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

28 Oct 2004

      ...
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