Question

// erasing from map
#include <iostream>
#include <map>
using namespace std;

int main ()
{
  map<char,int> mymap;
  map<char,int>::iterator it(mymap.begin());

  // insert some values:
  mymap['a']=10;
  mymap['b']=20;
  mymap['c']=30;
  mymap['d']=40;
  mymap['e']=50;
  mymap['f']=60;

  it=mymap.find('a');
  mymap.erase (it);                   // erasing by iterator

  // show content:
  for (; it != mymap.end(); it++ )
    cout << (*it).first << " => " << (*it).second << endl;
  return 0;
}

Why does this give an output like

a => 10
b => 20
c => 30
d => 40
e => 50
f => 60

shouldn’t "a => 10" be deleted anyways, but if I declare it = mymap.begin() in the for loop, everything is perfect. why?

program adapted from : http://www.cplusplus.com/reference/stl/map/erase/

Answer 1

Erasing an element of a map invalidates iterators pointing to that element (after all that element has been deleted). You shouldn’t reuse that iterator.

Since C++11 erase() returns a new iterator pointing to the next element, which can be used to continue iterating:

it = mymap.begin();
while (it != mymap.end()) {
   if (something)
      it = mymap.erase(it);
   else
      it++;
}

Before C++11 you would have to manually advance the iterator to the next element before the deletion takes place, for example like this:

mymap.erase(it++);

This works because the post-increment side-effect of it++ happens before erase() deletes the element. Since this is maybe not immediately obvious, the C++11 variant above should be preferred.