Re: Containers for derived classes

Daniel Wallin <[email protected]> writes:

> At 02:51 2003-08-11, David Abrahams wrote:
>>Daniel Wallin <[email protected]> writes:
>>
>> > BTW. What's the motivation for holding objects by shared_ptr<..>?
>>
>>A very good question.  I'm not sure what *motivates* it, but you do
>>get certain capabilities which you wouldn't get otherwise.  For
>>example, you can convert the Python object to shared_ptr<...>&, and
>>then the shared_ptr itself can be modified.  Whether or not that's
>>actually useful is another question.
>
> Ok.
>
>>If we'd like to always be able to use your adopt() policy without worrying
>> about a runtime failure, I think we'd want to hold all C++ objects
>> by auto_ptr
>>instead.
>
> It should be noted here that our implementation differs from yours on how
> ownership of the object is handled. In luabind we can tell the object_rep
> (kind of like the holder..) to give up ownership, meaning it won't delete
> it's held object when collected.

I plan to adopt that approach, but my point is that it has certain
limitations because you can never tell a group of sharing shared_ptr's
to give up ownership.  So you can null out the shared_ptr in the
holder, but you still have to hang onto something... a shared_ptr, if
you want to keep the object alive.

> We should share your idea with holders though, but maybe the holder
> type could be extended to allow more complex control?
>
> struct holder
> {
>    virtual void release() { assert(0, "unable to release object"); }
>    virtual void* holds(const type_info&) const = 0;
> };
>
> This could mean smart pointer types could potentially be used
> with adopt()..

I think it might be simpler just to use an auto_ptr holder as the
default mechanism.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com



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