# iter_swap(std::common_iterator)

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< cpp‎ | iterator‎ | common iterator

template< std::indirectly_swappable<I> I2, class S2 > friend constexpr void iter_swap( const common_iterator& x, const std::common_iterator<I2, S2>& y ) noexcept(/*see below*/); |  | (since C++20)

Swaps the objects pointed to by two underlying iterators. The behavior is undefined if `x` does not hold an `I` object or `y` does not hold an `I2` object (i.e. at least one of `x` and `y` does not hold an iterator).

The function body is equivalent to ranges::iter_swap(std::get<I>(x.var), std::get<I2>(y.var)).

This function template is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when `std::common_iterator<I,S>` is an associated class of the arguments.

### Parameters

x, y | - | the iterators to the elements to swap

### Return value

(none)

### Complexity

Constant.

### Exceptions

`noexcept` specification:
noexcept(noexcept(ranges::iter_swap(std::declval<const I&>(), std::declval<const I2&>())))

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <string>
    #include <vector>

    int main()
    {
        std::vector<std::string> v1 { "1", "2", "3", "4", "5" },
                                 v2 { "α", "β", "γ", "δ", "ε" };

        using CI = std::common_iterator<
                       std::counted_iterator<std::vector<std::string>::iterator>,
                       std::default_sentinel_t
                       >;

        CI first1 { std::counted_iterator { v1.begin(), 3 } };
        CI first2 { std::counted_iterator { v2.begin(), 4 } };
        CI last   { std::default_sentinel };

        auto print = [&] (auto rem) {
            std::cout << rem << "v1 = ";
            std::ranges::copy(v1, std::ostream_iterator<std::string>{std::cout, " "});
            std::cout << "\nv2 = ";
            std::ranges::copy(v2, std::ostream_iterator<std::string>{std::cout, " "});
            std::cout << '\n';
        };

        print("Before iter_swap:\n");

        for (; first1 != last && first2 != last; ++first1, ++first2) {
            iter_swap( first1, first2 ); // ADL
        }

        print("After iter_swap:\n");
    }

Output:

    Before iter_swap:
    v1 = 1 2 3 4 5
    v2 = α β γ δ ε
    After iter_swap:
    v1 = α β γ 4 5
    v2 = 1 2 3 δ ε

### 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 3574 | C++20 | `variant` was fully constexpr (P2231R1) but `common_iterator` was not | also made constexpr

### See also

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

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