C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Pointers and vectors          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Philippe Gagnon Guest Posted: Wed May 16, 2007 4:44 am    Post subject: Pointers and vectors Hello, If I have a pointer to an object contained in a vector, and that vector is resized, would the pointer be invalidated? Thanks for your answers. --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Alf P. Steinbach Guest Posted: Wed May 16, 2007 9:10 am    Post subject: Re: Pointers and vectors * Philippe Gagnon: Quote: If I have a pointer to an object contained in a vector, and that vector is resized, would the pointer be invalidated? Depends (a more precise question would allow a more precise answer). The vector's buffer can be replaced in two ways: reallocation and swap. A reallocation happens if and only if a capacity greater than the current capacity() is required for some operation. If and only if the buffer is replaced, the pointer will be invalid as a pointer. However, assuming no buffer replacement, if p >= &v[0]+v.size() after your operation(s), then p effectively points to raw storage: the pointer itself is still valid, e.g. can be compared, but points to garbage. -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Alberto Ganesh Barbati Guest Posted: Wed May 16, 2007 10:31 pm    Post subject: Re: Pointers and vectors Philippe Gagnon ha scritto: Quote: Hello, If I have a pointer to an object contained in a vector, and that vector is resized, would the pointer be invalidated? resize() might or might not trigger reallocation. If reallocation occurs then pointers to all contained objects will be invalidated. Reallocation occurs if and only if you call resize(n) with n > capacity(). You can set the value of capacity() by calling reserve(). You should call reserve() *before* inserting the elements, because reserve() may (and probably will) trigger reallocation. The idea is to call reserve() when the vector is empty (thus when reallocation doesn't bother you) and then do whatever you want with the vector by just being careful not to exceed capacity(). Notice that if resize() is actually shrinking the vector then some objects will be erased and pointers to them will be invalidated even if reallocation doesn't occur. HTH, Ganesh --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Guest Posted: Thu May 17, 2007 6:59 am    Post subject: Re: Pointers and vectors On 5ÔÂ16ÈÕ, ÏÂÎç12ʱ44·Ö, Philippe Gagnon <heel...@avicora.com> wrote: Quote: Hello, If I have a pointer to an object contained in a vector, and that vector is resized, would the pointer be invalidated? Thanks for your answers. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-...@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ:http://www.comeaucomputing.com/csc/faq.html ] unless reallocation happens, all the pointer or iterators in the vector will not invalidate. insert and erase-remove idiom can cause pointer and iterator invalidate as well. --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Alf P. Steinbach Guest Posted: Thu May 17, 2007 9:10 am    Post subject: Re: Pointers and vectors * Alberto Ganesh Barbati: Quote: Philippe Gagnon ha scritto: Hello, If I have a pointer to an object contained in a vector, and that vector is resized, would the pointer be invalidated? resize() might or might not trigger reallocation. If reallocation occurs then pointers to all contained objects will be invalidated. Reallocation occurs if and only if you call resize(n) with n > capacity(). I think you mean reserve(). Anyway, the resize() above is incorrect, whereas reserve() would be correct. resize() may call reserve(), reserve() doesn't call resize(). And it's necessary to consider swapping in addition to reallocation. But in my posting in this thread where I mentioned that, I erred in the same way as you, using too precise language. For swapping doesn't necessarily invalidate a pointer in the sense of rendering it invalid for all purposes. It /may/ invalidate a pointer (simplest example is the swap-with-temporary idiom for clearing), and it /does/ invalidate certain pointer operations such as subsequenctly comparing p > &v[0]. Sorry for stating that a swap will always invalidate pointers. But as Alan Turing noted, the potential for at least temporarily holding false, contradictory beliefs and communicating false statements is necessary (although not sufficient) for intelligence... I cling to that. [snip] Quote: Notice that if resize() is actually shrinking the vector then some objects will be erased and pointers to them will be invalidated even if reallocation doesn't occur. The pointers themselves will not be invalid (e.g. can still be safely compared with no UB), but will point to garbage. However, the standard uses this language for references, saying they will be "invalidated", and there the story is much the same as for pointers: the references themselves won't be completely invalid, e.g. &r can be compared if there's no user-defined address operator, but until perhaps the vector is resized up again they refer to garbage. The problem here, if there is one, is simply that the standard tries too hard to be abstract where the opposite is called for, doing the opposite of what we did, namely it's using too wooly language, which is probably connected to the fact that a vector's buffer didn't become guaranteed contiguous until C++2003 (TC1). For with a contiguous buffer and guarantees about when the buffer isn't replaced, which are now in place, there's no need to consult the standard at all, except perhaps to ascertain that those guarantees are there. Simple logic suffices, which is as it should be: it should be possible to /understand/the language. -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Alberto Ganesh Barbati Guest Posted: Fri May 18, 2007 9:11 pm    Post subject: Re: Pointers and vectors Alf P. Steinbach ha scritto: Quote: * Alberto Ganesh Barbati: Philippe Gagnon ha scritto: Hello, If I have a pointer to an object contained in a vector, and that vector is resized, would the pointer be invalidated? resize() might or might not trigger reallocation. If reallocation occurs then pointers to all contained objects will be invalidated. Reallocation occurs if and only if you call resize(n) with n > capacity(). I think you mean reserve(). Anyway, the resize() above is incorrect, whereas reserve() would be correct. resize() may call reserve(), reserve() doesn't call resize(). No no, I really meant resize(). In the statement "Reallocation occurs if and only if you call resize(n) with with n > capacity()" what I meant was "*As an effect of a call to resize()* reallocation occurs if and only if you call resize(n) with with n > capacity()". Of course, also calling reserve() as well as insert() can trigger reallocation under similar circumstances. BTW, while resize() and insert() *may* call reserve() to perform reallocation, they may achieve the same task *without* calling it (for example the reallocation task might be factored out in a private function as part of an optimization). So replacing "resize()" with "reserve()" in the statement above doesn't make it more correct. Regards, Ganesh --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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