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Showing content from https://en.cppreference.com/w/cpp/algorithm/../iterator/next.html below:

std::next - cppreference.com

Return the n^th successor (or -n^th predecessor if n is negative) of iterator it.

An iterator of type InputIt that holds the n^th successor (or -n^th predecessor if n is negative) of iterator it.

Linear.

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

template<class InputIt>
constexpr // since C++17
InputIt next(InputIt it, typename std::iterator_traits<InputIt>::difference_type n = 1)
{
    std::advance(it, n);
    return it;
}

Although the expression ++c.begin() often compiles, it is not guaranteed to do so: c.begin() is an rvalue expression, and there is no LegacyInputIterator requirement that specifies that increment of an rvalue is guaranteed to work. In particular, when iterators are implemented as pointers or its operator++ is lvalue-ref-qualified, ++c.begin() does not compile, while std::next(c.begin()) does.

#include <iostream>
#include <iterator>
#include <vector>
 
int main()
{
    std::vector<int> v{4, 5, 6};
 
    auto it = v.begin();
    auto nx = std::next(it, 2);
    std::cout << *it << ' ' << *nx << '\n';
 
    it = v.end();
    nx = std::next(it, -2);
    std::cout << ' ' << *nx << '\n';
}

Output:

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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