Vector iterator incompatible ? Same vector

Question

I have the following code

 vector<Interval> insert(vector<Interval>& intervals, Interval newInterval) 
    {
        vector<Interval> res;
        vector<Interval>::iterator it;
        for (it = intervals.begin(); it != intervals.end(); it++) 
        {
            if (newInterval.start < (*it).start) 
            {
                intervals.insert(it, newInterval);
                break;
            }
        }
        if (it == intervals.end())  //---->vector iterator incompatible
        {
            intervals.insert(it, newInterval); 
        }
}

I am getting an error of vector iterator incompatible at the statement

    if (it == intervals.end())  //---->vector iterator incompatible
    {
        intervals.insert(it, newInterval); 
    }

Can anyone please clarify why this is happening and how I can fix this? Also this happens even when insert is not called. This is the error that i get

Answer 1

The iterator is invalidated with insert (For vector, an insertion may cause the memory reallocation, all elements are moved to a new address. The iterator is just a pointer, so it can't be used to compare with the new end of a vector, it's meaningless), as the comment mentioned, you need to reassign the return value to the it value. Or just simplified the code with std::find_if , it's preferred to use STL algorithms over hand write loop. To make code clear and more maintainable.

#include <algorithm>

vector<Interval> insert(vector<Interval>& intervals, Interval newInterval) {
  vector<Interval> res;
  if (auto itr = std::find_if(intervals.begin(), intervals.end(),
                              [&newInterval](const Interval& val) {
                                return newInterval.start < val.start;
                              });
      itr != intervals.end()) {
    intervals.insert(itr, std::move(newInterval));
  } else {
    intervals.push_back(std::move(newInterval));
  }
  // omitted
  return res;
}

Or just simplify as:

#include <algorithm>
vector<Interval> insert2(vector<Interval>& intervals, Interval newInterval) {
  vector<Interval> res;
  auto itr = std::find_if(intervals.begin(), intervals.end(),
                          [&newInterval](const Interval& val) {
                            return newInterval.start < val.start;
                          });
  intervals.insert(itr, std::move(newInterval));

  // omitted
  return res;
}