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