std::prev

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

Return the countth predecessor (or -countth successor if count is negative) of iterator iter.

Parameters

Return value

As described above.

Complexity

Linear in count.

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

Possible implementation

template<class BidirIt>
constexpr // since C++17
BidirIt prev(BidirIt iter,
             typename std::iterator_traits<BidirIt>::difference_type count = 1)
{
    std::advance(iter, -count);
    return iter;
}

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

#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';
}

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See also