std::prev - cppreference.com (original) (raw)

| Defined in header | | | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | --------------------------- | | 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 nth predecessor (or -nth successor if n is negative) of iterator it.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Possible implementation
• 5 Notes
• 6 Example
• 7 See also

[edit] Parameters

[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 constexpr // since C++17 BidirIt prev(BidirIt it, typename std::iterator_traits::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

#include #include #include   int main() { std::vector 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:

[edit] See also

| | increment an iterator (function template) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | | advances an iterator by given distance (function template) [edit] | | | returns the distance between two iterators (function template) [edit] | | | decrement an iterator by a given distance or to a bound(algorithm function object)[edit] |