NAME

std::prev - std::prev

Synopsis

Defined in header <iterator>
template< class BidirIt > (since C++11)
BidirIt prev( BidirIt it, typename (until C++17)
std::iterator_traits<BidirIt>::difference_type n = 1 );
template< class BidirIt >

constexpr (since C++17)

BidirIt prev( BidirIt it, typename
std::iterator_traits<BidirIt>::difference_type n = 1 );

Return the n^th predecessor (or -n^th successor if n is negative) 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

An iterator of type BidirIt that holds the n^th predecessor (or -n^th successor if n
is negative) 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