Re: [boost] [RFC][threads] Header/namespace policy draft

From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2007-12-05 03:48:19

David Abrahams <dave <at> boost-consulting.com> writes:

> on Mon Dec 03 2007, Anthony Williams <anthony_w.geo-AT-yahoo.com> wrote:
> 
> > David Abrahams <dave <at> boost-consulting.com> writes:
> >
> >> on Mon Dec 03 2007, Anthony Williams <anthony_w.geo-AT-yahoo.com> wrote:
> >>
> >>> However, I did post a message asking for a fast-track review of it so 
> >>> that it
> >>> could become a documented part of boost. The only response I got was 
> >>> "there's
> >>> another implementation in the sandbox".
> >>
> >> And that implementation is fatally flawed, I'm ashamed to say.  We
> >> should be using http://opensource.adobe.com/group__move__related.html
> >
> > How does that help with a movable-but-not-copyable type (such as
> > boost::thread)?
> 
> I'm not sure, frankly; the author of that library believes all types
> are intrinsically both movable and copiable.  Maybe for that purpose
> we need something else.

Can I (again) propose the simple scheme I'm using for boost::thread?

In boost/thread/detail/move.hpp we (now) have

boost::detail::thread_move_t<T> as the transfer type
and boost::detail::thread_move(T&) as the move-from-lvalue helper

boost::thread then looks like this (move-related parts only)

class thread
{
private:
    thread(thread&); // no copy. Note no const
    thread& operator=(thread&); // no assign. Note no const

public:
    thread(detail::thread_move_t<thread>); // move constructor
    thread& operator=(detail::thread_move_t<thread>); // move assignment
    operator detail::thread_move_t<thread>(); // allow conversion to the 
                                              //transfer type
    detail::thread_move_t<thread> move(); // make it explicit
};

If you try and construct a thread object from an rvalue then it won't bind to
the (private) copy constructor, as it can't bind to a non-const reference.
However, a single user-defined conversion (to detail::thread_move_t<thread>)
allows it to bind to the move constructor. Likewise for assignment.

It works for gcc and MSVC, but not for Borland. When the test results come
through for the new test_thread_move test, we'll know which other compilers it
works on, too.

Anthony
-- 
Anthony Williams
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