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c++deque

Erase element iterating in a while loop


I try to remove elements in deque iterating a while loop. However seems that after erasing one element,the loop skip the next element . The code is the following

 Example program
#include <iostream>
#include <string>
#include <deque>
#include <algorithm>
using namespace std;

deque<int*> queue;
int main()
{
  int * a1 = new int(0);
  int * a2 = new int(2);
  int * a3 = new int(3);
  int * a4 = new int(4);
  int * a5 = new int(5);
  queue.push_back(a1);
  queue.push_back(a2);
  queue.push_back(a3);
  queue.push_back(a4);
  queue.push_back(a5);

    deque<int * >::iterator it = queue.begin();

    while(it != queue.end())
    {
       std::cout << "Element :" << **it << std::endl;
       //remove all elements are divided by 2
       if((**it&1) == 0)
       {
        deque<int * >::iterator cur = (it+1);
        std::cout << "Erase " << **it << std::endl;
        queue.erase(std::remove(queue.begin(), queue.end(), *it), queue.end());
        delete *it;
        it = cur;
       }
       else
       {
           ++it;
       }
    }
    std::cout << "End" << std::endl;     
}

The output is the following

*Element :0 Erase 0

Element :3

Element :4 Erase 4

End*

Seems that the the element 2 and 5 were skipped.


Solution

  • You have undefined behaviour, because you are dereferencing an invalid iterator at delete *it;. To fix your immediate problem, you need something like

    int * to_del = *it;
    it = queue.erase(it);
    delete to_del;
    

    However, you should also avoid using int * where int is sufficient, and writing loops when there is something applicable in <algorithm>

    bool is_even(int i) { return (i % 2) == 0; }
    
    std::deque<int> queue { 0, 2, 3, 4, 5 };
    queue.erase(std::remove_if(queue.begin(), queue.end(), is_even), queue.end());