std::ranges::distance
Defined in header <iterator> |
||
|---|---|---|
| Call signature | ||
template< std::input_or_output_iterator I, std::sentinel_for<I> S >
requires (!std::sized_sentinel_for<S, I>)
constexpr std::iter_difference_t<I> distance( I first, S last ); |
(1) | (since C++20) |
template< std::input_or_output_iterator I, std::sized_sentinel_for<I> S > constexpr std::iter_difference_t<I> distance( const I& first, const S& last ); |
(2) | (since C++20) |
template< ranges::range R > constexpr ranges::range_difference_t<R> distance( R&& r ); |
(3) | (since C++20) |
1,2) Returns the number of hops from
first to
last.
3) Returns the size of
r as a signed integer.
The function-like entities described on this page are niebloids, that is:
- Explicit template argument lists may not be specified when calling any of them.
- None of them is visible to argument-dependent lookup.
- When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.
In practice, they may be implemented as function objects, or with special compiler extensions.
Parameters
| first | - | iterator pointing to the first element |
| last | - | sentinel denoting the end of the range first is an iterator to |
| r | - | range to calculate the distance of |
Return value
1) The number of increments needed to go from
first to
last.
2)
last - first.
3) If
R models
ranges::sized_range, returns
ranges::size(r); otherwise
ranges::distance(ranges::begin(r), ranges::end(r)).
Complexity
1) Linear.
2) Constant.
2) If
R models
ranges::sized_range or if
std::sized_sentinel_for<ranges::sentinel_t<R>, ranges::iterator_t<R>> is modeled, complexity is constant; otherwise linear.
Possible implementation
struct distance_fn {
template<std::input_or_output_iterator I, std::sentinel_for<I> S>
requires (!std::sized_sentinel_for<S, I>)
constexpr std::iter_difference_t<I> operator()(I first, S last) const
{
std::iter_difference_t<I> result = 0;
while (first != last) {
++first;
++result;
}
return result;
}
template<std::input_or_output_iterator I, std::sized_sentinel_for<I> S>
constexpr std::iter_difference_t<I> operator()(const I& first, const S& last) const
{
return last - first;
}
template<ranges::range R>
constexpr ranges::range_difference_t<R> operator()(R&& r) const
{
if constexpr (ranges::sized_range<std::remove_cvref_t<R>>) {
return static_cast<ranges::range_difference_t<R>>(ranges::size(r));
}
else {
return (*this)(ranges::begin(r), ranges::end(r));
}
}
};
inline constexpr auto distance = distance_fn{};
|
Example
#include <iostream>
#include <iterator>
#include <vector>
int main()
{
std::vector<int> v{ 3, 1, 4 };
namespace ranges = std::ranges;
std::cout << "distance(first, last) = "
<< ranges::distance(v.begin(), v.end()) << '\n'
<< "distance(last, first) = "
<< ranges::distance(v.end(), v.begin()) << '\n'
<< "distance(v) = " << ranges::distance(v) << '\n';
}
Output:
distance(first, last) = 3 distance(last, first) = -3 distance(v) = 3
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3392 | C++20 | distance takes iterator by value, thus rejecting move-only iterator lvalue with a sized sentinel |
by reference overload added |
See also
|
(C++20)
|
advances an iterator by given distance or to a given bound (niebloid) |
|
(C++20)(C++20)
|
returns the number of elements satisfying specific criteria (niebloid) |
| returns the distance between two iterators (function template) |
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/iterator/ranges/distance
