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

From: Oleg Fedtchenko (BoostOleg_at_[hidden])
Date: 2004-10-28 03:44:10

> Peter Dimov wrote:
> 
> This is a bit harder.
> 
> class A
> {
> private:
>     WORD m_wData;
> 
> private:
>     A();
> 
> public:
>     static shared_ptr<A> CreateA() { return shared_ptr<A>(new A); }
> 
>     friend shared_ptr<WORD> GetData( shared_ptr<A> this_ )
>     {
>         return shared_ptr<WORD>(
>             &this_->m_wData,
>             boost::bind( &shared_ptr<A>::reset, this_ )
>         );
>     }
> };
> 
> int main()
> {
>     shared_ptr<A> pa = A::CreateA();
>     shared_ptr<WORD> pw = GetData(pa);
>     pa.reset();
>     std::cout << "*pw: " << *pw << '\n';
> }

Parameter this_ in the function (above)

     friend shared_ptr<WORD> GetData( shared_ptr<A> this_ )

is an owner of a data member this_->m_wData.

But the owner may be not an immediate 
class-container-of-the-member or its derivative but any 
arbitrary class like this:

class A
{
public:
    WORD m_wData;
};

class B
{
public:
    A a;

    friend shared_ptr<WORD> GetData( shared_ptr<B> this_ )
    {
        return shared_ptr<WORD>(
            &this_->a.m_wData,
            boost::bind( &shared_ptr<A>::reset, this_ )
        );
    }
};

In this case we must create overridden functions for every 
owner type to correctly specify pointer to a member.
And what about when A::m_wData is private?

Don't you think this is the better variant:

    template<class Z>
    shared_ptr<WORD> A::GetData( shared_ptr<Z> owner )
    {
        return shared_ptr<WORD>(
            &m_wData,
            boost::bind( &shared_ptr<Z>::reset, owner )
        );
    }

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/msg72943.php