std::advance(3)

NAME
std::advance - std::advance

Synopsis
   Defined in header <iterator>
   template< class InputIt, class Distance > (until C++17)
   void advance( InputIt& it, Distance n );
   template< class InputIt, class Distance > (since C++17)
   constexpr void advance( InputIt& it, Distance n );
   Increments given iterator it by n elements.
   If n is negative, the iterator is decremented. In this case, InputIt must meet the
   requirements of LegacyBidirectionalIterator, otherwise the behavior is undefined.

Parameters
   it - iterator to be advanced
   n - number of elements it should be advanced

Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.

Return value
   (none)

Complexity
   Linear.
   However, if InputIt additionally meets the requirements of
   LegacyRandomAccessIterator, complexity is constant.

Notes
   The behavior is undefined if the specified sequence of increments or decrements
   would require that a non-incrementable iterator (such as the past-the-end iterator)
   is incremented, or that a non-decrementable iterator (such as the front iterator or
   the singular iterator) is decremented.

Possible implementation
   See also the implementations in libstdc++ and libc++.

First version
   // implementation via tag dispatch, available in C++98 with constexpr removed
   namespace detail {
   template<class It>
   constexpr // required since C++17
   void do_advance(It& it, typename std::iterator_traits<It>::difference_type n,
                   std::input_iterator_tag)
   {
       while (n > 0) {
           --n;
           ++it;
       }
   }
   template<class It>
   constexpr // required since C++17
   void do_advance(It& it, typename std::iterator_traits<It>::difference_type n,
                   std::bidirectional_iterator_tag)
   {
       while (n > 0) {
           --n;
           ++it;
       }
       while (n < 0) {
           ++n;
           --it;
       }
   }
   template<class It>
   constexpr // required since C++17
   void do_advance(It& it, typename std::iterator_traits<It>::difference_type n,
                   std::random_access_iterator_tag)
   {
       it += n;
   }
   } // namespace detail
   template<class It, class Distance>
   constexpr // since C++17
   void advance(It& it, Distance n)
   {
       detail::do_advance(it, typename std::iterator_traits<It>::difference_type(n),
                        typename std::iterator_traits<It>::iterator_category());
   }

Second version
   // implementation via constexpr if, available in C++17
   template<class It, class Distance>
   constexpr void advance(It& it, Distance n)
   {
       using category = typename std::iterator_traits<It>::iterator_category;
       static_assert(std::is_base_of_v<std::input_iterator_tag, category>);
       auto dist = typename std::iterator_traits<It>::difference_type(n);
       if constexpr (std::is_base_of_v<std::random_access_iterator_tag, category>)
           it += dist;
       else {
           while (dist > 0) {
               --dist;
               ++it;
           }
           if constexpr (std::is_base_of_v<std::bidirectional_iterator_tag, category>) {
               while (dist < 0) {
                   ++dist;
                   --it;
               }
           }
       }
   }

Example
// Run this code
 #include <iostream>
 #include <iterator>
 #include <vector>
 int main()
 {
     std::vector<int> v{ 3, 1, 4 };
     auto vi = v.begin();
     std::advance(vi, 2);
     std::cout << *vi << ' ';
     vi = v.end();
     std::advance(vi, -2);
     std::cout << *vi << '\n';
 }

Output:
 4 1

See also
   next increment an iterator
   (C++11) (function template)
   prev decrement an iterator
   (C++11) (function template)
   distance returns the distance between two iterators
                   (function template)
   ranges::advance advances an iterator by given distance or to a given bound
   (C++20) (niebloid)