std::swap

#include <algorithm>
#include <iostream>
 
namespace Ns
{
 class A
 {
 int id {};
 
 friend void swap(A& lhs, A& rhs)
 {
 std::cout << "swap(" << lhs << ", " << rhs << ")\n";
 std::swap(lhs.id, rhs.id);
 }
 
 friend std::ostream & operator<<(std::ostream & os, A const& a)
 {
 return os << "A::id=" << a.id;
 }
 
 public:
 A(int i) : id {i} {}
 A(A const&) = delete;
 A& operator = (A const&) = delete;
 };
}
 
int main()
{
 int a = 5, b = 3;
 std::cout << a << ' ' << b << '\n';
 std::swap(a, b);
 std::cout << a << ' ' << b << '\n';
 
 Ns::A p {6}, q {9};
 std::cout << p << ' ' << q << '\n';
// std::swap(p, q); // error, type requirements are not satisfied
 swap(p, q); // OK, ADL finds the appropriate friend `swap`
 std::cout << p << ' ' << q << '\n';
}

5 3
3 5
A::id=6 A::id=9
swap(A::id=6, A::id=9)
A::id=9 A::id=6