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

From:

Louis Lavery <Louis@LaverREMOVE.demon.co.uk>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 17 Jul 2007 16:25:50 CST

Message-ID:

<f7j79p$g2b$1$8300dec7@news.demon.co.uk>

Carl Barron wrote:

In article <f7e40e$ccn$1$8302bc10@news.demon.co.uk>, Louis Lavery
<Louis@LaverREMOVE.demon.co.uk> wrote:

     so I don't see anything more concise than:
      for the erase function call in original code.
      {
         set<unsigned int>::iterator it = ri++.base();

Isn't ri.base() now one before it so...

ri++ returns a copy of ri before the increment and increments ri.
therefore ri++.base() returns the base iterator that ri had before
it was incremented.

Yes. So ri.base() is now one before it. I.e. ri.base() == --it.

The second version eliminates the need of a copy of ri produced with
operator ++(int). [most likely, op ++(int) creates a copy, especially
for a set<...>::reverse_iterator.]. But the final result is the same
with either version I provided in the post you refer to.

Yes and they both result in invalidating ri by erasing the item its
base iterator points to. But you don't have to believe me, it's easy
to ask your compilers.

Your example code...

     but i'd probably
      {
         set<unsigned int>::iterator it = ri.base();
         ++ri;
         erase(--it);
      }
     to avoid copying ri more than needed.

....is equivalent to...

      {
         set<unsigned int>::iterator it = ri.base();
         ++ri;
         --it;
         erase(it);
      }

....can be tested via...

#include <cassert>
#include <set>

int main()
{
    std::set<int unsigned> uis;
    uis.insert(1);
    std::set<int unsigned>::reverse_iterator ri = uis.rbegin();

    std::set<int unsigned>::iterator it = ri.base();
    ++ri;
    --it;
    assert(ri.base() != it); // Ask the compiler what it thinks.
    uis.erase(it);

    return 0;
}

....try it.

Louis.

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