# std::iter_swap

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class ForwardIt1, class ForwardIt2 > void iter_swap( ForwardIt1 a, ForwardIt2 b ); |  | (constexpr since C++20)

Swaps the values of the elements the given iterators are pointing to.

If any of the following conditions is satisfied, the behavior is undefined:

- a or b is not dereferenceable.
- *a is not Swappable with *b.

### Parameters

a, b | - | iterators to the elements to swap
Type requirements
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.

### Return value

(none)

### Complexity

Constant.

### Notes

This function template models the semantics of the swap operation given by Swappable. That is, overloads of swap found by ADL and the fall back of std::swap are considered.

### Possible implementation

    template<class ForwardIt1, class ForwardIt2>
    constexpr //< since C++20
    void iter_swap(ForwardIt1 a, ForwardIt2 b)
    {
        using std::swap;
        swap(*a, *b);
    }

### Example

The following is an implementation of selection sort in C++.

Run this code

    #include <algorithm>
    #include <iostream>
    #include <random>
    #include <string_view>
    #include <vector>

    template<class ForwardIt>
    void selection_sort(ForwardIt begin, ForwardIt end)
    {
        for (ForwardIt it = begin; it != end; ++it)
            std::iter_swap(it, std::min_element(it, end));
    }

    void println(std::string_view rem, std::vector<int> const& v)
    {
        std::cout << rem;
        for (int e : v)
            std::cout << e << ' ';
        std::cout << '\n';
    }

    template<int min, int max>
    int rand_int()
    {
        static std::uniform_int_distribution dist(min, max);
        static std::mt19937 gen(std::random_device{}());
        return dist(gen);
    }

    int main()
    {
        std::vector<int> v;
        std::generate_n(std::back_inserter(v), 20, rand_int<-9, +9>);

        std::cout << std::showpos;
        println("Before sort: ", v);
        selection_sort(v.begin(), v.end());
        println("After sort:  ", v);
    }

Possible output:

    Before sort: -9 -3 +2 -8 +0 -1 +8 -4 -5 +1 -4 -5 +4 -9 -8 -6 -6 +8 -4 -6
    After sort:  -9 -9 -8 -8 -6 -6 -6 -5 -5 -4 -4 -4 -3 -1 +0 +1 +2 +4 +8 +8

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 187 | C++98 | it was unspecified whether swap is used | the effect is equivalent to swap(*a, *b)

### See also

swap | swaps the values of two objects (function template)
swap_ranges | swaps two ranges of elements (function template)
iter_swap (C++20) | swaps the objects pointed to by two adjusted underlying iterators (function template)
iter_swap (C++20) | swaps the objects pointed to by two underlying iterators (function template)
iter_swap (C++20) | swaps the values referenced by two dereferenceable objects (customization point object)

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