C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  clearing capacity af a vector          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Lasse Skyum Guest Posted: Thu Oct 30, 2003 3:53 pm    Post subject: clearing capacity af a vector We had a thread earlies about how to clear the capacity af a std::vector... how about this: std::vector<int> v; v.~vector<int>(); // destruct it (don't know if I need the <int> here?) new (&v)vector<int>(); // construct it again -- Lasse Back to top Victor Bazarov Guest Posted: Thu Oct 30, 2003 4:01 pm    Post subject: Re: clearing capacity af a vector "Lasse Skyum" <no spam> wrote... Quote: We had a thread earlies about how to clear the capacity af a std::vector... how about this: std::vector<int> v; v.~vector<int>(); // destruct it (don't know if I need the <int> here?) new (&v)vector<int>(); // construct it again While it _might_ work (and even according some twisted reading of the Standard, it is allowed to work), I'd suggest v.swap(std::vector<int>()); Victor Back to top Chris Theis Guest Posted: Thu Oct 30, 2003 4:03 pm    Post subject: Re: clearing capacity af a vector "Lasse Skyum" <no spam> wrote Quote: We had a thread earlies about how to clear the capacity af a std::vector... how about this: std::vector<int> v; v.~vector<int>(); // destruct it (don't know if I need the <int> here?) new (&v)vector<int>(); // construct it again -- As Victor already said - this might work. But IMHO using placement new with any real need to is not such a hot idea. BTW what's the problem with using the "normal" solution with the swap function? Chris Back to top Andrew Koenig Guest Posted: Thu Oct 30, 2003 4:13 pm    Post subject: Re: clearing capacity af a vector Quote: While it _might_ work (and even according some twisted reading of the Standard, it is allowed to work), I'd suggest v.swap(std::vector<int>()); This technique almost works, but not quite. The trouble is that std::vector<int>() is an rvalue, which means that it can't be used as the argument to swap, which requires an lvalue. You can make the technique work with a slightly sneaky trick: std::vector<int>().swap(v); This code relies on the fact that it is permissible to call a member function that changes the contents of an rvalue. Back to top Victor Bazarov Guest Posted: Thu Oct 30, 2003 5:11 pm    Post subject: Re: clearing capacity af a vector "Andrew Koenig" <ark (AT) acm (DOT) org> wrote... Quote: While it _might_ work (and even according some twisted reading of the Standard, it is allowed to work), I'd suggest v.swap(std::vector<int>()); This technique almost works, but not quite. The trouble is that std::vector<int>() is an rvalue, which means that it can't be used as the argument to swap, which requires an lvalue. You can make the technique work with a slightly sneaky trick: std::vector<int>().swap(v); This code relies on the fact that it is permissible to call a member function that changes the contents of an rvalue. Thank you for the correction. I ought to check before posting. Victor Back to top Lasse Skyum Guest Posted: Thu Oct 30, 2003 6:01 pm    Post subject: Re: clearing capacity af a vector Quote: As Victor already said - this might work. But IMHO using placement new with any real need to is not such a hot idea. BTW what's the problem with using the "normal" solution with the swap function? The only reason was, that I was told that the swap method was not guarenteed to work by the standard. I thought this method would... I don't really understand why it wouldn't work... the resulting object should be a newly initialized vector. Just because it uses the same piece of memory that once was used for another vector should not affect that...or? Maybe it would be inefficient compared to the swap method? (Suppose it was something that happened verry often) -- Lasse Back to top Victor Bazarov Guest Posted: Thu Oct 30, 2003 6:33 pm    Post subject: Re: clearing capacity af a vector "Lasse Skyum" <no spam> wrote... Quote: As Victor already said - this might work. But IMHO using placement new with any real need to is not such a hot idea. BTW what's the problem with using the "normal" solution with the swap function? The only reason was, that I was told that the swap method was not guarenteed to work by the standard. I thought this method would... I don't really understand why it wouldn't work... the resulting object should be a newly initialized vector. Just because it uses the same piece of memory that once was used for another vector should not affect that...or? Maybe it would be inefficient compared to the swap method? (Suppose it was something that happened verry often) No, the efficiency of those two methods is very close. Swapping variation is basically the same as constructing an empty one and destroying the original one, with the addition of swapping some data members, so swapping may actually be just a tad slower. Victor Back to top Lasse Skyum Guest Posted: Thu Oct 30, 2003 6:47 pm    Post subject: Re: clearing capacity af a vector Quote: No, the efficiency of those two methods is very close. Swapping variation is basically the same as constructing an empty one and destroying the original one, with the addition of swapping some data members, so swapping may actually be just a tad slower. Victor Okay, seems strange that people don't do it that way then... I still don't see why it wouldn't work? -- Lasse Back to top Victor Bazarov Guest Posted: Thu Oct 30, 2003 7:25 pm    Post subject: Re: clearing capacity af a vector "Lasse Skyum" <no spam> wrote... Quote: No, the efficiency of those two methods is very close. Swapping variation is basically the same as constructing an empty one and destroying the original one, with the addition of swapping some data members, so swapping may actually be just a tad slower. Victor Okay, seems strange that people don't do it that way then... I still don't see why it wouldn't work? Well, I don't want this to turn into another discussion why using a destructor and then placement new isn't such a good idea, you can find it on Google Groups, if you want to see what people think. In short: if you have to call a destructor directly on something you didn't create using placement new, you might need to rethink your design or implementation. IMHO. Victor Back to top Lasse Skyum Guest Posted: Thu Oct 30, 2003 7:50 pm    Post subject: Re: clearing capacity af a vector Quote: In short: if you have to call a destructor directly on something you didn't create using placement new, you might need to rethink your design or implementation. IMHO. Well that kind of makes sence... a little anyway :-) -- Lasse Back to top Howard Hinnant Guest Posted: Mon Nov 03, 2003 9:02 pm    Post subject: Re: clearing capacity af a vector In article <jDbob.47350$275.116409@attbi_s53>, "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote: Quote: "Andrew Koenig" <ark (AT) acm (DOT) org> wrote... While it _might_ work (and even according some twisted reading of the Standard, it is allowed to work), I'd suggest v.swap(std::vector<int>()); This technique almost works, but not quite. The trouble is that std::vector<int>() is an rvalue, which means that it can't be used as the argument to swap, which requires an lvalue. You can make the technique work with a slightly sneaky trick: std::vector<int>().swap(v); This code relies on the fact that it is permissible to call a member function that changes the contents of an rvalue. Thank you for the correction. I ought to check before posting. That's a common mistake, even for experts such as yourself. For that reason, I'd love to see: template <class T> void swap(T&&, T&&); (see move proposal: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1377.htm ) which would allow expressions such as: swap(v, std::vector<int>()); or: swap(std::vector<int>(), v); which I've personally grown to like (ymmv). -Howard 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 (comp.lang.c++) 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.