NAME¶

std::prev - std::prev

Synopsis¶

Defined in header <iterator>
template< class BidirIt >

BidirIt prev( (since C++11)
BidirIt it, (until C++17)

typename std::iterator_traits<BidirIt>::difference_type n = 1 );
template< class BidirIt >

constexpr BidirIt prev( (since C++17)
BidirIt it,

typename std::iterator_traits<BidirIt>::difference_type n = 1 );

Return the nth predecessor of iterator it.

Parameters¶

it - an iterator
n - number of elements it should be descended

Type requirements¶

-
BidirIt must meet the requirements of LegacyBidirectionalIterator.

Return value¶

The nth predecessor of iterator it.

Complexity¶

Linear.

However, if BidirIt additionally meets the requirements of
LegacyRandomAccessIterator, complexity is constant.

Possible implementation¶

template<class BidirIt>
constexpr // since C++17
BidirIt prev(BidirIt it, typename std::iterator_traits<BidirIt>::difference_type n = 1)
{
std::advance(it, -n);
return it;
}

Notes¶

Although the expression --c.end() often compiles, it is not guaranteed to do so:
c.end() is an rvalue expression, and there is no iterator requirement that specifies
that decrement of an rvalue is guaranteed to work. In particular, when iterators are
implemented as pointers or its operator-- is lvalue-ref-qualified, --c.end() does
not compile, while std::prev(c.end()) does.

Example¶

// Run this code

#include <iostream>
#include <iterator>
#include <vector>

int main()
{
std::vector<int> v{ 3, 1, 4 };

auto it = v.end();
auto pv = std::prev(it, 2);
std::cout << *pv << '\n';

it = v.begin();
pv = std::prev(it, -2);
std::cout << *pv << '\n';
}

Output:¶

1
4

See also¶

next increment an iterator
(C++11) (function template)
advance advances an iterator by given distance
(function template)
distance returns the distance between two iterators
(function template)
ranges::prev decrement an iterator by a given distance or to a bound
(C++20) (niebloid)