Exer09_22.cpp

#include <iostream>
#include <vector>
using std::vector;
int main()
{
    int some_val = 3;
    vector<int> iv{2, 3, 5, 8, 9};
    vector<int>::iterator mid = iv.begin() + iv.size() / 2;
    vector<int>::difference_type dif = mid - iv.begin();
    for(vector<int>::iterator iter = iv.begin(); iter != mid; ++iter)
    {
        if(*iter == some_val)
        {
            ++dif;
            // output element mid refers to before insertion
            std::cerr << *mid << " " << std::endl;
            ++(iter = iv.insert(iter, 2 * some_val)); // if not, iter also becomes invalid
            // output element mid refers to after insertion
            std::cerr << *mid << " " << std::endl;
            mid = dif + iv.begin();
            // reassign mid
            std::cerr << *mid << " " << std::endl;
        }
    }
    return 0;
}
// Note: from the result, we can see after insertion, the whole vector is moved
// to another place in memory, so that mid refers to an invalid address. To fix
// this problem, we have reassign mid to refer to the original mid point of iv.