Re: [boost] interest in generic cyclic visitor?

Subject: Re: [boost] interest in generic cyclic visitor?
From: Jan Herrmann (jherrmann79_at_[hidden])
Date: 2012-08-06 12:03:11

On 03.08.2012 13:58, Sohail Somani wrote:
> On 03/08/2012 3:17 AM, Jan Herrmann wrote:
>> Additionally it can be used as a base for multimethod implementation.
>
> This is much more interesting to me. Can you elaborate or give some
> sample code?
>

Here is an example. In contrast to other examples I prefer hiding 
visitor objects in normal functions to place them in different 
translation units and make them easier to use. The main aspect of my
multimethod (and visitor) implementation is the forwarding to a non 
polymorphic function object preserving the constness of its arguments. 
It can be templated and you can use all aspects of static polymorphism 
for implementing operator(). At compile time it generates a polynomial 
amount of code to forward to the user specified function object (for 
this example 3 polymorphic classes, 3 intermediate static visitors and 
in sum 6 virtual functions). The dispatch actually needs 2 virtual calls 
per polymorphic parameter (in this example 4 calls). It will detect all 
ambiguities at compile time and if you add new classes to a hierarchy 
you have to recompile everything. Depending on your static visitors 
(function objects) it will use base classes, a generic solution or 
compilation will fail.

Please be aware that my intention for my library was an  generic 
implementation of an GoF visitor. The multimethod implementation was not 
my primary concern but at this time it is implemented within 65 lines of 
(yet undocumented) code. My multimethod implementation needs only a 
special templated accept function which can be implemented for cyclic 
visitable classes, boost::variant, an wrapper for boost::any and other 
types with polymorphic behavior. If there is interest in my library I 
will implement a more generic way of using these visitors. Additional to 
the dispatching mechanism it will handle parameter and return value 
transportation.

Jan Herrmann