std::ranges::advance

Defined in header `<iterator>`
Call signature
template< std::input_or_output_iterator I > constexpr void advance( I& i, std::iter_difference_t<I> n );	(1)	(since C++20)
template< std::input_or_output_iterator I, std::sentinel_for<I> S > constexpr void advance( I& i, S bound );	(2)	(since C++20)
template< std::input_or_output_iterator I, std::sentinel_for<I> S > constexpr std::iter_difference_t<I> advance( I& i, std::iter_difference_t<I> n, S bound );	(3)	(since C++20)

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

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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