Map iterator not deferencable

Question

I have to find in mytextmap the closest value to keymap, for all keymap's elements and attach it to each other in a new map <char, char> called key(key of key is key of keymap, value is key of mytextmap).

Here my attempt:

for (auto i = keymap.begin(); i != keymap.end();){
    int j = ((i->second) / sumkey)*sum;
    auto tmp = find_value(mytextmap.begin(), mytextmap.end(), j);
    while (tmp->second != j)
    {
        j--;
        tmp = find_value(mytextmap.begin(), mytextmap.end(), j);
    }
    key[(*tmp).second] = i->second;
    i=keymap.erase(i);
    mytextmap.erase(tmp);
}

find_value - returns an itterator for element in (map.begin(), map.end()) with value(like j in my code).

There is an error, somewhere. Crash error: "expression map/set iterator not dereferencable.
Problem is not in erase(already fixed). On while-line it crahes. Sting before works, string after - no.

You're invalidating your iterator by removing it from the map. — Fred Larson, Feb 04 '15 at 22:53
Is there any way to remove an element with some itterator? I need 2 find an element, attach them, and them made them removed from my maps. — Misha Tihonov, Feb 04 '15 at 22:55
Sure. Use `i = keymap.erase(i);` and don't increment the iterator. — Dietmar Kühl, Feb 04 '15 at 22:57

R Sahu · Accepted Answer · 2015-02-04T23:15:58.553

1

After the line

keymap.erase(i);

the iterator, i, is invalid.

Try:

for (auto i = keymap.begin(); i != keymap.end(); )
{
   int j = ((i->second) / sumkey)*sum;
   auto tmp = find_value(mytextmap.begin(), mytextmap.end(), j);
   while (tmp->second != j)
   {
      j--;
      tmp = find_value(mytextmap.begin(), mytextmap.end(), j);
   }
   key[(*tmp).second] = i->second;
   auto next = i;
   std::advance(next, 1);
   keymap.erase(i);
   i = next;
   mytextmap.erase(tmp);
}

If you are using a C++11 compiler, you can use:

for (auto i = keymap.begin(); i != keymap.end(); )
{
   int j = ((i->second) / sumkey)*sum;
   auto tmp = find_value(mytextmap.begin(), mytextmap.end(), j);
   while (tmp->second != j)
   {
      j--;
      tmp = find_value(mytextmap.begin(), mytextmap.end(), j);
   }
   key[(*tmp).second] = i->second;
   i = keymap.erase(i);
   mytextmap.erase(tmp);
}

Update, in response to OP's comment

I think the problem lies in the lines:

   while (tmp->second != j)
   {
      j--;
      tmp = find_value(mytextmap.begin(), mytextmap.end(), j);
   }
   key[(*tmp).second] = i->second;

What happens when tmp == mytextmap.end()?

I don't understand what you are trying to accomplish in those lines. I suggest you add some checks that make sure that you don't access tmp when tmp == mytextmap.end().

edited Feb 04 '15 at 23:15

answered Feb 04 '15 at 22:57

R Sahu

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and so it still doesn't work. Same mistake. – Misha Tihonov Feb 04 '15 at 23:04
@MishaTihonov, did you make sure to remove the `++i` in the `for` line? – R Sahu Feb 04 '15 at 23:08
@R Sahu so now ive just copied ur code to be sure. Still nothing. – Misha Tihonov Feb 04 '15 at 23:09
@R Sahu, it crashes on while-line. Before erasing. – Misha Tihonov Feb 04 '15 at 23:19
@MishaTihonov, that makes sense now. See my update. – R Sahu Feb 04 '15 at 23:20
@R Sahu, yeap, ure right. I had mistake there. Just changed my while into while (tmp == mytextmap.end()) and now it works. Thank you! – Misha Tihonov Feb 04 '15 at 23:29

Map iterator not deferencable

1 Answers1