std::ranges::swap(3)

NAME
std::ranges::swap - std::ranges::swap

Synopsis
   Defined in header <concepts>
   namespace ranges {
       inline namespace /* unspecified */ {
           inline constexpr /* unspecified */ swap = /* (since C++20)
   unspecified */; (customization point object)
       }
   }
   Call signature
   template< class T, class U >
   constexpr void ranges::swap( T&& t, U&& u ) (since C++20)
   noexcept(/* see below */);
   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 specifies that a type can be swapped or that two types can be swapped
   swappable_with with each other
   (C++20) (concept)
   swap swaps the values of two objects
                  (function template)