Re: [boost] [proto]howto generate extended tree top down instead of bottom up?

From: Eric Niebler (eric_at_[hidden])
Date: 2007-07-07 12:49:47

Larry Evans wrote:
> http://boost-sandbox.sourceforge.net/libs/proto/doc/html/boost_proto/users_guide/expression_extension/expression_generators.html
> 
> contains:
> 
>    we want all expressions that involve the calculator placeholders to be
>    calculators
> 
> which means, IIUC, that calulator expressions are built bottom up.
> IOW, to build a calculator expressions, you must start with a
> proto terminal calculator expression.  However, I would like
> to start from a non-terminal expression (e.g. any binary
> or unary expression)  and convert all the nodes to calculator 
> expressions.  What's the simplest way to do this?  I've tried
> understanding trans::construct and other trans templates; however,
> I can't figure it out so far :(
> 
> TIA for any clues.

If you have an expression tree, and you want to wrap each node in some 
wrapper, the construct transform is what you want. For instance:

     #include <boost/xpressive/proto/proto.hpp>
     #include <boost/xpressive/proto/transform/construct.hpp>

     using namespace boost;
     using namespace proto;

     template<typename Expr>
     struct wrap
     {
         explicit wrap(Expr const &expr)
         {}
     };

     struct WrapItUp
       : transform::construct<
             or_<
                 terminal<_>
               , nary_expr<_, vararg<WrapItUp> >
             >
           , wrap<_>(_)
         >
     {};

     terminal<int>::type const _1 = {1};
     terminal<int>::type const _2 = {2};

     int main()
     {
         int i = 0;
         WrapItUp::call( _1 + _2, i, i );

         return 0;
     }

This little example works, but if the wrap struct inherits from 
proto::extends, it doesn't compile because of a bug in construct<> which 
causes the eager instantiation of wrap<_>. You could write your own 
as_wrap transform which doesn't have that problem and use that instead 
of construct.

HTH,

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com