std::ranges::advance
From cppreference.com
| Defined in header <iterator>
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| Call signature |
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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 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.
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();
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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
See also
(C++20) |
increment an iterator by a given distance or to a bound (algorithm function object) |
(C++20) |
decrement an iterator by a given distance or to a bound (algorithm function object) |
(C++20) |
returns the distance between an iterator and a sentinel, or between the beginning and end of a range (algorithm function object) |
| advances an iterator by given distance (function template) |