[Boost-users] Performing static_cast<shared_ptr<base>*> on a shared_ptr<derived>*. Any guarantees?

From: Pete Bartlett (pete_at_[hidden])
Date: 2007-06-11 18:02:16

I have taken over some code that boils down to

 

 

///begin code

 

#include <iostream>

#include <boost/shared_ptr.hpp>

 

struct base {

      int i;

      base(int i_):i(i_) {}

};

 

struct derived : base

{

      derived(int i): base(i) {}

};

 

int main()

{

      boost::shared_ptr<derived> pd(new derived(5));

      void* ptr_to_pd = &pd;

      boost::shared_ptr<base>* ptr_to_pb 

            = static_cast< boost::shared_ptr<base>* >(ptr_to_pd);

      

      std::cout << "\n" << (**ptr_to_pb).i ; //Prints 5

}

 

 

///End code

 

Now shared_ptr<B> and shared_ptr<A> are unrelated types, so IIUC there
is no guarantee that this will work. Is that right? I know from the
shared_ptr documentation that I should refactor in the direction of
dynamic_ptr_cast (though complications not shown in the code above make
that harder that). However for the time being this code appears to be
working. Is that just a fluke? I.e. I am relying on the author of
boost::shared_ptr and/or my compiler creators not to do something such
that it doesn't work. Is there the possibility that any memory could be
deleted twice? Or I am worrying over nothing and in fact there is a
reason why the above is safe?  

 

Many thanks for any advice you can provide,

Peter Bartlett