# std::prev

From cppreference.com
< cpp‎ | iterator

Defined in header `<iterator>` |  |
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 `n`th predecessor (or `-n`th 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.

### Return value

An iterator of type `BidirIt` that holds the `n`th predecessor (or `-n`th 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

next (C++11) | increment an iterator (function template)
advance | advances an iterator by given distance (function template)
distance | returns the distance between two iterators (function template)
ranges::prev (C++20) | decrement an iterator by a given distance or to a bound (niebloid)

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