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Subject: Re: [proto] The proper way to compose function returning expressions
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-04-26 12:40:21

On Thu, Apr 26, 2012 at 9:02 AM, Eric Niebler <eric_at_[hidden]> wrote:

> On 4/25/2012 1:41 PM, Mathias Gaunard wrote:
> > On 24/04/12 22:31, Eric Niebler wrote:
> >> On 4/23/2012 10:17 PM, Joel Falcou wrote:
> >>> On 04/24/2012 12:15 AM, Eric Niebler wrote:
> >>>
> >>> I think this is an important issues to solve as far as Proto
> grokability
> >>> does.
> >>
> >> Agreed. It would be very nice to have. But you still have to know when
> >> to use it.
> >>
> >>> One of my coworker on NT2 tried to do just this (the norm2 thingy) and
> >>> he get puzzled by the random crash.
> >>>
> > [...]
> >>
> >> The implicit_expr code lived in a detail namespace in past versions of
> >> proto. You can find it if you dig through subversion history. I'm not
> >> going to do that work for you because the code was broken in subtle ways
> >> having to do with the consistency of terminal handling. Repeated
> >> attempts to close the holes just opened new ones. It really should be
> >> left for dead. I'd rather see what you come up with on your own.
> >
> > The issue Joel had in NT2 was probably unrelated to this. In NT2 we hold
> > all expressions by value unless the tag is boost::proto::tag::terminal.
> > This was done by modifying as_child in our domain.
> >
> > I strongly recommend doing this for most proto-based DSLs. It makes auto
> > foo = some_proto_expression work as expected, and allows expression
> > rewriting of the style that was shown in the thread without any problem.
> >
> > There is probably a slight compile-time cost associated to it, though.
> Interesting. I avoided this design because I was uncertain whether the
> compiler would be able to optimize out all the copies of the
> intermediate nodes. You're saying NT2 does it this way and doesn't
> suffer performance problems? And you've hand-checked the generated code
> and found it to be optimal? That would certainly change things.

FWIW, some years back, I had created a rather simple expression template
engine, tried storing all intermediate nodes by value, and, IIRC, was
surprised to find (via putting print statements into the copy constructor)
that MSVC8 elided all the intermediate copying.

And, because of this, I certainly took notice that, when Proto came out, by
default, all intermediate Proto nodes were held by reference.

But...I could be remembering incorrectly :(

- Jeff

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