Re: [Boost-users] shared_ptr invalid covariant return type

From: Daniel Krügler (dsp_at_[hidden])
Date: 2008-06-25 09:45:20

Khandelwal, Amit wrote:
> I read the discussion and tried it using gcc version 3.2.3 I am getting the following error. 
> 
> clone2.cpp:34: sorry, not implemented: adjusting pointers for covariant returns
> clone2.cpp:34: sorry, not implemented: adjusting pointers for covariant returns
> clone2.cpp:34: sorry, not implemented: adjusting pointers for covariant returns
> clone2.cpp: In member function `std::auto_ptr<Derived> 
>    Cloneable<Derived>::clone() const [with Derived = A]':
> clone2.cpp:50:   instantiated from here
> 
> clone2.cpp:17: invalid static_cast from type `CloneableBase*' to type `A*
> 
> 
> I think my code is right. Any thoughts?
> 
> 
> #include <memory>
> 
> class CloneableBase {
> public:
>         virtual ~CloneableBase() {};
> 
> protected:
>         virtual CloneableBase* doClone() const = 0 ;
> };
> 
> 
> template < typename Derived >
> class Cloneable : public virtual CloneableBase {
> public:
>         typedef std::auto_ptr<Derived> AutoPtr;
>         AutoPtr clone() const {
>             return AutoPtr( static_cast<Derived *>( doClone() ) );
>         }
> };
> 
> 
> template < typename Derived >
> class NaturallyCloneable : public virtual CloneableBase {
> protected:
> 
>         virtual CloneableBase* doClone() const {
>             return new Derived( static_cast< const Derived& >(*this) );
>         }
> };
> 
> 
> class A : public Cloneable<A> {}; // abstract base for B and C 
> class B : public A, public NaturallyCloneable<B> {};
> class C : public A {
>     virtual A* doClone() const {
>             return new C;
>     }
> };
> 
> 
> int main() {
>     {
>         B b;
>         C c;
>         b.clone();
>         c.clone();
>     }
> } 

This problem was discussed in the thread and lead to the final 
implementation that uses dynamic_cast instead of static_cast
inside of Cloneable. The final solution also provides a nice
SmartPointer policy, which solves the problem of the return type
(Note that the pointer policy does even support naked pointers
as well).

Greetings from Bremen,

Daniel