std::prev

 

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Stream iterators istream_iterator    ostream_iterator istreambuf_iterator ostreambuf_iterator	Iterator customization points ranges::iter_move (C++20) ranges::iter_swap (C++20)
Iterator operations advance distance prev (C++11)   next (C++11) ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)	Range access begincbegin (C++11)(C++14) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

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