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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.


[edit] Parameters

x, y - the iterators to the elements to swap

[edit] Return value


[edit] Complexity


[edit] Exceptions

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

[edit] Example

#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<
    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");


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

[edit] 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

[edit] See also

swaps the values of two objects
(function template) [edit]
swaps two ranges of elements
(function template) [edit]
swaps the elements pointed to by two iterators
(function template) [edit]
swaps the values referenced by two dereferenceable objects
(customization point object) [edit]
swaps the objects pointed to by two underlying iterators
(function template) [edit]