std::iter_swap in C++

void iter_swap (ForwardIterator1 a, ForwardIterator2 b);

Here, a and b are forward iterators.

Returns: It has a void return type, so it does not 
return any value.

// C++ program to demonstrate the use of std::iter_swap
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
int main()
{
    // Declaring first vector
    vector<int> v1;
    int i;

    for (i = 0; i < 10; ++i) {
        v1.push_back(i);
    }
    // v1 contains 0 1 2 3 4 5 6 7 8 9

    vector<int>::iterator i1, i2;

    i1 = v1.begin();
    i2 = v1.end() - 1;

    // Performing swap between first and last element
    // of vector
    std::iter_swap(i1, i2);

    // Displaying v1 after swapping
    for (i = 0; i < 10; ++i) {
        cout << v1[i] << " ";
    }

    return 0;
}

9 1 2 3 4 5 6 7 8 0

std::iter_swap vs std::swap

• Optimization for node-based sequences: Most STL algorithms operate on iterator ranges. It therefore makes sense to use iter_swap when swapping elements within those ranges, swapping the elements pointed to by two iterators. This allows optimization for node-based sequences such as std::list, whereby the nodes are just relinked, rather than the data actually being swapped.
• Use in STL definition: Some STL algorithm like std::reverse involve the use of std::iter_swap in its definition. Therefore, one should have knowledge about this in order to understand these definition. CPP

  // Definition of std::reverse()
  template void reverse(BidirectionalIterator first, 
                         BidirectionalIterator last)
  {
      while ((first != last) && (first != --last)) 
      {
          std::iter_swap(first, last);
          ++first;
      }
  }
  

• Providing abstraction: iter_swap usefully encapsulates the part of the swappable interface which you would otherwise implement every time.