std::ranges::advance
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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)
constexpr void advance( I& i, std::iter_difference_t <I> n );
template< std::input_or_output_iterator I, std::sentinel_for <I> S >
constexpr void advance( I& i, S bound );
(2)
(since C++20)
constexpr void advance( I& i, S bound );
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)
constexpr std::iter_difference_t <I> advance( I& i, std::iter_difference_t <I> n, S bound );
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 algorithm function objects (informally known as 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.
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>) (*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();
[edit] Example
Run this code
#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
[edit] See also
(C++20)
(algorithm function object)[edit]