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std::ranges::advance

From cppreference.com
< cpp‎ | iterator
 
 
Iterator library
Iterator concepts
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Indirect callable concepts
Common algorithm requirements
Utilities
Iterator adaptors
Stream iterators
Iterator customization points
Iterator operations
(C++11)
(C++11)
ranges::advance
(C++20)
Range access
(C++11)(C++14)
(C++11)(C++14)
(C++17)(C++20)
(C++14)(C++14)
(C++14)(C++14)
(C++17)
(C++17)
 
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:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] 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

[edit] Return value

3) The difference between n and the actual distance i traversed.

[edit] 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.

[edit] 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.

[edit] 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>) {
        ranges::advance(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
        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) {
            ranges::advance(i, bound);
            return -dist;
        }
 
        ranges::advance(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();

[edit] Example

#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

[edit] See also

increment an iterator by a given distance or to a bound
(niebloid) [edit]
decrement an iterator by a given distance or to a bound
(niebloid) [edit]
returns the distance between an iterator and a sentinel, or between the beginning and end of a range
(niebloid) [edit]
advances an iterator by given distance
(function template) [edit]