Unordered_map iterator invalidation

Question

i have this iterator loop,

typedef  boost::unordered_map<std::pair<int, int>, NavigationNode> NodesMap;
NodesMap nodes;
for (NodesMap::iterator it= nodes.begin(); it != nodes.end() ; ++it)
{

  if(it->second.type == NavigationNodeType_Walkable)
  {
    ConnectNode(&it->second);
  }

}

ConnectNode function seems to be invalidating the iterator. It pushes new elements inside the NavigationNode and modifies existing members of the NavigationNode.

i have two questions

Is passing it->second as pointer bad?
What's the best way to iterate through this container?

thank you.

edit:

does accessing container's elements like this

   nodes[intpair(x, y)]

inside ConnectNode function would cause this problem?

edit2 yes it does.

Why is that? and How would i get around it?

Iterators to an unordered map are only invalidated if the container itself is modified, either by erasing the element to which you had the iterator, or when rehashing occurs. — Kerrek SB, Apr 22 '12 at 13:06
i suspect it, when i comment out ConnectNode function it works fine. — mikbal, Apr 22 '12 at 13:07
if this is not a iterator invalidation. do you know anything that would cause this? — mikbal, Apr 22 '12 at 13:10
@mikbal: It sounds like `ConnectNode` does something to `nodes`, but without seeing the definition of that function we can only guess. — Mike Seymour, Apr 22 '12 at 13:26

juanchopanza · Accepted Answer · 2012-04-22T13:28:22.460

1

Is passing it->second as pointer bad?

It depends on what the function that takes the pointer itself does. Taken in isolation, there is nothing inherently wrong with passing a pointer.
What's the best way to iterate through this container?

The way that you are using is fine. Using begin() and end() to iterate is pretty standard.

So I think the problem must be with the ConnectNode, and it is likely that you do not have an iterator invalidation problem, but something else.

"does accessing container's elements like this"

nodes[intpair(x, y)]

This will add a new entry to the map is one with key intpair(x,y) doesn't exist, so yes, this could mess up the iteration. You can avoid this by checking if the element exists for that key before accessing with the [] operator.

edited Apr 22 '12 at 13:28

answered Apr 22 '12 at 13:19

juanchopanza

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accessing nodes map inside ConnectNode function caused it.(check edit) do you know why? – mikbal Apr 22 '12 at 13:21
@mikbal accessing the nodes like that would add new elements if they don't exist, so yes, it could mess things up. – juanchopanza Apr 22 '12 at 13:26
ok. i should first check if it exists, then try to access it. thank you. that was it. – mikbal Apr 22 '12 at 13:31

score 1 · Answer 2 · answered Apr 22 '12 at 13:28

According to your update, ConnectNode can modify the map you're iterating over.

nodes[intpair(x, y)]

If that key doesn't already exist in the map, then a new element will be inserted. That might cause the map to be rehashed, invalidating all iterators.

To avoid modifying the map, then you could use find() or at(). find() will indicate whether the key exists via its return value, and at() will throw if the key doesn't exist.

If you do need to add new elements to the map during ConnectNode, then things are rather more tricky. You could perhaps put them in a separate container, and then add them to nodes after your loop.

Unordered_map iterator invalidation

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