C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  swapping std::pairs?          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Azumanga Guest Posted: Tue Sep 14, 2004 3:19 pm    Post subject: swapping std::pairs? Why doesn't std::pair have an overload of swap, which swaps both of it's elements? This would seem to be a natural thing to do, and allow for a more efficent implementation of swap when the elements of the pair can themselves be more efficently swapped. In a similar note, the new "tuple" class in tr1 has a similar problem. I can't see how this could lead to inefficency or a problem, but I assume it must be possible? Thank you, 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 Daniel Krügler Guest Posted: Thu Sep 16, 2004 3:11 pm    Post subject: Re: swapping std::pairs? Hello Azumanga, Azumanga schrieb: Quote: Why doesn't std::pair have an overload of swap, which swaps both of it's elements? This would seem to be a natural thing to do, and allow for a more efficent implementation of swap when the elements of the pair can themselves be more efficently swapped. In a similar note, the new "tuple" class in tr1 has a similar problem. I can't see how this could lead to inefficency or a problem, but I assume it must be possible? Both std::pair and tr1::tuple are heterogenous containers which store their elements by value (and **not** via a handle/pointer). How do you propose swapping two instances of arbitrary types? If you solve the problem to implement a generally valid heterogenous swap template <typename T1, typename T2> void swap(T1& a, T2& b); you also have solved the missing swap problems of pair and tuple ;-) Furtheron, lets assume, you provide such a solution, e.g. using boost::lexical_cast or similar stuff: What is the general advantage of such a function? To my opinion we will end with something like template <typename Policy, typename T1, typename T2> void swap(T1& a, T2& b); of at least one policy to decide, in which way the transaction shall take place (what is the intermediate storage?). Now considering the fact that a class-defined swap member function is usually of special interest, if at least some of the class internals are of handle-like nature, I don't see this advantage in case of value-based heterogenous containers like pair and tuple. Greetings from Bremen, Daniel Krügler --- [ 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 Joe Gottman Guest Posted: Thu Sep 16, 2004 9:13 pm    Post subject: Re: swapping std::pairs? """Daniel Krügler (ne Spangenberg)""" <dsp (AT) bdal (DOT) de> wrote Quote: Both std::pair and tr1::tuple are heterogenous containers which store their elements by value (and **not** via a handle/pointer). How do you propose swapping two instances of arbitrary types? Delegate to the types themselves. template <class X, class Y> void swap(std::pair<X,Y> &p1, std::pair<X, Y> &p2) { swap(p1.first, p2.first); swap(p1.second, p2.second); } If either X or Y has a specialized swap() function defined, this should call that function. Joe Gottman --- [ 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 Guest Posted: Fri Sep 17, 2004 6:15 am    Post subject: Re: swapping std::pairs? Daniel Krügler (ne Spangenberg) wrote: Quote: Hello Azumanga, Azumanga schrieb: Why doesn't std::pair have an overload of swap, which swaps both of it's elements? This would seem to be a natural thing to do, and allow for a more efficent implementation of swap when the elements of the pair can themselves be more efficently swapped. In a similar note, the new "tuple" class in tr1 has a similar problem. I can't see how this could lead to inefficency or a problem, but I assume it must be possible? Both std::pair and tr1::tuple are heterogenous containers which store their elements by value (and **not** via a handle/pointer). How do you propose swapping two instances of arbitrary types? If you solve the problem to implement a generally valid heterogenous swap template <typename T1, typename T2 void swap(T1& a, T2& b); you also have solved the missing swap problems of pair and tuple ;-) Furtheron, lets assume, you provide such a solution, e.g. using boost::lexical_cast or similar stuff: What is the general advantage of such a function? To my opinion we will end with something like template void swap(T1& a, T2& b); of at least one policy to decide, in which way the transaction shall take place (what is the intermediate storage?). Now considering the fact that a class-defined swap member function is usually of special interest, if at least some of the class internals are of handle-like nature, I don't see this advantage in case of value-based heterogenous containers like pair and tuple. I'm not sure if one of us is confused.. it might be me what I am suggesting is introducing into std something much like (sorry if this isn't actually correct code, but I hope it's fairly close!) template void swap(pair<T,U> &a,pair<T,U> &b) { swap(a.first,b.first); swap(a.second,b.second); } At the moment if I call swap on a pair, it simply falls through to the "basic" swap, which does {pair<T,U> temp=a; a=b; b=temp;}. I can't see how adding this specialised swap could ever hurt matters, and if one or both of T or U has it's own specialised swap, then this would improve performance... --- [ 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 Alberto Barbati Guest Posted: Fri Sep 17, 2004 6:15 am    Post subject: Re: swapping std::pairs? Daniel Krügler (ne Spangenberg) wrote: Quote: Hello Azumanga, Azumanga schrieb: Why doesn't std::pair have an overload of swap, which swaps both of it's elements? This would seem to be a natural thing to do, and allow for a more efficent implementation of swap when the elements of the pair can themselves be more efficently swapped. In a similar note, the new "tuple" class in tr1 has a similar problem. I can't see how this could lead to inefficency or a problem, but I assume it must be possible? Both std::pair and tr1::tuple are heterogenous containers which store their elements by value (and **not** via a handle/pointer). How do you propose swapping two instances of arbitrary types? If you solve the problem to implement a generally valid heterogenous swap template <typename T1, typename T2 void swap(T1& a, T2& b); you also have solved the missing swap problems of pair and tuple ;-) IIUC, the OP did not intend to swap the two members of one pair, but to swap two pairs (of the same type) memberwise. That is: template void swap(pair<T1, T2>& p1, pair<T1, T2>& p2) { swap(p1.first, p2.first); swap(p1.second, p2.second); } Notice that there are no swaps between heterogenous types. The OP is surprised, as I am, that such an overload of std::swap is not standardized. Without this overload swapping pairs falls back on the generic std::swap that requires copies of both arguments even if one or both have a more efficient way of swapping. Alberto --- [ 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 Daniel Krügler Guest Posted: Fri Sep 17, 2004 4:26 pm    Post subject: Re: swapping std::pairs? Hello Joe Gottman, Joe Gottman schrieb: Quote: """Daniel Krügler (ne Spangenberg)""" <dsp (AT) bdal (DOT) de> wrote in message news:4147EBC2.5000208 (AT) bdal (DOT) de... Both std::pair and tr1::tuple are heterogenous containers which store their elements by value (and **not** via a handle/pointer). How do you propose swapping two instances of arbitrary types? Delegate to the types themselves. template <class X, class Y void swap(std::pair { swap(p1.first, p2.first); swap(p1.second, p2.second); } If either X or Y has a specialized swap() function defined, this should call that function. OK, I obviously misunderstood the OP's intend, because I interpreted him as wishing a heterogenous swap between first and second. Thanks for clarification, Daniel --- [ 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 Jonathan Biggar Guest Posted: Fri Sep 17, 2004 6:13 pm    Post subject: Re: swapping std::pairs? Alberto Barbati wrote: Quote: IIUC, the OP did not intend to swap the two members of one pair, but to swap two pairs (of the same type) memberwise. That is: template <class T1, class T2 void swap(pair { swap(p1.first, p2.first); swap(p1.second, p2.second); } Notice that there are no swaps between heterogenous types. The OP is surprised, as I am, that such an overload of std::swap is not standardized. Without this overload swapping pairs falls back on the generic std::swap that requires copies of both arguments even if one or both have a more efficient way of swapping. One thing that complicates the issue is exception safety. If the second swap() operation throws in your template function, it leaves things in an inconsistent state, with neither p1 nor p2 containing a consistent pair. The generic std::swap does not have this problem. -- Jon Biggar Floorboard Software [email]jon (AT) floorboard (DOT) com[/email] [email]jon (AT) biggar (DOT) org[/email] --- [ 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 David B. Held Guest Posted: Sat Sep 18, 2004 12:03 am    Post subject: Re: swapping std::pairs? Jonathan Biggar wrote: Quote: [...] One thing that complicates the issue is exception safety. If the second swap() operation throws in your template function, it leaves things in an inconsistent state, with neither p1 nor p2 containing a consistent pair. The generic std::swap does not have this problem. Swap should probably not throw in the first place, and I can't think of a sane implementation that does. Dave --- [ 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 Guest Posted: Sat Sep 18, 2004 4:56 am    Post subject: Re: swapping std::pairs? Jonathan Biggar wrote: Quote: Alberto Barbati wrote: IIUC, the OP did not intend to swap the two members of one pair, but to swap two pairs (of the same type) memberwise. That is: template <class T1, class T2 void swap(pair { swap(p1.first, p2.first); swap(p1.second, p2.second); } Notice that there are no swaps between heterogenous types. The OP is surprised, as I am, that such an overload of std::swap is not standardized. Without this overload swapping pairs falls back on the generic std::swap that requires copies of both arguments even if one or both have a more efficient way of swapping. One thing that complicates the issue is exception safety. If the second swap() operation throws in your template function, it leaves things in an inconsistent state, with neither p1 nor p2 containing a consistent pair. The generic std::swap does not have this problem. While this swap would have this problem, normal swap can still have the problem that when you try to swap a and b you end up with both being assigned the value of b, which is surely no better? --- [ 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 Alberto Barbati Guest Posted: Sat Sep 18, 2004 4:56 am    Post subject: Re: swapping std::pairs? Jonathan Biggar wrote: Quote: Alberto Barbati wrote: One thing that complicates the issue is exception safety. If the second swap() operation throws in your template function, it leaves things in an inconsistent state, with neither p1 nor p2 containing a consistent pair. Correct. Quote: The generic std::swap does not have this problem. Incorrect. The generic std::swap still suffer this problem. The generic swap looks like this: template <class T> void swap(T& a, T& b) { T t = a; a = b; b = t; } and pairs are copied memberwise. So swapping two pairs is performed like this: 1) t.first = a.first 2) t.second = a.second 3) a.first = b.first 4) a.second = b.second 5) b.first = t.first 6) b.second = t.second If one of the assignments in steps 4 or 6 throws an exception, you are left with one of the two pairs in an unconsistent state. Yes, one broken pair is better than two broken pairs, but it's still no consolation. A memberwise swap is not less secure than the generic one. In fact, I expect it to be more secure in the general case, because it is usually easier to write a non-throwing swap than it is to write a non-throwing assignment operator. Alberto --- [ 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 Jon Biggar Guest Posted: Mon Sep 20, 2004 5:22 am    Post subject: Re: swapping std::pairs? chris wrote: Quote: One thing that complicates the issue is exception safety. If the second swap() operation throws in your template function, it leaves things in an inconsistent state, with neither p1 nor p2 containing a consistent pair. The generic std::swap does not have this problem. While this swap would have this problem, normal swap can still have the problem that when you try to swap a and b you end up with both being assigned the value of b, which is surely no better? Sure it's better. At least the first and second in both copies of b are consistent with each other. With the proposed template specialized version, you get a's first with b's second and vice versa. -- Jon Biggar Floorboard Software [email]jon (AT) floorboard (DOT) com[/email] [email]jon (AT) biggar (DOT) org[/email] --- [ 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 Daniel Krügler Guest Posted: Mon Sep 20, 2004 10:44 am    Post subject: Re: swapping std::pairs? Hello David Held, David B. Held schrieb: Quote: Jonathan Biggar wrote: [...] One thing that complicates the issue is exception safety. If the second swap() operation throws in your template function, it leaves things in an inconsistent state, with neither p1 nor p2 containing a consistent pair. The generic std::swap does not have this problem. Swap should probably not throw in the first place, and I can't think of a sane implementation that does. That is not true. It is not so unusual, that a copy constructor or copy assignment operator might throw under special circumstances. This usually happens, if the corresponding class uses some (limited) resources, and where the "allocation" for such a resource fails. Since the generic swap is implemented in terms of these members, it might throw. In most cases such a throwing swap can be prevented by providing a user-defined spezialized swap for the given class, but I don't think that we can enforce that requirement for any user-defined class ("If you don't provide such a non-throwing swap for your class, you will have undefined behaviour"). Greetings from Bremen, Daniel --- [ 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 Peter Dimov Guest Posted: Mon Sep 20, 2004 1:07 pm    Post subject: Re: swapping std::pairs? [email]jon (AT) floorboard (DOT) com[/email] (Jon Biggar) wrote in message news:<BR%2d.21110$By7.18254 (AT) newssvr27 (DOT) news.prodigy.com>... Quote: chris wrote: One thing that complicates the issue is exception safety. If the second swap() operation throws in your template function, it leaves things in an inconsistent state, with neither p1 nor p2 containing a consistent pair. The generic std::swap does not have this problem. While this swap would have this problem, normal swap can still have the problem that when you try to swap a and b you end up with both being assigned the value of b, which is surely no better? Sure it's better. At least the first and second in both copies of b are consistent with each other. void swap( pair<T1, T2> & p1, pair<T1, T2> & p2 ) { pair<T1, T2> tmp( p1 ); // OK p1 = p2; // p1.second = p2.second throws p2 = tmp; } Now you have p1.first == p2.first, and the state of p1.second is unspecified (basic) or unchanged (strong). No? --- [ 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 Alberto Barbati Guest Posted: Tue Sep 21, 2004 5:22 am    Post subject: Re: swapping std::pairs? Jon Biggar wrote: Quote: chris wrote: One thing that complicates the issue is exception safety. If the second swap() operation throws in your template function, it leaves things in an inconsistent state, with neither p1 nor p2 containing a consistent pair. The generic std::swap does not have this problem. While this swap would have this problem, normal swap can still have the problem that when you try to swap a and b you end up with both being assigned the value of b, which is surely no better? Sure it's better. At least the first and second in both copies of b are consistent with each other. That's not entirely correct. "Normal" swap can indeed break one of two copies (but not both). See my other post for details. Alberto --- [ 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.