Re: [boost] shared_ptr: unsafe, or am I crazy?

From: Luigi Ballabio (ballabio_at_[hidden])
Date: 2002-01-14 12:24:16

At 08:56 AM 1/14/02 -0500, you wrote:
>struct Base { Base(); ~Base(); };
>struct Derived : Base { Derived(); ~Derived(); };
>
>void unsafe()
>{
>     shared_ptr<Derived> d(new Derived);
>     shared_ptr<Base> b(d);
>     d.reset();
>     b.reset(); // equivalent to delete (Base*)(new Derived)!!
>};
>
>Isn't shared_ptr just screaming for the application of John Maddock's new
>is_polymorphic<> trait?

Maybe I'm just about to embarass myself, but isn't Base just screaming for 
having a virtual destructor instead, if you want to use it this way?

IMHO the point here is that a virtual or a non-virtual destructor are a 
strong statement: a virtual destructor means "My derived classes are meant 
to be used polymorphically: go ahead and upcast", while a non-virtual one 
means "My derived classes are meant to be used as such: pray do not upcast."

If the user wants to upcast, there's nothing in the language to prevent 
him. But if he does, then he's on his own since he disregarded an explicit 
(well, IMHO anyway) hint by the class writer.

Bye,
         Luigi