Re: [boost] [fusion] mpl::clear doesn't work on fusion views

Subject: Re: [boost] [fusion] mpl::clear doesn't work on fusion views
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2011-07-29 16:36:01

On Fri, Jul 29, 2011 at 12:52 PM, Eric Niebler <eric_at_[hidden]> wrote:

> Today I ran into a limitation in Fusion/MPL interoperability. I couldn't
> use an mpl algorithm on a Fusion sequence because mpl::clear wasn't
> defined on it. The sequence was a joint_view. I see that
> fusion::detail::clear is only defined for Fusion cons, map, set, vector
> and deque. Can we get it defined for all Fusion sequences? I think for
> everything else, an empty Fusion vector would do. That is all
> fusion::clear does, btw.
>
> (Aside: it seems strange to me that mpl::clear of a fusion::set gives an
> empty set, but that fusion::clear of an empty set gives an empty vector.
> This should probably be made consistent.)
>

I'm confused.  Why would fusion::detail::clear (which I presume is
associated with fusion::clear) have anything to do with mpl::clear?

Also, mpl::clear requires an MPL Extensible Sequence, which...I'm guessing
fusion::joint_view doesn't satisfy...?  So either make fusion::joint_view an
MPL Extensible Sequence, or relax the requirements on mpl::clear...?  I'm
not sure either make sense, really.

I guess there's some reason you don't want to or can't copy the types in the
joint_view into an mpl::vector and operate on that instead?

- Jeff