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 12:26:05

On 03.08.2012 15:24, Larry Evans wrote:
> On 08/03/12 07:58, 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?
>>
>>
> Then you might be interested in the multimethods discussion
> in a July 2010 thread:
>
>    http://thread.gmane.org/gmane.comp.lib.boost.devel/206203
Thank you for this link.
>
> As shown elsewhere in that thread, there's multimethod code available
> already in the boost sandbox.  That code:
>
> http://svn.boost.org/svn/boost/sandbox/variadic_templates/libs/composite_storage/sandbox/pack/predator_prey.cpp
>
> is one example.  Another, more complete example, is here:
>
> http://svn.boost.org/svn/boost/sandbox/variadic_templates/libs/composite_storage/sandbox/pack/one_of_multiple_dispatch.test.cpp
Thank you for these links, too.
>
> The more complete example shows the method can be used both with virtual
> functions and discriminated unions, such as boost:: variant
> or that in:
>
>
> http://svn.boost.org/svn/boost/sandbox/variadic_templates/boost/composite_storage/pack/container_one_of_maybe.hpp
>
> Unfortunately, the multimethod using the one_of_maybe discriminated
> union can be a compile-time hog (based on some simple tests I've run).
> I've no idea why :(
I have included visiting variants, too. It was really simple. I 
furthermore can visiting anys and polymorphic base classes with a 
wrapper. It uses any_cast or dynamic cast and works like Alexandrescus 
static dispatcher. The only problem I have and which i cant solve is the 
polynomial amount of code. The compiler has simply to build a lot of 
classes and virtual functions for every case.

>
> HTH.
>
> -Larry
>

My main idea is to provide (normally) 2 templated functions for all 
visitable types:
template<class static_visitor>
void accept(MyClass const&, static_visitor& sv);
template<class static_visitor>
void accept(MyClass & static_visitor& sv);
As long as these functions can be found you can build a const correct 
multimethod implementation on top of them without knowing anything about 
MyClass. Please look at 
https://raw.github.com/janherrmann/visitor/master/dispatch.hpp .

Best regards
Jan Herrmann