std::prev

template< class BidirIt >
BidirIt prev(
  BidirIt it, 
  typename std::iterator_traits<BidirIt>::difference_type n = 1 );

template< class BidirIt >
constexpr BidirIt prev(
  BidirIt it, 
  typename std::iterator_traits<BidirIt>::difference_type n = 1 );

Return the nth predecessor of iterator it.

Parameters

Return value

The nth predecessor of iterator it.

Complexity

Linear.

However, if BidirIt additionally meets the requirements of LegacyRandomAccessIterator, complexity is constant.

Possible implementation

template<class BidirIt>
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, --c.end() does not compile, while std::prev(c.end()) does.

Example

#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';
}

1

See also