Proto for Boost mailing list page: Re: [proto] Expression as *other* fusion sequence

Subject: Re: [proto] Expression as *other* fusion sequence
From: Eric Niebler (eric_at_[hidden])
Date: 2011-06-17 02:25:39

On 6/17/2011 7:30 AM, Joel falcou wrote:
> proto expression are fusion sequence that iterates over the node
> children. All fine and dandy.
> 
> Now here is my use case. I have expression whose terminal are fusion
> sequence that access tot he terminal values (think terminal holding a
> std:;array for example) and I wished to have expression of the terminal
> be fusion sequence themselves so i can do stuff like :
> 
> at_c<0>( x + y * 3 )
> 
> where x and y are such terminals, this statement returning me the
> equivalent of :
> 
> at_c<0>( x ) + at_c<0>( y ) * 3
> 
> Obviously, no candy as both fusion registration conflicts with each
> others. My Fusion-fu beign quite weak, is there a way to have this AND
> still have proto expressions behave as they should in other context ?

Doable, but not easy. The problem you'll have is that all Proto
expression types have a nested fusion_tag that is a typedef for
proto::tag::proto_expr. That is how Fusion figures out how to iterate
over Proto expressions. You'll need to define your own tag, use
proto::extends (not BOOST_PROTO_EXTENDS) to define an expression
extension, and hide the fusion_tag typedef in the base with your own.
Then you'll need to implement the necessary Fusion hooks for your custom
Fusion tag type.

HTH,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com