std::prev

 

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

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

See also

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