[iterator.operations]

# 24 Iterators library [[iterators]](./#iterators)

## 24.4 Iterator primitives [[iterator.primitives]](iterator.primitives#iterator.operations)

### 24.4.3 Iterator operations [iterator.operations]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2834)

Since only random access iterators provide+ and- operators, the library provides two
function templatesadvance anddistance[.](#1.sentence-1)

These
function templates
use+ and- for random access iterators (and are, therefore, constant
time for them); for input, forward and bidirectional iterators they use++ to provide linear time
implementations[.](#1.sentence-2)

[🔗](#lib:advance)

`template<class InputIterator, class Distance>
  constexpr void advance(InputIterator& i, Distance n);
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2863)

*Preconditions*: n is negative only for bidirectional iterators[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2868)

*Effects*: Increments i by n if n is non-negative, and
decrements i by -n otherwise[.](#3.sentence-1)

[🔗](#lib:distance)

`template<class InputIterator>
  constexpr typename iterator_traits<InputIterator>::difference_type
    distance(InputIterator first, InputIterator last);
`

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2882)

*Preconditions*: last is reachable from first, orInputIterator meets
the *Cpp17RandomAccessIterator* requirements andfirst is reachable from last[.](#4.sentence-1)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2889)

*Effects*: If InputIterator meets the *Cpp17RandomAccessIterator* requirements,
returns (last - first); otherwise, incrementsfirst until last is reached and returns
the number of increments[.](#5.sentence-1)

[🔗](#lib:next)

`template<class InputIterator>
  constexpr InputIterator next(InputIterator x,
    typename iterator_traits<InputIterator>::difference_type n = 1);
`

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2905)

*Effects*: Equivalent to: advance(x, n); return x;

[🔗](#lib:prev)

`template<class BidirectionalIterator>
  constexpr BidirectionalIterator prev(BidirectionalIterator x,
    typename iterator_traits<BidirectionalIterator>::difference_type n = 1);
`

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2918)

*Effects*: Equivalent to: advance(x, -n); return x;