Implementation of shared_ptr

Hi group!

I've got a question on the implementation of std::tr1::shared_ptr. In
the book titled Beyond C++ Standard Library, a simple technique to
prevent the deletion of raw pointer of shared_ptr is presented:

#include <iostream>
#include <tr1/memory>

using namespace std;
using namespace std::tr1;

class A
{
protected:
     virtual ~A() { }
};

class B: public A
{
public:
     virtual ~B() { }
};

int main()
{
     shared_ptr<A> a(new B());
     A *raw_a = a.get();
     delete raw_a;

     return EXIT_SUCCESS;
}

In this example, delete raw_a raises a compile time error because A's
destructor is protected. However, the following successfully compiles
and run:

int main()
{
     shared_ptr<A> a(new B());
     return EXIT_SUCCESS;
}

This means that, in shared_ptr, something like the following is
happening:

B *b = dynamic_cast<B*>(a.get());
delete b;

I searched for the source code, but I could not even find the
destructor. Moreover, AFAIK, shared_ptr is implemented like:

template<typename T>
class shared_ptr
{
     T *t_;

public:
     shared_ptr(T *t): t_(t) { }

     template<typename Y>
     shared_ptr(Y *y): t_(y) { }
      ...
};

The second construtor, which takes Y *y as an argument, is provided for
the situation where type conversion can happen like the one I've shown
above (storing new B() into shared_ptr<A>). And this implies that the
type information of Y is completely lost after y is assigned to t_.
That being the case, shared_ptr<A> a(new B()) can not deallocate the
memory because the deletition of a raw pointer of type A is a compile
time error.

So, I'm curious how shared_ptr is handling the deallocation of Y *y.
Could anybody give me some pointers?

Sincerely,
Minkoo Seo

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