Re: [Boost-users] [Proto] Recreating an expression

Subject: Re: [Boost-users] [Proto] Recreating an expression
From: Eric Niebler (eric_at_[hidden])
Date: 2009-07-03 18:41:59

Here's a little extra feedback that I forgot include in my first reply.

christophe henry wrote:
> These terminals are used with a simple grammar:
> struct BuildGuards
>    : proto::or_<
>         proto::when<
>                     proto::logical_or<BuildGuards,BuildGuards >,
>                     
> GuardOR<BuildGuards(proto::_left),BuildGuards(proto::_right)>()
>                 >,
>         proto::when<
>                     proto::logical_and<BuildGuards,BuildGuards >,
>                     
> GuardAND<BuildGuards(proto::_left),BuildGuards(proto::_right)>()
>                 >,
>         proto::when<
>                     proto::logical_not<BuildGuards >,
>                     GuardNOT<BuildGuards(proto::_value)>()

Use proto::_child instead of proto::_value here. proto::_value returns 
the value of a terminal node. proto::_child returns the child of a unary 
expression tree node. It's really just luck that this is working for you.

>                 >,
>          proto::when <
>                     proto::terminal<guard_tag>,
>                     proto::_()

I would use proto::_ here instead of proto::_(). They mean the same 
thing, but proto::_ is more idiomatic. See the following section for an 
explanation of why proto::_ and proto::_() are equivalent when used as 
transforms:

http://www.boost.org/doc/libs/1_39_0/doc/html/proto/users_guide.html#boost_proto.users_guide.back_end.expression_transformation.data.implicit_parameters_to_primitive_transforms

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