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std::distance

From cppreference.com
< cpp‎ | iterator
Defined in header <iterator>
template< class InputIt >

typename std::iterator_traits<InputIt>::difference_type

    distance( InputIt first, InputIt last );
(until C++17)
template< class InputIt >

constexpr typename std::iterator_traits<InputIt>::difference_type

    distance( InputIt first, InputIt last );
(since C++17)

Returns the number of elements between first and last.

The behavior is undefined if last is not reachable from first by (possibly repeatedly) incrementing first.

(until C++11)

If InputIt is not RandomAccessIterator, the behavior is undefined if last is not reachable from first by (possibly repeatedly) incrementing first. If InputIt is RandomAccessIterator, the behavior is undefined if last is not reachable from first and first is not reachable from last.

(since C++11)

Contents

[edit] Parameters

first - iterator pointing to the first element
last - iterator pointing to the end of the range
Type requirements
-
InputIt must meet the requirements of InputIterator. The operation is more efficient if InputIt additionally meets the requirements of RandomAccessIterator

[edit] Return value

The number of increments needed to go from first to last. The value may be negative if random-access iterators are used and first is reachable from last (since C++11)

[edit] Complexity

Linear.

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

[edit] Example

#include <iostream>
#include <iterator>
#include <vector>
 
int main() 
{
    std::vector<int> v{ 3, 1, 4 };
    std::cout << "distance(first, last) = "
              << std::distance(v.begin(), v.end()) << '\n'
              << "distance(last, first) = "
              << std::distance(v.end(), v.begin()) << '\n';
}

Output:

distance(first, last) = 3
distance(last, first) = -3

[edit] See also

advances an iterator by given distance
(function) [edit]