std::prev
From cppreference.com
| Defined in header <iterator>
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template< class BidirIt > BidirIt prev( BidirIt it, typename std::iterator_traits<BidirIt>::difference_type n = 1 ); |
(since C++11) (until C++17) |
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| template< class BidirIt > constexpr |
(since C++17) | |
Return the nth predecessor (or -nth 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.
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Return value
An iterator of type BidirIt that holds the nth predecessor (or -nth 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
| (C++11) |
increment an iterator (function template) |
| advances an iterator by given distance (function template) | |
| returns the distance between two iterators (function template) | |
| (C++20) |
decrement an iterator by a given distance or to a bound (algorithm function object) |