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

From: Khandelwal, Amit (amit.khandelwal_at_[hidden])
Date: 2008-06-25 07:49:14

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();
    }
} 

-----Original Message-----
From: boost-users-bounces_at_[hidden] [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Daniel Krügler
Sent: Wednesday, June 25, 2008 6:00 AM
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] shared_ptr invalid covariant return type

John wrote:
> Thorsten Ottosen wrote:
> 
>> John skrev:
>>
>>> This pattern/idiom is well-established:
>>>
>>>   struct A {
>>>       virtual A*  clone () = 0;
>>>   };
>>>
>>>   struct B : public A {
>>>       virtual B*  clone ();
>>>   };
>>>
>>>
>>> But the above does not work when the pointers A* and B* are replaced 
>>> by shared_ptr<A> and shared_ptr<B>:
>>>
>>>   struct  A {
>>>      virtual shared_ptr<A> clone () = 0;
>>>   };
>>>
>>>   struct  B : public A {
>>>      virtual shared_ptr<B> clone ();
>>>   };
>>>
>>>
>>> Is there a workaround? Another way to solve the same problem? Thanks!
>>
>>
>> Well, don't use shared_ptr here. A newly created clone is not shared, 
>> and it makes the interface less usable in other context to return a 
>> shared ptr here.
>>
>> -Thorsten
> 
> 
> 
> Hah! Well, yes, I guess it's true that there's always an option not to 
> use smart pointers at all. :)  Perhaps people will make too many 
> assumptions about the method based on the name, "clone".  Instead, 
> pretend the clone() method is named something else, without any other 
> assumptions.
> 
> For example, pretend it's a get_row() method for a database cursor 
> abstraction. The method might trigger a fetch, in which case the 
> returned object is not shared, or it might return a previously fecthed 
> row.  Also, whether this method is usable in non-smart-pointer context 
> is not a concern. -John

If you are interested in a more generalized approach of the same problem, you should have alook at

http://preview.tinyurl.com/3w6elg

Greetings from Bremen,

Daniel Krügler

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