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std:: advance

From cppreference.net
C++
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
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(C++17) (C++20)
(C++17)
(C++17)
Defined in header <iterator>
template < class InputIt, class Distance >
void advance ( InputIt & it, Distance n ) ;
(until C++17)
template < class InputIt, class Distance >
constexpr void advance ( InputIt & it, Distance n ) ;
(since C++17)

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.

Contents

Parameters

it - iterator to be advanced
n - number of elements it should be advanced
Type requirements
-
InputIt must meet the requirements of LegacyInputIterator .

Return value

(none)

Complexity

Linear.

However, if InputIt additionally meets the requirements of LegacyRandomAccessIterator , 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

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


Non-constexpr version 
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());
}
constexpr version 
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;
            }
    }
}

Example

Run this code 
#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: 

4 1

See also

(C++11)
increment an iterator
(function template)
(C++11)
decrement an iterator
(function template)
returns the distance between two iterators
(function template)
(C++20)
advances an iterator by given distance or to a given bound
(algorithm function object)