Removing elements from a C++ map through a for-loop

My STL is a b开发者_开发百科it rusty, so forgive me for asking a possibly trivial question. Consider the following piece of code:

map<int,int> m;
...
for (auto itr = m.begin(); itr != m.end(); ++itr) {
    if (itr->second == 0) {
        m.erase(itr);
    }
}

The question is: Is it safe to erase elements while looping over the map?

Yes, but not the way you do it. You're invalidating itr when you erase, then incrementing the invalid iterator.

auto itr = m.begin();
while (itr != m.end()) {
  if (itr->first == 0) {
    m.erase(itr++);
  } else {
    ++itr;
  }
}

I think that you shouldn't use removed iterator at all - in case of lists this causes serious problems, shouldn't be different for maps.

EDIT by Matthieu M: this code is well-formed in C++0x and allowed as an extension by MSVC.

map<int,int> m;
...
auto itr = m.begin();
while (itr != m.end())
{
    if (itr->second == 0) {
        itr = m.erase(itr);
    }
    else 
    {
        itr++;
    }
}

For the example given, It would actually be easier to use the erase overload that takes a key as an argument. This function erases all elements in the map with the given key (for a map, this is always either zero or one element)

map<int,int> m; 
// ...
m.erase(0); // erase all elements with key equivalent to 0