[boost] Re: Request for data-member smart pointer

From: Oleg Fedtchenko (BoostOleg_at_[hidden])
Date: 2004-10-18 14:18:42

 >> Oleg Fedtchenko wrote: 
 >>
 >>     shared_ptr<Tree> Parse( shared_ptr<DocActiveX> ptrOwner)
 >>     {
 >>         shared_ptr<Tree> ptrMember( &m_Tree, ptrOwner);
 >>         return ptrMember;
 >>     }
 >>
 >> And corresponding ctor that passes over a deletable pointer to an
 >> owner (ptrOwner) of a class member (p):
 >>
 >>
 >>         template <class Y, class Z>
 >>         shared_ptr( Y * p, shared_ptr<Z>& ptrOwner)
 >>         : px(p), pn(ptrOwner.pn)
 >>         {...}
 >>
 >> we must specify an owner every time and easily can 
 >> pass in a shared_ptr other than pointing to the right 
 >> owner:
 
 > Larry Evans wrote:
 > 
 > Could you define a new shared_ptr templated CTOR which takes a pointer
 > to member:

 >     template<typename Member>
 >     struct shared_ptr
 >     {
 >        template<typename Owner, Member Owner::*pm_type>
 >        shared_ptr(shared_ptr<Owner>* a_owner, pm_type a_pm)
 >        : px(a_owner.get()->a_pm), pn(???)
 >        {}
 >     };

 > I haven't compiled this; hence, the syntax may not be right.
 > How would this compare with member_ptr, and the others?

I also did not compile this piece of code.
But if I caught the idea behind it right, then to place constrains on 
acceptable combinations *member-owner* looks surprising.

An owner knows all of it members exposed through shared_ptr.
And it contains all of those acceptable combinations 
*type of owner and its corresponding acceptable type of member*.

But I had a task of hierarchy of several classes two of which were
used as owners for the same type of member exposed through a
smart pointer. One of those owners had a pointer to another one.
If I had used a CTOR with both pointers to a member and its owner
(as at the beginning)
I could have passed in the pointer to another (wrong) owner 
instead of *this*. And the type *member-owner* constrains above
would have not prevented an access violation.

But member_ptr keeping a pointer to an owner in its base class 
does not allow to make a mistake.

Oleg Fedtchenko

The proposed smart pointer member_ptr is described at
http://boostoleg.narod.ru/member_ptr.html

thread entry
http://lists.boost.org/MailArchives/boost/msg73168.php