std::weak_ptr<T>::swap

Since C++11

void swap( weak_ptr& r ) noexcept;

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

#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();   
}

Result

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