter by reference. They then operate (and possibly alter) some change) a range based on that parameter. Consider for example the following code: A[]={0,1,0,1}; printf("%d,%d,%d",std::remove(A,A+4,*A)-A,A[0],A[1]); It might be expected this would return "2,1,1", meaning that it left two values, both of them one. On g++ 3.3 however, it returns "2,1,0", because the after the first delete operation the value at *A is changed. The only reasonable implementation (I believe) is to simply say that if any parameters passed to a mutating algorithm could be changed during its execution, then the behaviour is undefined. The main I ask this is that I would like to add a test to a debugging C++ standard library implementation which checks exactly this condition, but wanted to first of all check that it should infact be undefined behaviour :) Chris --- [ 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://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Howard Hinnant Guest Posted: Tue Mar 01, 2005 6:20 pm    Post subject: Re: Possible Defect report in various mutating algorithms In article <cvipf2$egq$1 (AT) pump1 (DOT) york.ac.uk>, [email]caj (AT) cs (DOT) york.ac.uk[/email] (Chris Jefferson) wrote: Quote: I'm fairly sure this is a fairly simple defect report to various mutating algorithms, but I thought I would post before reporting it in an "offical" defect report for comments. Various algorithms (including remove and list::remove if passed an element of the list, and possibly others) take a parameter by reference. They then operate (and possibly alter) some change) a range based on that parameter. Consider for example the following code: A[]={0,1,0,1}; printf("%d,%d,%d",std::remove(A,A+4,*A)-A,A[0],A[1]); It might be expected this would return "2,1,1", meaning that it left two values, both of them one. On g++ 3.3 however, it returns "2,1,0", because the after the first delete operation the value at *A is changed. The only reasonable implementation (I believe) is to simply say that if any parameters passed to a mutating algorithm could be changed during its execution, then the behaviour is undefined. The main I ask this is that I would like to add a test to a debugging C++ standard library implementation which checks exactly this condition, but wanted to first of all check that it should infact be undefined behaviour Imho your report and resolution are on target (modulo minor wording issues). -Howard --- [ 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://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Thorsten Ottosen Guest Posted: Thu Mar 03, 2005 6:48 am    Post subject: Re: Possible Defect report in various mutating algorithms "Chris Jefferson" <caj (AT) cs (DOT) york.ac.uk> wrote Quote: I'm fairly sure this is a fairly simple defect report to various mutating algorithms, but I thought I would post before reporting it in an "offical" defect report for comments. Various algorithms (including remove and list::remove if passed an element of the list, and possibly others) take a parameter by reference. They then operate (and possibly alter) some change) a range based on that parameter. Consider for example the following code: A[]={0,1,0,1}; printf("%d,%d,%d",std::remove(A,A+4,*A)-A,A[0],A[1]); It might be expected this would return "2,1,1", meaning that it left two values, both of them one. On g++ 3.3 however, it returns "2,1,0", because the after the first delete operation the value at *A is changed. hm... AFAICT, the above code gives implementation defined behavior because you are not guaranteed that evaluation occurs from left to right in a function call. -Thorsten --- [ 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://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Chris Jefferson Guest Posted: Sat Mar 05, 2005 12:12 am    Post subject: Re: Possible Defect report in various mutating algorithms Thorsten Ottosen wrote: Quote: "Chris Jefferson" <caj (AT) cs (DOT) york.ac.uk> wrote in message news:cvipf2$egq$1 (AT) pump1 (DOT) york.ac.uk... | I'm fairly sure this is a fairly simple defect report to various | mutating algorithms, but I thought I would post before reporting it in | an "offical" defect report for comments. | | Various algorithms (including remove and list::remove if passed an | element of the list, and possibly others) take a parameter by reference. | They then operate (and possibly alter) some change) a range based on | that parameter. | | Consider for example the following code: | | A[]={0,1,0,1}; | printf("%d,%d,%d",std::remove(A,A+4,*A)-A,A[0],A[1]); | | It might be expected this would return "2,1,1", meaning that it left two | values, both of them one. On g++ 3.3 however, it returns "2,1,0", | because the after the first delete operation the value at *A is changed. hm... AFAICT, the above code gives implementation defined behavior because you are not guaranteed that evaluation occurs from left to right in a function call. Woops! Thats true.. sorry about that! However, the same problem still occurs from: A[]={0,1,0,1}; std::remove(A,A+4,*A); printf("%d,%d",A[0],A[1]); Chris --- [ 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://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Pete Becker Guest Posted: Sat Mar 05, 2005 12:12 am    Post subject: Re: Possible Defect report in various mutating algorithms Thorsten Ottosen wrote: Quote: hm... AFAICT, the above code gives implementation defined behavior because you are not guaranteed that evaluation occurs from left to right in a function call. Sorry to sound picky, but it's not implementation defined, it's unspecified. The difference being that the former requires the implementation to document its behavior; the latter does not. In both cases, though, the code is valid. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) --- [ 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://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language, library and standards All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.