C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Iterator Question Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Bob Bell Guest Posted: Tue Oct 14, 2003 9:10 am    Post subject: Iterator Question All- Are default-constructed standard container iterators guaranteed to compare equal? As in #include <vector> void F() { std::vector<int>::iterator a, b; if (a == b) { // ... } } Does the standard say whether "// ..." will be executed or not? Bob [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Tue Oct 14, 2003 8:55 pm    Post subject: Re: Iterator Question [email]belvis (AT) pacbell (DOT) net[/email] (Bob Bell) writes: Quote: All- Are default-constructed standard container iterators guaranteed to compare equal? As in Unfortunately, no Quote: #include <vector void F() { std::vector if (a == b) { That's undefined behavior. Quote: // ... } } Does the standard say whether "// ..." will be executed or not? Comments are all executed at program startup time, but their effects are, well, somewhat limited ;-> -- Dave Abrahams Boost Consulting www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maciej Sobczak Guest Posted: Tue Oct 14, 2003 9:08 pm    Post subject: Re: Iterator Question Hi, Bob Bell wrote: Quote: Are default-constructed standard container iterators guaranteed to compare equal? No. Such a rule would not be really useful - I mean it would not give you much more than you could gain by following good coding guidelines and not using uninitialized variables at all. Implementing this rule would also be a cost that not everyone would be willing to pay. Iterators are supposed to be implementable in as efficient way as possible. On the extreme, vector<>::iterator can be a plain pointer, so: Quote: std::vector<int>::iterator a, b; can be no different than: int *a, *b; Quote: if (a == b) { Now nobody knows what happens - it is not a simple true/false instruction. On the extreme, it can be undefined behavior, resulting in whatever you expect or anything you do not expect. Just do not use uninitialized variables in your code. -- Maciej Sobczak : http://www.msobczak.com/ Programming : http://www.msobczak.com/prog/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Potter Guest Posted: Tue Oct 14, 2003 9:15 pm    Post subject: Re: Iterator Question On 14 Oct 2003 05:10:12 -0400, [email]belvis (AT) pacbell (DOT) net[/email] (Bob Bell) wrote: Quote: Are default-constructed standard container iterators guaranteed to compare equal? std::vector<int>::iterator a, b; if (a == b) { Identical to: int* p; int* q; if (p == q) Just looking at an uninitialized iterator is undefined behavior. The standard offers no protection for your hard drive in the above code. See 24.1/5 and TC. John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top johnchx Guest Posted: Tue Oct 14, 2003 9:19 pm    Post subject: Re: Iterator Question [email]belvis (AT) pacbell (DOT) net[/email] (Bob Bell) wrote Quote: Are default-constructed standard container iterators guaranteed to compare equal? As in Is there a requirement that such iterators can be default-constructed at all? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Joshua Lehrer Guest Posted: Wed Oct 15, 2003 10:44 am    Post subject: Re: Iterator Question [email]belvis (AT) pacbell (DOT) net[/email] (Bob Bell) wrote in message news:<c87c1cfb.0310131409.59a88f4e (AT) posting (DOT) google.com>... Quote: All- Are default-constructed standard container iterators guaranteed to compare equal? As in #include <vector void F() { std::vector if (a == b) { // ... } } Does the standard say whether "// ..." will be executed or not? Replace "std::vector<int>::iterator" with "int*", which it is allowed to be, and you get your answer: char * a,b; if (a==b) { } 'a' and 'b' are uninitialized and therefore the comparison has undefined results. joshua lehrer factset research systems NYSE:FDS [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Potter Guest Posted: Wed Oct 15, 2003 10:45 am    Post subject: Re: Iterator Question On 14 Oct 2003 17:19:45 -0400, [email]johnchx2 (AT) yahoo (DOT) com[/email] (johnchx) wrote: Quote: belvis (AT) pacbell (DOT) net (Bob Bell) wrote Are default-constructed standard container iterators guaranteed to compare equal? As in Is there a requirement that such iterators can be default-constructed at all? Yes. All standard containers have at least forward iterators and forward iterators must be default constructable. See table 74 and note that they might be singular (undefined lvalue to rvalue conversion). John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Joshua Lehrer Guest Posted: Wed Oct 15, 2003 12:52 pm    Post subject: Re: Iterator Question [email]johnchx2 (AT) yahoo (DOT) com[/email] (johnchx) wrote in message news:<4fb4137d.0310140959.3a926535 (AT) posting (DOT) google.com>... Quote: belvis (AT) pacbell (DOT) net (Bob Bell) wrote Are default-constructed standard container iterators guaranteed to compare equal? As in Is there a requirement that such iterators can be default-constructed at all? Yes. A Random Access Itertor is a refinement of Bidirectional Iterator which is a refinement of Forward Iterator which is a refinement of Input Iterator which is a refinement of Trivial Iterator which is a refinement of Default Constructible. http://www.sgi.com/tech/stl/RandomAccessIterator.html http://www.sgi.com/tech/stl/BidirectionalIterator.html http://www.sgi.com/tech/stl/ForwardIterator.html http://www.sgi.com/tech/stl/InputIterator.html http://www.sgi.com/tech/stl/trivial.html http://www.sgi.com/tech/stl/DefaultConstructible.html Thanks SGI. joshua lehrer factset research systems NYSE:FDS [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Michael Birck Guest Posted: Wed Oct 15, 2003 10:02 pm    Post subject: Re: Iterator Question [email]johnchx2 (AT) yahoo (DOT) com[/email] (johnchx) wrote in news:4fb4137d.0310140959.3a926535 @posting.google.com: Quote: belvis (AT) pacbell (DOT) net (Bob Bell) wrote Is there a requirement that such iterators can be default-constructed at all? Yes - otherwise you couldn't put iterators in a standard container. /Michael [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Potter Guest Posted: Thu Oct 16, 2003 2:10 pm    Post subject: Re: Iterator Question On 15 Oct 2003 08:52:27 -0400, [email]usenet_cpp (AT) lehrerfamily (DOT) com[/email] (Joshua Lehrer) wrote: Quote: johnchx2 (AT) yahoo (DOT) com (johnchx) wrote in message news:<4fb4137d.0310140959.3a926535 (AT) posting (DOT) google.com>... [email]belvis (AT) pacbell (DOT) net[/email] (Bob Bell) wrote Are default-constructed standard container iterators guaranteed to compare equal? As in Is there a requirement that such iterators can be default-constructed at all? Yes. A Random Access Itertor is a refinement of Bidirectional Iterator which is a refinement of Forward Iterator which is a refinement of Input Iterator which is a refinement of Trivial Iterator which is a refinement of Default Constructible. I don't know whether you misread SGI or they misspoke. Neither input (table 72) nor output (table 73) iterator are required to be default constructable. One of the refinements made by forward (table 74) iterator is default constructable. John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Potter Guest Posted: Thu Oct 16, 2003 2:14 pm    Post subject: Re: Iterator Question On 15 Oct 2003 18:02:12 -0400, Michael Birck <LASTNAME (AT) mail (DOT) dk> wrote: Quote: johnchx2 (AT) yahoo (DOT) com (johnchx) wrote in news:4fb4137d.0310140959.3a926535 @posting.google.com: [email]belvis (AT) pacbell (DOT) net[/email] (Bob Bell) wrote Is there a requirement that such iterators can be default-constructed at all? Yes - otherwise you couldn't put iterators in a standard container. No. The standard was purposely written to allow non-default constructable objects in all containers. The requirements are copy constructable and copy assignable. // back_insert_iterator<vector i; // error no ctor vector<back_insert_iterator > v; // valid vector<int> vi; v.push_back(back_inserter(vi)); // amusing v[0] = 42; assert(vi.size() == 1); Here is an iterator that is not default constructable; however, it is not a standard container :: iterator. A standard container of them is still valid. Some operations, such as reserve, are not allowed. John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Siemel Naran Guest Posted: Thu Oct 16, 2003 2:25 pm    Post subject: Re: Iterator Question "Michael Birck" <LASTNAME (AT) mail (DOT) dk> wrote in message Quote: johnchx2 (AT) yahoo (DOT) com (johnchx) wrote in news:4fb4137d.0310140959.3a926535 Is there a requirement that such iterators can be default-constructed at all? Yes - otherwise you couldn't put iterators in a standard container. Standard containers can hold objects that have no default constructor. The following program should compile. #include <vector> class Thing { public: Thing(int, int); }; int main() { std::vector<Thing> vector; vector.push_back(Thing(1,2)); } -- +++++++++++ Siemel Naran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Thu Oct 16, 2003 2:49 pm    Post subject: Re: Iterator Question Michael Birck <LASTNAME (AT) mail (DOT) dk> wrote Quote: johnchx2 (AT) yahoo (DOT) com (johnchx) wrote in news:4fb4137d.0310140959.3a926535 @posting.google.com: Is there a requirement that such iterators can be default-constructed at all? Yes - otherwise you couldn't put iterators in a standard container. The answer is yet, because the standard says so. None of the standard containers require the contained object to be default constructable, however, and you can (and I sometimes do) use containers of objects which do not have default constructors. -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France, +33 (0)1 30 23 45 16 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top johnchx Guest Posted: Thu Oct 16, 2003 2:54 pm    Post subject: Re: Iterator Question [email]usenet_cpp (AT) lehrerfamily (DOT) com[/email] (Joshua Lehrer) wrote Quote: johnchx2 (AT) yahoo (DOT) com (johnchx) wrote Is there a requirement that such iterators can be default-constructed at all? Yes. A Random Access Itertor is a refinement of Bidirectional Iterator which is a refinement of Forward Iterator which is a refinement of Input Iterator which is a refinement of Trivial Iterator which is a refinement of Default Constructible. Once perhaps, but no longer -- input iterators need not be default constructable (table 72). The requirement is introduced for forward iterators (table 74) and "above". Which, for the purposes of my question, boils down to the same thing, of course. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top johnchx Guest Posted: Thu Oct 16, 2003 2:54 pm    Post subject: Re: Iterator Question Michael Birck <LASTNAME (AT) mail (DOT) dk> wrote Quote: johnchx2 (AT) yahoo (DOT) com (johnchx) wrote Is there a requirement that such iterators can be default-constructed at all? Yes - otherwise you couldn't put iterators in a standard container. The standard containers require their elements to be copy constructible and assignable, not necessarily default constructible. The answer to my question is indeed "yes," but that's not the reason. :-) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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