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Re: list::begin() iterator comportement after push_back operations

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Hizo wrote:
> Hi there,
>
> I have a problem with the begin iterator of STL Lists.
> Indeed, if we keep the begin iterator of an empty list when we test
> it after multiple push_back operations it becomes the end iterator.
> Here is my code:
>
> -------------------------------------------
> #include <iostream>
> using std::cout;
> using std::endl;
> using std::boolalpha;
>
> #include <list>
> using std::list;
>
> int main(int argc, char * argv[])
> {
> list<int> l;
> list<int>::const_iterator it = l.begin();
> list<int>::const_reverse_iterator rit = l.rbegin();
>
> l.push_back(1);
> l.push_back(2);
>
> cout << boolalpha << (it == l.end()) << endl;
> cout << boolalpha << (rit == l.rend()) << endl;
>
> return 0;
> }
> -------------------------------------------
>
> It actually returns:
> true
> false
>
> with gcc version 4.3.4 (Gentoo 4.3.4 p1.0, pie-10.1.5)
>
> Is it possible to keep in memory the begin iterator of a list (not
> using reverse iterators) which will really point to the begin of the
> list after push_back operations on the list (obviously I am not able
> to use l.begin() after (because it is an initial state in my
> algorithm and I then update the iterator that pointed to the begin
> iterator initialy))
>
> Thanks for your help.

Short answer: No.

All containers start out with c.begin() == c.end(), as that is one way 
of seeing that the container is empty.

When you add elements to the container, some or all iterators will be 
invalidated. A little different for each container type, but 
definitely the begin() iterator will change when you add an element to 
the start of the container (which of course happens when you add to an 
empty container).

Reverse iterators will not help either, as they will be equally 
invalidated.


Bo Persson

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list::begin() iterator comportement after push_back operations Hizo <hizostudio@gmail.com> - 2011-04-02 04:46 -0700
  Re: list::begin() iterator comportement after push_back operations "Bo Persson" <bop@gmb.dk> - 2011-04-02 13:57 +0200
    Re: list::begin() iterator comportement after push_back operations Hizo <hizostudio@gmail.com> - 2011-04-02 05:23 -0700
    Re: list::begin() iterator comportement after push_back operations Hizo <hizostudio@gmail.com> - 2011-04-02 05:24 -0700
    Re: list::begin() iterator comportement after push_back operations Hizo <hizostudio@gmail.com> - 2011-04-02 05:28 -0700
    Re: list::begin() iterator comportement after push_back operations Hizo <hizostudio@gmail.com> - 2011-04-02 05:32 -0700
      Re: list::begin() iterator comportement after push_back operations "Bo Persson" <bop@gmb.dk> - 2011-04-02 15:37 +0200
      Re: list::begin() iterator comportement after push_back operations James Kanze <james.kanze@gmail.com> - 2011-04-03 10:01 -0700

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