Please help, .lock() doesn't work

[krutuk]

I learn smart pointers. And right now I am learning reference loop. I found some code in net and changed it, but it doesn't work. Please help. Function .lock() does nothing

#include <iostream>
#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>

class B;

class A
{
public:
    A() {}
    boost::weak_ptr<B> b;
};

class B
{
public:
    B() {}
    boost::shared_ptr<A> a;
};

int main()
{
    boost::shared_ptr<A> _A(new A); // A == 1
    boost::shared_ptr<B> _B(new B); // B == 1

    std::cout << "Created" << std::endl;
    std::cout << "A " << _A.use_count() << std::endl; // A == 1
    std::cout << "B " << _B.use_count() << std::endl; // B == 1

    _A->b = _B; // B != +1
    _B->a = _A; // A +1

    std::cout << "Increased" << std::endl;
    std::cout << "A " << _A.use_count() << std::endl; // A == 2
    std::cout << "B " << _B.use_count() << std::endl; // B == 1

    _B = _A->b.lock(); // B +1 if I will need it
    
    std::cout << "Locked" << std::endl;
    std::cout << "A " << _A.use_count() << std::endl; // A == 2
    std::cout << "B " << _B.use_count() << std::endl; // B == 1, but should be 2

    return 0;
}
  

[Peter87]

_B = _A->b.lock();

This doesn't really do anything because _B and _A->b are already pointing to the same object. Only one shared_ptr points to the object so use_count() returns 1 as expected.

[krutuk]

As I understood, lock() increases use_counter() when shared_ptr and weak_ptr point to another objects?

[Peter87]

lock() returns a temporary shared_ptr object. When this object is created the use count is increased by 1 because there is now one more shared_ptr pointing to the object.

The temporary shared_ptr object is then assigned to _B. The use count of the object pointed to by _B before the assignment will be decreased by 1 (if it drops to 0 it will be deleted) and then the use count of the assigned object will be increased by 1. In this case _B points to the same object before and after so the use count stays the same (-1+1=0).

At the end of the line the temporary shared_ptr object is destroyed so the use count is decreased by 1.

In total the use count didn't change.

[krutuk]

Peter87, thanks man, you really helped!