Re: [boost] Propose adding Clone Smart Pointer (clone_ptr) to theboost library

From: Axter (boost_at_[hidden])
Date: 2005-08-16 20:01:09

----- Original Message ----- 
From: "Jonathan Wakely" <cow_at_[hidden]>
Newsgroups: gmane.comp.lib.boost.devel
Sent: Tuesday, August 16, 2005 10:39 AM
Subject: Re: Propose adding Clone Smart Pointer (clone_ptr) to theboost 
library

> David Abrahams wrote:
>
>> Martin Bonner <martin.bonner_at_[hidden]> writes:
>>
>> > That looked a very clever trick.  I couldn't see how clone_ptr was 
>> > going to
>> > copy the dynamic type of the argument to the constructor without any
>> > information about that dynamic type.
>> >
>> > Having looked at the code, I don't think it does.
>>
>> Right, I went through the same thing.  The documentation claims a bit
>> too much for this library.  I expected to find some magic in the
>> library but instead I found what I already knew to be the practical
>> limitation of C++: the copying logic is captured based on the static
>> type of the pointer with which the original smart pointer was
>> initialized, not on the dynamic type of the object referred to.
>
> Which is why ptr_container requires a clone() member to ensure the
> dynamic type is available when copying. Clone_ptr has no way to access
> the dynamic type.

It does not need a clone member if you apply strict pointer ownership logic, 
which is what a clone pointer normally does.

If you create the object by passing it directly to the constructor, it will 
be able to clone itself with no problems.