Problem with std::map::iterator after calling erase()

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.

Answer 2

Calling erase() invalidates the iterator. In this case, what's happening is the iterator points to the residual value left behind in memory (but don't rely on this undefined behaviour!). Reset the iterator with it=mymap.begin() before the loop for the desired results.

http://codepad.org/zVFRtoV5

This answer shows how to erase elements while iterating over an std::map:

for(map<T, S*>::iterator it = T2pS.begin(); it != T2pS.end(); T2pS.erase(it++)) {
    // wilhelmtell in the comments is right: no need to check for NULL. 
    // delete of a NULL pointer is a no-op.
    if(it->second != NULL) {
        delete it->second;
            it->second = NULL;
    }
}