std::ranges::advance(3)

NAME
std::ranges::advance - std::ranges::advance

Synopsis
   Defined in header <iterator>
   Call signature
   template< std::input_or_output_iterator I > (1) (since C++20)
   constexpr void advance( I& i, std::iter_difference_t<I> n );
   template< std::input_or_output_iterator I, std::sentinel_for<I> S
   > (2) (since C++20)
   constexpr void advance( I& i, S bound );
   template< std::input_or_output_iterator I, std::sentinel_for<I> S
   > (3) (since C++20)
   constexpr std::iter_difference_t<I> advance( I& i,
   std::iter_difference_t<I> n, S bound );
   1) Increments given iterator i for n times.
   2) Increments given iterator i until i == bound.
   3) Increments given iterator i for n times, or until i == bound, whichever comes
   first.
   If n is negative, the iterator is decremented. In this case, I must model
   std::bidirectional_iterator, and S must be the same type as I if bound is provided,
   otherwise the behavior is undefined.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   i - iterator to be advanced
   bound - sentinel denoting the end of the range i is an iterator to
   n - number of maximal increments of i

Return value
   3) The difference between n and the actual distance i traversed.

Complexity
   Linear.
   However, if I additionally models std::random_access_iterator, or S models
   std::sized_sentinel_for<I>, or I and S model std::assignable_from<I&, S>, 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
  struct advance_fn
  {
      template<std::input_or_output_iterator I>
      constexpr void operator()(I& i, std::iter_difference_t<I> n) const
      {
          if constexpr (std::random_access_iterator<I>)
              i += n;
          else
          {
              while (n > 0)
              {
                  --n;
                  ++i;
              }
              if constexpr (std::bidirectional_iterator<I>)
              {
                  while (n < 0)
                  {
                      ++n;
                      --i;
                  }
              }
          }
      }
      template<std::input_or_output_iterator I, std::sentinel_for<I> S>
      constexpr void operator()(I& i, S bound) const
      {
          if constexpr (std::assignable_from<I&, S>)
              i = std::move(bound);
          else if constexpr (std::sized_sentinel_for<S, I>)
              (*this)(i, bound - i);
          else
              while (i != bound)
                  ++i;
      }
      template<std::input_or_output_iterator I, std::sentinel_for<I> S>
      constexpr std::iter_difference_t<I>
      operator()(I& i, std::iter_difference_t<I> n, S bound) const
      {
          if constexpr (std::sized_sentinel_for<S, I>)
          {
              // std::abs is not constexpr until C++23
              auto abs = [](const std::iter_difference_t<I> x) { return x < 0 ? -x : x; };
              if (const auto dist = abs(n) - abs(bound - i); dist < 0)
              {
                  (*this)(i, bound);
                  return -dist;
              }
              (*this)(i, n);
              return 0;
          }
          else
          {
              while (n > 0 && i != bound)
              {
                  --n;
                  ++i;
              }
              if constexpr (std::bidirectional_iterator<I>)
              {
                  while (n < 0 && i != bound)
                  {
                      ++n;
                      --i;
                  }
              }
              return n;
          }
      }
  };
  inline constexpr auto advance = advance_fn();

Example
// Run this code
 #include <iostream>
 #include <iterator>
 #include <vector>
 int main()
 {
     std::vector<int> v {3, 1, 4};
     auto vi = v.begin();
     std::ranges::advance(vi, 2);
     std::cout << "1) value: " << *vi << '\n' << std::boolalpha;
     std::ranges::advance(vi, v.end());
     std::cout << "2) vi == v.end(): " << (vi == v.end()) << '\n';
     std::ranges::advance(vi, -3);
     std::cout << "3) value: " << *vi << '\n';
     std::cout << "4) diff: " << std::ranges::advance(vi, 2, v.end())
               << ", value: " << *vi << '\n';
     std::cout << "5) diff: " << std::ranges::advance(vi, 4, v.end())
               << ", vi == v.end(): " << (vi == v.end()) << '\n';
 }

Output:
 1) value: 4
 2) vi == v.end(): true
 3) value: 3
 4) diff: 0, value: 4
 5) diff: 3, vi == v.end(): true

See also
   ranges::next increment an iterator by a given distance or to a bound
   (C++20) (niebloid)
   ranges::prev decrement an iterator by a given distance or to a bound
   (C++20) (niebloid)
   ranges::distance returns the distance between an iterator and a sentinel, or between
   (C++20) the beginning and end of a range
                    (niebloid)
   advance advances an iterator by given distance
                    (function template)