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

From: Daniel Krügler (dsp_at_[hidden])
Date: 2008-06-25 06:00:23

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