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std::advance - cppreference.com

template< class InputIt, class Distance >
void advance( InputIt& it, Distance n );

template< class InputIt, class Distance >
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.

(none)

Linear.

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

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.

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

namespace detail
{
    template<class It>
    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>
    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>
    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>
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());
}

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;
            }
    }
}

#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:

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