std::iter_swap

 

C++

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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)