Re: Using std::set::erase with iterators

From:

Abhishek Padmanabh <abhishek.padmanabh@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 3 Mar 2008 06:39:18 -0800 (PST)

Message-ID:

<049fef7d-5f6f-4bdf-9141-4a32bd5d77dc@s12g2000prg.googlegroups.com>

On Mar 3, 2:43 pm, mathieu <mathieu.malate...@gmail.com> wrote:

Hi,

  I do not understand how I am supposed to use erase when looping over
elements in a set, here is what I am trying to do (*). Could someone
please indicate how was I supposed to do it properly ? All I can think
of is something like this (very cumbersome):

    if( *it % 2 )
      {
      std::set<int>::const_iterator duplicate = it;
      ++it;
      s.erase( duplicate );
      }
    else
      {
      ++it;
      }

You could use the returned iterator out of the erase call. Like this:

for( std::set<int>::iterator it = s.begin(); it != s.end(); )
{
  if( *it % 2 )
    it = s.erase( it );
  else
    ++it;
}

std::set is node-base, so erasing an element does not invalidate all
iterators, it just makes the removed item's iterator invalid and so
you cannot advance it with operator++() as your original for-loop is
doing:

for( std::set<int>::const_iterator it = s.begin(); it != s.end(); +
+it)
{
  if( *it % 2 )
    s.erase( it ); //--->> Here you erase it and then use the same
'it' to advance as ++it.
}