Getting run time error between cross- plateform with same C++ code

Question

I am erasing element from the C++ multimap using following code which is running perfectly under Linux, but throwing expression set/map is not incrementable on Windows.

void delete_entries(multimap<int, pair<int, int> > &m, int i, int j) {
    // This function deletes entries from multimap where box number == i and Ball index ==j

    multimap<int, pair<int, int> > ::iterator it = m.begin();
    while (it != m.end()) {
        if ((*it).second.first == i || (*it).second.second == j)
            m.erase(it);
        it++;
    }
}

You are ignoring the [iterator invalidation rules](http://stackoverflow.com/questions/6438086/iterator-invalidation-rules) — NathanOliver, Jul 25 '16 at 12:15
@NathanOliver i know problem in code but my question why it run on linux but not running in windows. — Ashuotsh Pandey, Jul 25 '16 at 12:16
Undefined behavior is undefined. It can do whatever it wants. — NathanOliver, Jul 25 '16 at 12:16

score 2 · Accepted Answer · edited May 23 '17 at 12:16

while (it != m.end()) {
    if ((*it).second.first == i || (*it).second.second == j)
        m.erase(it);
    it++;   //if the previous condition holds true, 'it' is invalidated
}

An erased iterator becomes invalidated. And incrementing an invalidated iterator is Undefined behavior.

Undefined behavior means that even if it seemingly works, there are no more guarantees to the behavior of your entire program. (this applies to the seemingly working one in linux too; you should be really happy that it crashed on MSVC)

For the correct way to erase, see the other answers here and this question: How to remove from a map while iterating it?

score 2 · Answer 2 · answered Jul 25 '16 at 12:26

Since C++11, erase returns the iterator following the last removed element.

void delete_entries(multimap<int, pair<int, int> > &m, int i, int j) {
// this function deletes entries from multimap where box number == i and Ball index ==j 

  multimap<int, pair<int, int> > ::iterator it = m.begin();
  while (it != m.end()) {
    if ((*it).second.first == i || (*it).second.second == j)
      it = m.erase(it);
    else
      it++;
  }
}

score 1 · Answer 3 · answered Jul 25 '16 at 12:21

1

The iterator becomes invalid after erase. Since C++11, you can do:

while (it != m.end()) {
    if ((*it).second.first == i || (*it).second.second == j)
        it = m.erase(it);
    else
        it++;
}

answered Jul 25 '16 at 12:21

rocambille

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Getting run time error between cross- plateform with same C++ code

3 Answers3