# std::ranges::swap

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Defined in header `<concepts>` |  |
namespace ranges { inline namespace /* unspecified */ { inline constexpr /* unspecified */ swap = /* unspecified */; } } |  | (since C++20) (customization point object)
Call signature |  |
template< class T, class U > constexpr void ranges::swap( T&& t, U&& u ) noexcept(/* see below */); |  | (since C++20)

Exchanges the values referenced by t and u.

ranges::swap(t, u) is expression-equivalent to:

1. (void)swap(t, u), if t or u has class or enumeration type, and that expression is valid, where the overload resolution is performed within namespace `std::ranges` with the additional candidate template<class T> void swap(T&, T&) = delete;.
   - If the function selected by overload resolution does not exchange the values referenced by t and u, the program is ill-formed; no diagnostic required.
2. Otherwise, (void)ranges::swap_ranges(t, u), if t and u are lvalue arrays of equal extent (but possibly different element types) and ranges::swap(*t, *u) is a valid expression, except that noexcept((void)ranges::swap_ranges(t, u)) is equal to noexcept(ranges::swap(*t, *u)).
3. Otherwise, an expression which exchanges the referenced values of t and u, if they are both lvalues of the same type `V` that models std::move_constructible<V> and std::assignable_from<V&, V>.
   - The result of applying the `noexcept` operator to that expression is equal to std::is_nothrow_move_constructible_v<V> && std::is_nothrow_move_assignable_v<V>.
   - That expression is a constant expression if
     - `V` is a LiteralType,
     - both t = std::move(u)) and u = std::move(t) are constant subexpressions, and
     - the full-expressions of the initializers in the following declarations are constant subexpressions:
       - V v1(std::move(t));
       - V v2(std::move(u));
4. Otherwise, ranges::swap(t, u) is ill-formed, which can result in substitution failure when ranges::swap(t, u) appears in the immediate context of a template instantiation.

### Customization point objects

The name `ranges::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 `__swap_fn`.

All instances of `__swap_fn` are equal. The effects of invoking different instances of type `__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::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::swap` above, `__swap_fn` models

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

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

### Example

Run this code

    #include <array>
    #include <concepts>
    #include <iostream>
    #include <ranges>
    #include <string_view>
    #include <vector>

    void print(std::string_view name,
               std::ranges::common_range auto const& p,
               std::ranges::common_range auto const& q)
    {
        std::cout << name << "1{ ";
        for (auto const& i : p)
            std::cout << i << ' ';
        std::cout << "}, " << name << "2{ ";
        for (auto const& i : q)
            std::cout << i << ' ';
        std::cout << "}\n";
    }

    void print(std::string_view name, int p, int q)
    {
        std::cout << name << "1 = " << p << ", " << name << "2 = " << q << '\n';
    }

    struct IntLike
    {
        int v;
    };

    void swap(IntLike& lhs, int& rhs)
    {
        std::swap(lhs.v, rhs);
    }

    void swap(int& lhs, IntLike& rhs)
    {
        std::swap(lhs, rhs.v);
    }

    std::ostream& operator<<(std::ostream& out, IntLike i)
    {
        return out << i.v;
    }

    int main()
    {
        std::vector a1{10, 11, 12}, a2{13, 14};
        std::ranges::swap(a1, a2);
        print("a", a1, a2);

        std::array b1{15, 16, 17}, b2{18, 19, 20};
        std::ranges::swap(b1, b2);
        print("b", b1, b2);

        // std::array c1{1, 2, 3}; std::array c2{4, 5};
        // std::ranges::swap(c1, c2); // error: no swap found by ADL

        int d1[]{21, 22, 23}, d2[]{24, 25, 26};
        std::ranges::swap(d1, d2);
        print("d", d1, d2);

        // int e1[]{1, 2, 3}, e2[]{4, 5};
        // std::ranges::swap(e1, e2); // error: extents mismatch

        // char f1[]{1, 2, 3};
        // int  f2[]{4, 5, 6};
        // std::ranges::swap(f1, f2); // error: no swap(*f1, *f2) found by ADL

        IntLike g1[]{1, 2, 3};
        int     g2[]{4, 5, 6};
        std::ranges::swap(g1, g2); // heterogeneous swap supported
        print("g", g1, g2);

        int h1{27}, h2{28};
        std::ranges::swap(h1, h2);
        print("h", h1, h2);
    }

Output:

    a1{ 13 14 }, a2{ 10 11 12 }
    b1{ 18 19 20 }, b2{ 15 16 17 }
    d1{ 24 25 26 }, d2{ 21 22 23 }
    g1{ 4 5 6 }, g2{ 1 2 3 }
    h1 = 28, h2 = 27

### See also

swappable swappable_with (C++20) | specifies that a type can be swapped or that two types can be swapped with each other (concept)
swap | swaps the values of two objects (function template)

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