# std::ranges::iter_swap

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< cpp‎ | iterator

Defined in header `<iterator>` |  |
namespace ranges { inline namespace /* unspecified */ { inline constexpr /* unspecified */ iter_swap = /* unspecified */; } } |  | (since C++20) (customization point object)
Call signature |  |
template< class I1, class I2 > constexpr void iter_swap( I1&& i1, I2&& i2 ) noexcept(/* see below */); |  | (since C++20)
Helper function |  |
template< class X, class Y > constexpr std::iter_value_t<X> iter-exchange-move( X&& x, Y&& y ) noexcept(noexcept(std::iter_value_t<X>(std::ranges::iter_move(x))) && noexcept(*x = std::ranges::iter_move(y))); |  | (exposition only*)

Swaps values denoted by two iterators.

The effect of the exposition-only helper function `iter-exchange-move` is equivalent to

    std::iter_value_t<X> old(std::ranges::iter_move(x));
    *x = std::ranges::iter_move(y);
    return old;

ranges::iter_swap(i1, i2) is expression-equivalent to:

1. (void)iter_swap(i1, i2), if i1 or i2 has a class or enumeration type and the expression is well-formed, where the overload resolution of `iter_swap` is performed with the additional candidate void iter_swap(auto, auto) = delete;[1], excluding `std::ranges::iter_swap` itself.
   - If the selected overload does not exchange the value denoted by i1 and i2, the program is ill-formed, no diagnostic required.
2. Otherwise, ranges::swap(*i1, *i2) if both `I1` and `I2` model `indirectly_readable` and if std::iter_reference_t<I1> and std::iter_reference_t<I2> model `swappable_with`.
3. Otherwise, (void)(*i1 =`` `iter-exchange-move`(i2, i1)), if std::indirectly_movable_storable<I1, I2> and std::indirectly_movable_storable<I2, I1> are both modeled, except that i1 is only evaluated once.
4. Otherwise, ranges::iter_swap(i1, i2) is ill-formed, which can result in substitution failure when ranges::iter_swap(i1, i2) appears in the immediate context of a template instantiation.

1. ↑ This precludes calling unconstrained std::iter_swap.

### Customization point objects

The name `ranges::iter_swap` denotes a customization point object, which is a const function object of a literal `semiregular` class type. For exposition purposes, the cv-unqualified version of its type is denoted as `__iter_swap_fn`.

All instances of `__iter_swap_fn` are equal. The effects of invoking different instances of type `__iter_swap_fn` on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, `ranges::iter_swap` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval<Args>()... meet the requirements for arguments to `ranges::iter_swap` above, `__iter_swap_fn` models

- std::invocable<__iter_swap_fn, Args...>,
- std::invocable<const __iter_swap_fn, Args...>,
- std::invocable<__iter_swap_fn&, Args...>, and
- std::invocable<const __iter_swap_fn&, Args...>.

Otherwise, no function call operator of `__iter_swap_fn` participates in overload resolution.

### Example

| This section is incomplete Reason: no example

### See also

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 | swaps the elements pointed to by two iterators (function template)

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