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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-06-11 18:00:40

Pete Bartlett wrote:

...

> 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
> }

The obvious question is why don't you just use

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

?

> 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.

Yes, you are relying on the author of boost::shared_ptr to not break this 
code. It so happens that boost::shared_ptr<X> is binary compatible with 
boost::shared_ptr<Y> whenever X* is binary compatible with Y*, but this 
isn't guaranteed for an arbitrary implementation of shared_ptr.

Even so, this construct is inherently dangerous since it subverts the type 
system. You can assign to *ptr_to_pb a boost::shared_ptr<base> that doesn't 
point to a 'derived', but the portion of the code that holds pd will still 
think that its target is a 'derived' and may crash trying to access the 
nonexistent members of *pd.