std::distance

C++

Language

Headers

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Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Iterator library

Iterator primitives
iterator_traits
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tag
iterator
Iterator adaptors
reverse_iterator move_iterator (C++11) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) back_inserter front_inserter inserter
Stream iterators
istream_iterator
ostream_iterator
istreambuf_iterator
ostreambuf_iterator
Iterator operations
advance
distance
prev (C++11)
next (C++11)
Range access
begincbegin (C++11)(C++14)
endcend (C++11)(C++14)
rbegincrbegin (C++14)(C++14)
rendcrend (C++14)(C++14)
Container access
size (C++17)
empty (C++17)
data (C++17)

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 hops from first to last.

Parameters

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)

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

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)

Complexity

Linear.

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

Example

Run this code

#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';
               //the behavior is undefined (until C++11)
}

Output:

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

See also

advance	advances an iterator by given distance (function)
countcount_if	returns the number of elements satisfying specific criteria (function template)