Why doesn't shared_ptr generate a cycle?

       Consider the following code using boost::shared_ptr:
#include<iostream>
#include<boost/shared_ptr.hpp>
#include<boost/weak_ptr.hpp>
using namespace std;

class B;

class A{
private:
    boost::shared_ptr<B> x;
public:
    A();
    void create(B* bb);
};

class B{
private:
    boost::shared_ptr<A> y;
public:
    B(A* aa){
        y=boost::shared_ptr<A>(aa);
    }
};

A::A(){ }

void A::create(B* bb){
    x=boost::shared_ptr<B>(bb);
}

int main(){
    A *a= new A;
    B *b= new B(a);
    a->create(b);

    return 1;
}
      In the main function , we create two dynamic objects *a and *b. *a
contains a shared_ptr<B> which points to *b, whereas *b contains a
shared_ptr<A> which points to *a. Before the main function returns ,it
should call A and B's destruction functions. But since A contains a
shared_ptr<B>,calling to A's destruction function will cause a call to
B's destruction function. Then it becomes a cycle.
      I have run the above program in VC8.0 and it generates no errors.
It doesn't accord with my expectation. I wonder why.
     Any explanations or opinions will be appreciated.

HaoZhuo

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