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) BidirIt prev( BidirIt it, typename std::iterator_traits <BidirIt>::difference_type n = 1 );
(until C++17)
template< class BidirIt >
(since C++17)
constexpr
Return the nth predecessor (or -nth successor if n is negative) of iterator it.
Contents
[edit] Parameters
it
-
an iterator
n
-
number of elements it should be descended
Type requirements
-
BidirIt must meet the requirements of LegacyBidirectionalIterator.
[edit] Return value
An iterator of type BidirIt that holds the nth predecessor (or -nth successor if n is negative) of iterator it.
[edit] Complexity
Linear.
However, if BidirIt additionally meets the requirements of LegacyRandomAccessIterator, complexity is constant.
[edit] 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; }
[edit] 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.
[edit] 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