Re: Problems removing an element from a std::set using a reverse_iterator

From: (Carl Barron)
Sat, 14 Jul 2007 14:32:35 CST
irotas <> wrote:

The article states that option 1 is fine for all standard containers
*except* for std::string and std::vector, which often implement
iterators as built-in pointers (C++ imposes the constraint that
pointers returned from functions may not be modified).

   chapter and verse please. this implies that
    char *p = "abzxycd";
   if(*(strchr(p,'z')+3) !='c') {/* */}
    is illegal , [not safe but it is legal]

However, the
article claims that option 2 works for all standard containers, and
therefore is the preferred technique.

   I do see a huge problem with your code. You are invalidating the
reverse_iterator by invalidating its 'base iterator'.

    first you find the last entry . thats ok then you erase that entry
   noe you increment the reverse iterator , decrementing the base
iterator so it no points to the erased element. that is you are doing
this under the hood:
    std::set<unsigned int>::iterator it = my_set.end();
    while (it != my_set.begin()
        std::set<unsigned int>::iterator tmp = it; --tmp;
        --it; // Ouch it now points to erased element if it was erased.

    You need to do something like.

    template <class Cont, class Pred>
    void erase_backward_if(Cont &c, Pred pred)
       typename Cont::iterator it;
       typename Cont::reverse_iterator rit;

       // find the first to remove
       rit = std::find_if(c.rbegin(),c.rend(), pred);
       // while any to remove.
       while(rit != c.rend())
         // copy the base iterator to erase to it for safe keeping
          it = --(rit.begin());
          // 'advance' the reverse iterator
          rit = std::find_if(++rit,c.rend(),pred());
          // now erase the elment pointed to by it.

   now the data is not removed from the container until the
reverse_iterator has moved to the 'next position' and beyond.

Of course this is O(N) but apparently that is not a problem.

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