# std::ranges::advance

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< cpp‎ | iterator

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 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 <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 << "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 (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)

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