Re: [boost] [proto] small breaking change

Subject: Re: [boost] [proto] small breaking change
From: joel falcou (joel.falcou_at_[hidden])
Date: 2010-06-14 16:09:57

Eric Niebler wrote:
> I was mistaken about falling foul of RVO. Here is what is happening. You
> have two domains in play, both with different generators. You compose
> some expressions in different domains, A and B. Proto decides the
> resulting expression R should be in domain B. Proto prefers things to be
> nice and uniform, so before making A a child of R, it puts it also into
> B's domain by applying B's generator. That causes the extra copy you're
> seeing. If A and B had had the same generator, no extra invocation of
> the generator would be performed.
>
> Why does Proto put expression A into B's domain? Good question. It just
> seemed like the right thing to do when I wrote the code. Right now, that
> looks like a dubious decision. Lemme think on it for a bit

Don't know if it'll help you but, back in the day where subdomain didn't 
exist, NT2 used a
as(a,b) function to make a comaptible with b. It was like :

  template<class Y,class C,class X> static inline
  typename boost::proto::result_of::
  make_expr < tags::as_
            , containers::domain<typename Y::nt2_category_tag>
            , containers::expr<X,C> const&
            >::type const
  as( containers::expr<X,C> const& xpr, Y const& )
  {
    return  boost::proto::
            make_expr < tags::as_
                      , containers::domain<typename Y::nt2_category_tag>
                      > ( boost::cref(xpr) );
  }

and it worked without any copy as a was stored as a reference in a 
special node.