C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  std::map questions          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message andy.canfield@gmail.com Guest Posted: Thu Oct 27, 2005 9:29 am    Post subject: std::map questions I have two questions about std::map. First question: If I have a std::map with an iterator pointing to begin() and I insert an element into the map using insert() is my iterator garaunteed to still be pointing at the same place it was when I set it to begin()? I have the book "The C++ Standard Library: A tutorial and reference" but I can't find anywhere where it says yes or no on this. Second question: If I traverse a std::map using an iterator and the map sort criteria was "less than" am I garaunteed that it will traverse from least to greatest if I traverse from begin() to end() or is it only garaunteed that I will touch all elements in the map? Thank you, Andy [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ko van der Sloot Guest Posted: Thu Oct 27, 2005 11:15 am    Post subject: Re: std::map questions [email]andy.canfield (AT) gmail (DOT) com[/email] wrote: Quote: I have two questions about std::map. First question: If I have a std::map with an iterator pointing to begin() and I insert an element into the map using insert() is my iterator garaunteed to still be pointing at the same place it was when I set it to begin()? I have the book "The C++ Standard Library: A tutorial and reference" but I can't find anywhere where it says yes or no on this. did you see table 6.33 on page 227? Quote: Second question: If I traverse a std::map using an iterator and the map sort criteria was "less than" am I garaunteed that it will traverse from least to greatest if I traverse from begin() to end() or is it only garaunteed that I will touch all elements in the map? I would be very suprised if the traverse would be unsorted. Can't find the right wording in Josuttis right now. Quote: Thank you, Andy Ko [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tony Delroy Guest Posted: Thu Oct 27, 2005 11:16 am    Post subject: Re: std::map questions Hi Andy, SGI provide an excellent reference to their implementation of the STL, see http://www.sgi.com/tech/stl/Map.html for the map page. Note their promise: Quote: Map has the important property that inserting a new element into a map does not invalidate iterators that point to existing elements. Erasing an element from a map also does not invalidate any iterators, except, of course, for iterators that actually point to the element that is being erased. So, regarding your first question: the iterator you assigned from begin() will still point to the same element (though that may not be the beginning element any more) after an insert() IF IT EVER POINTED TO AN ELEMENT IN THE FIRST PLACE. If the map had been empty, begin() would have returned the save value end() would have, and this could be virtually anything. In summary, don't use the original value unless you know it wasn't equal to end() at the time it was retrieved. Re Q2. The sort order is guaranteed. Regards, Tony [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Thu Oct 27, 2005 11:16 am    Post subject: Re: std::map questions [email]andy.canfield (AT) gmail (DOT) com[/email] wrote: Quote: First question: If I have a std::map with an iterator pointing to begin() and I insert an element into the map using insert() is my iterator garaunteed to still be pointing at the same place it was when I set it to begin()? 1. The iterator still points to the same element. 2. If the new element sorts before the one it points to, it will not point to the beginning of the sequence, but I don't think that's what you meant. Quote: I have the book "The C++ Standard Library: A tutorial and reference" but I can't find anywhere where it says yes or no on this. I don't know that book, but maybe you just don't see it, the thing is called 'iterator invalidation'. Quote: Second question: If I traverse a std::map using an iterator and the map sort criteria was "less than" am I garaunteed that it will traverse from least to greatest if I traverse from begin() to end() or is it only garaunteed that I will touch all elements in the map? Yes, this is guaranteed, not only for map but also multimap, set and multiset. Uli [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Thu Oct 27, 2005 2:05 pm    Post subject: Re: std::map questions In article <1130362302.342506.119480 (AT) g14g2000cwa (DOT) googlegroups.com>, [email]andy.canfield (AT) gmail (DOT) com[/email] writes Quote: First question: If I have a std::map with an iterator pointing to begin() and I insert an element into the map using insert() is my iterator garaunteed to still be pointing at the same place it was when I set it to begin()? Same place in what sense? You have no control on where a new element will be added and the result may need radical restructuring of the internal representation. I would expect the iterator to still locate some element of the map but not necessarily the one you might expect, and not always the one that is now identified by dereferencing the value returned by a new call to begin(). -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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