Re: [Boost-users] Proto transform using visitor

From: Eric Niebler (eric_at_[hidden])
Date: 2008-06-04 10:53:59

Joel Falcou wrote:
> Hello,
> 
> I'm currently trying to grasp the way proto trasnform are supposed to 
> work and I was wondering fio there is any use cases or examples of how 
> and why a Visitor can be used.

Hi,

In the proto/v4 branch, "Visitor" has been renamed "Data" to more 
accurately reflect its role as a blob of user-specified data to be 
passed along to each transform.

A very simple example is a lambda library with placeholders. Imagine you 
wrote a transform that evaluated lambdas with arguments. When evaluating 
a lambda expression such as (_1 + _2) with the arguments (3,4), you 
might pass the arguments to the transform as a tuple in the Data 
parameter (a.k.a Visitor). Then, when the transform has recursed to a 
placeholder terminal, say _1, the result would be the 0th element in the 
Data tuple.

I have reimplemented the Boost Lambda Library on top of proto using this 
technique. You can find it here:

http://svn.boost.org/trac/boost/browser/branches/proto/v4/libs/proto/example/lambda.hpp

HTH,

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