std::weak_ptr::swap(3)

NAME
std::weak_ptr::swap - std::weak_ptr::swap

Synopsis
   void swap( weak_ptr& r ) noexcept; (since C++11)
   Exchanges the stored pointer values and the ownerships of *this and r. Reference
   counts, if any, are not adjusted.

Parameters
   r - smart pointer to exchange the contents with

Return value
   (none)

Example
// Run this code
 #include <iostream>
 #include <memory>
 #include <string>
 struct Foo {
     Foo(int _val) : val(_val) { std::cout << "Foo...\n"; }
     ~Foo() { std::cout << "~Foo...\n"; }
     std::string print() { return std::to_string(val); }
     int val;
 };
 int main()
 {
     std::shared_ptr<Foo> sp1 = std::make_shared<Foo>(100);
     std::shared_ptr<Foo> sp2 = std::make_shared<Foo>(200);
     std::weak_ptr<Foo> wp1 = sp1;
     std::weak_ptr<Foo> wp2 = sp2;
     auto print = [&]() {
         auto p1 = wp1.lock();
         auto p2 = wp2.lock();
         std::cout << " p1=" << (p1 ? p1->print() : "nullptr");
         std::cout << " p2=" << (p2 ? p2->print() : "nullptr") << '\n';
     };
     print();
     wp1.swap(wp2);
     print();
     wp1.reset();
     print();
     wp1.swap(wp2);
     print();
 }

Output:
 Foo...
 Foo...
  p1=100 p2=200
  p1=200 p2=100
  p1=nullptr p2=100
  p1=100 p2=nullptr
 ~Foo...
 ~Foo...