#include <iostream>
#include <memory>
 
struct B;
struct A
{
    void print() const { std::cout << "A " << std::endl; }
    std::weak_ptr<B> pB;
};
 
struct B
{
    void print() const { std::cout << "B " << std::endl; }
    std::weak_ptr<A> pA;
};
 
int main()
{
    auto a = std::make_shared<A>(); 
    auto b = std::make_shared<B>();
 
    a->pB = b;
    b->pA = a;
    a->print();
    auto wb = a->pB.lock();
    if (wb) { wb->print(); } else { std::cout << "nullptr\n"; }
 
    a.reset();
 
    auto wa = b->pA.lock();
    if (wa) { wa->print(); } else { std::cout << "nullptr\n"; }
}
 

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