[boost] Re: Re: smarter auto_ptr

From: Jonathan Turkanis (technews_at_[hidden])
Date: 2004-01-18 18:16:56

"Daniel Wallin" <dalwan01_at_[hidden]> wrote in message
news:400B0CA9.4050209_at_student.umu.se...
> Howard Hinnant wrote:
> > <snip>
> >
> > Below is a brief, realistic demo of the dangers of moving with
copy
> > syntax from an lvalue:
> >
> ><snip>
> >
> > It is safe to move from lvalues with some syntax other than copy.
So
> > the functionality of auto_ptr can still be had, just not with the
same
> > syntax.  You might consider something like:
> >
> > http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/
> > n1377.htm#move_ptr%20Example
> >
> > This example relies on a language feature not available to you.
> > However, I believe you can emulate this behavior in today's C++
using
> > the same techniques you have demonstrated.
>
> How about:
>
>    template<class T>
>    class move_ptr
>    {
>    public:
>        move_ptr() : m_p(0) {}
>
>        template<class U>
>        move_ptr(const move_ptr<U>& p)
>            : m_p(const_cast<move_ptr<U>&>(p).release())
>        {}
>
>        explicit move_ptr(T* p) : m_p(p)
>        {}
>
>        ~move_ptr()
>        {
>          delete m_p;
>        }
>
>        move_ptr& operator=(move_ptr p)
>        {
>            p.swap(*this);
>            return *this;
>        }
>
>        void reset(T* p)
>        {
>            move_ptr(p).swap(*this);
>        }
>
>        T* release()
>        {
>            T* p = m_p;
>            m_p = 0;
>            return p;
>        }
>
>        void swap(move_ptr& p)
>        {
>            std::swap(m_p, p.m_p);
>        }
>
>        T& operator*() const
>        {
>            return *m_p;
>        }
>
>        T* operator->() const
>        {
>            return m_p;
>        }
>
>    private:
>        template<class U> struct cant_move_from_const;
>        template<class U> struct cant_move_from_const<const
move_ptr<U> >
>        {
>            typedef typename move_ptr<U>::error type;
>        };
>
>        template<class U>
>        move_ptr(U&, typename cant_move_from_const<U>::type = 0);
>
>        move_ptr(move_ptr&);
>        template<class U> move_ptr(move_ptr<U>&);
>
>        T* m_p;
>    };
>

I was going to try this later today. You beat me to it!

It looks good on VC7.1 and Intel7.1 (Windows). Codewarrior 8.0 can't
handle it, as expected.

Jonathan