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

From: Khandelwal, Amit (amit.khandelwal_at_[hidden])
Date: 2008-06-25 10:52:29

I am reading through the paper by B. Strousstrup on multiple inheritance and this is what is says.

A class can have an arbitrary number of virtual base classes.
One can cast from a derived class to a virtual base class, but not from a virtual base class to a derived
class. The former involves following the virtual base pointer; the latter cannot be done given the information
available at run time. Storing a ''back-pointer'' to the enclosing object(s) is non-trivial in general and
was considered unsuitable for C++ as was the alternative strategy of dynamically keeping track of the
objects ''for which'' a given member function invocation operates. The uses of such a back-pointer mechanism
did not seem to warrant the added implementation complexity, the extra space required in objects, and
the added run-time cost of initialization.

class Cloneable : public virtual CloneableBase { .. }

As per B. Strousstrup we cannot do it. Am I missing something obvious?

I have also seen the following error - something expected ..

 cannot convert from base `CloneableBase' to derived type `A' via 
   virtual base `CloneableBase

I will read through the entire chain again. 

Appreciate your time.

-----Original Message-----
From: boost-users-bounces_at_[hidden] [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Daniel Krügler
Sent: Wednesday, June 25, 2008 9:45 AM
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] shared_ptr invalid covariant return type

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

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