Insertion sort implementation using iterators and vectors - c++

Question

I'm trying to implement insertion sort algorithm using iterators and it doesn't seem to work as I thought... Do you have any ideas of how to implement it?
Also, I can't use code like this one: https://www.geeksforgeeks.org/insertion-sort-using-c-stl/ because I'm intending to make an animation and it will get more complicated.
This is my source code so far:
#include <iostream>
#include <vector>
#include <algorithm>
int main()
{
std::vector<int> seq = { 5, 4, 3, 2, 1 };
std::vector<int>::iterator itj;
std::vector<int>::iterator leftmost;
// insertion sort
for (std::vector<int>::iterator iti = seq.begin() + 1; iti != seq.end(); iti = std::next(iti))
{
itj = std::prev(iti);
leftmost = iti;
while (std::distance(seq.begin(), itj) >= 0 && *itj > *leftmost)
{
std::next(itj) = itj;
itj = prev(itj);
}
std::next(itj) = leftmost;
}
// printing
for (std::vector<int>::iterator iti = seq.begin(); iti != seq.end(); iti = std::next(iti))
{
std::cout << *iti << " ";
}
}

Answer 1

Here's a very elegant implementation of insertion sort using iterators lifted directly from the reference page on rotate:
// insertion sort
for (auto i = v.begin(); i != v.end(); ++i) {
std::rotate(std::upper_bound(v.begin(), i, *i), i, i+1);
}
All you have to do is understand how std::rotate works, and this becomes easy to understand. (rotate is anyway a really powerful algorithm that you should be comfortable with).

Answer 2

Here is an implementation taken from SGI STL1:
template<class Random_it, class T>
void unguarded_linear_insert(Random_it last, T val) {
auto next = last;
--next;
while (val < *next) {
*last = *next;
last = next;
--next;
}
*last = val;
}
template<class Random_it>
void linear_insert(Random_it first, Random_it last) {
auto val = *last;
if (val < *first) {
std::copy_backward(first, last, last + 1);
*first = val;
}
else
unguarded_linear_insert(last, val);
}
template<class Random_it>
void insertion_sort(Random_it first, Random_it last) {
if (first == last)
return;
for (auto i = first + 1; i != last; ++i)
linear_insert(first, i);
}
Note how the val < *first condition and std::copy_backward are used to simplify the loop inside unguarded_linear_insert: only one condition, namely val < *next can be checked in that loop.
1 The same implementation can be found in libstdc++.