Re: [boost] [gsoc] Pointer Plus Bits Behavior and Interface

Subject: Re: [boost] [gsoc] Pointer Plus Bits Behavior and Interface
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-07-16 07:36:28

vicente.botet wrote:
> From: "Stewart, Robert" <Robert.Stewart_at_[hidden]>
>
> > Now, we could rethink the extrinsic view approach and instead
> > provide two accessors that return a smart pointer and a
> > payload object.  That is, one calls a member function to get
> > the equivalent of Joaquin's pb:
> >
> >   x.pb.ptr->foo() vs x.pointer()->foo()
> >   x.pb.b          vs x.bits()
>
> The bitfield_tuple class will be extended so the user is able
> to pack a pointer and some bitfields as follows
>
> struct a;
> struct b;
> struct c;
> typedef bitfield_tuple<
>     pointer<foo, a>,
>     flag<b>,
>     flag<c>
> >  foo_bool_bool;
>
> This can be used as
>
> foo_bool_bool fbb = make_bitfield_tuple<foo_bool_bool>(new
> foo(), true, false);
>
> get<a>(fbb)->foo_member
>
> if (get<a>(fbb)!=0) {}
>
> This design gives the separated view of each one of  the
> components as suggested by Joaquin, Stefan.

Nice.

> The question was if we stop there or we provide some kind of
> smart pointer that stores a pointer but that allows to use
> the spare bits to store other info. Brian suggested me to
> rename pointer_plus_bits something  like twiddling_ptr or
> stuffling_ptr (Brian could you recall the term you used).
> This class would have as major interface the interface of a
> smart pointer, but would have also the interface as it was a
> bitfield_tuple. We can consider that
>
> twiddling_ptr<foo, flag<b>, flag<c> >
>
> is
>
> bitfield_tuple<
>     pointer<T, ptr_tag>,
>     flag<b>,
>     flag<c>
> >
>
> adding the smart pointer operations.

That's the wrong place to add the pointer knowledge.

> As the bitfield_tuple, can not have more than a pointer (at
> least not for now), we could think in providing a specific
> accessor so no tag is needed
>
> bitfield_tuple<
>     pointer<T>,
>     flag<b>,
>     flag<c>
> > pbb;
>
> pbb.pointer()->
>
> This function could be disabled (using SFINAE) if the
> bitfield_tuple has no pointer member.

That's an interesting approach.  However, a wrapper of bitfield_tuple could determine how (and how many of) the extra bits are used and provide an accessor or operators for pointer use.  That is, one wrapper might implement the smart pointer interface with accessors for the bits, while another might provide pointer() to access the pointer.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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