iter_swap(std::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

#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

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)