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

From: Jeff Flinn (TriumphSprint2000_at_[hidden])
Date: 2007-06-11 17:58:49

Pete Bartlett wrote:
> 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);

Why not:

        boost::shared_ptr<base> pb = static_pointer_cast<base>(pd);

>       std::cout << "\n" << 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?  

See http://www.boost.org/libs/smart_ptr/shared_ptr.htm#functions.

Jeff