What happens if you call erase() on a map element while iterating from begin to end?

In the following code I loop through a map and test if an element needs to be erased. Is it safe to erase the element and keep iterating or do I need to collect the keys in another container and do a second loop to call the erase()?

map<string, SerialdMsg::SerialFunction_t>::iterator pm_it;
for (pm_it = port_map.begin(); pm_it != port_map.end(); pm_it++)
{
    if (pm_it->second == delete_this_id) {
        port_map.erase(pm_it->first);
    }
}

UPDATE: Of course, I then read this question which I didn't think would be related but answers my question.

From stackoverflow

• Erasing elements in a map does not invalidate any iterators.
  (apart from iterators on the element that was deleted)

  Actually inserting or deleting does not invalidate any of the iterators:

  Also see this answer:
  http://stackoverflow.com/questions/180516/how-to-filter-items-from-a-stdmap

  But you do need to update your code:
  In your code you increment pm_it after calling erase. At this point it is too late and is already invalidated.

  map<string, SerialdMsg::SerialFunction_t>::iterator pm_it = port_map.begin();
  while(pm_it != port_map.end())
  {
      if (pm_it->second == delete_this_id)
      {
          port_map.erase(pm_it++);  // Use iterator.
                                    // Note the post increment.
                                    // Increments the iterator but returns the
                                    // original value for use by erase 
      }
      else
      {
          ++pm_it;           // Can use pre-increment in this case
                             // To make sure you have the efficient version
      }
  }
  

• You could iterate from end to beginning to avoid errors

  MSalters : Doesn't help at all. The only iterator invalidated is the iterator erased. You can neither increment it or decrement it afterwards. pm_it-- is equally wrong.

• This is how I would do it, approximately:

  bool is_remove( pair<string, SerialdMsg::SerialFunction_t> val )
  {
      return val.second == delete_this_id;
  }
  
  map<string, SerialdMsg::SerialFunction_t>::iterator new_end = 
      remove_if (port_map.begin( ), port_map.end( ), is_remove );
  
  port_map.erase (new_end, port_map.end( ) );
  

  There is something odd about

  val.second == delete_this_id
  

  but I just copied it from your example code.

  Martin York : Use the map value_type in the is_remove. Also prefer functor to function (Easier for the compiler to optimize) and you can bind different values to 'delete_this_id'

• Here's how I do that ...

  typedef map<string, string>   StringsMap;
  typedef StringsMap::iterator  StrinsMapIterator;
  
  StringsMap m_TheMap; // Your map, fill it up with data    
  
  bool IsTheOneToDelete(string str)
  {
       return true; // Add your deletion criteria logic here
  }
  
  void SelectiveDelete()
  {
       StringsMapIter itBegin = m_TheMap.begin();
       StringsMapIter itEnd   = m_TheMap.end();
       StringsMapIter itTemp;
  
       while (itBegin != itEnd)
       {
            if (IsTheOneToDelete(itBegin->second)) // Criteria checking here
            {
                 itTemp = itBegin;          // Keep a reference to the iter
                 ++itBegin;                 // Advance in the map
                 m_TheMap.erase(itTemp);    // Erase it !!!
            }
            else
                 ++itBegin;                 // Just move on ...
       }
  }