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 may not be specified when calling any of them.
     * None of them is visible to argument-dependent lookup.
     * When one of them is found by normal unqualified lookup for the name to the left
       of the function-call operator, it inhibits argument-dependent lookup.
   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 isn't constexpr until C++23
           auto abs = [](const std::iter_difference_t<I> x) { return x < 0 ? -x : x; };
           const auto dist = abs(n) - abs(bound - i);
           if (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 <iomanip>
 #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 << "value: " << *vi << '\n';
     {
         std::ranges::advance(vi, v.end());
         std::cout << std::boolalpha;
         std::cout << "vi == v.end(): " << (vi == v.end()) << '\n';
         std::ranges::advance(vi, -3);
         std::cout << "value: " << *vi << '\n';
         std::cout << "diff: " << std::ranges::advance(vi, 2, v.end()) << ", ";
         std::cout << "value: " << *vi << '\n';
         std::cout << "diff: " << std::ranges::advance(vi, 4, v.end()) << ", ";
         std::cout << "vi == v.end(): " << (vi == v.end()) << '\n';
         std::cout << std::noboolalpha;
     }
 }

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