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