Re: [Boost-users] [mpl][fusion] iteration over mutiple containers

Subject: Re: [Boost-users] [mpl][fusion] iteration over mutiple containers
From: Manfred Doudar (manfred.doudar_at_[hidden])
Date: 2010-06-01 07:44:31

I may have confused some with my last post, ..but the mpl::for_each
invocation doesn't align with the function call-operator of Func,
rather, you'd rewrite Func's callable operator appropriately to match
the for_each invocation.

What I have today is a call like:

   Func<MyVisitor> ftor(...);
   ftor.operator()<tag, Algo_type>(Ctr[index]);

Hope that kind of clarifies what may have been cause for some confusion.

Looking forward to what others can say here..

--Manfred

On Tue, 1 Jun 2010 21:37:33 +1000
Manfred Doudar <manfred.doudar_at_[hidden]> wrote:

> 
> Hello,
> 
> I've been using the Boost.PP lately, and everything now works rather
> nicely, but there's been a thing in the back of my head that has
> always been telling me, that the problem is better suited to
> mpl/fusion. The problem in this case is I have "two things" that need
> to be iterated over, one a series of tags (aka "types"), and the the
> other, a runtime index.
> 
> I'll express what I'm after with mpl::for_each, because the issue
> becomes clearer that way:
> 
> 
>   using namespace boost;
> 
>   //Func:  is an object that facilitates visitation on algorithms
> 
>   template <template <typename> Visitor>
>   struct Func
>   {
>       typedef void result_type;
> 
>       template <typename R, typename A>
>       result_type
>       operator()(A* algo)
>       {
>            // do work.. visit algo, with Visitor<R>
> 
>            apply_visitor(std::move(Visitor<R>()), *algo, ...); 
>       }
>   }
> 
> 
> 
>   // tags are the return type on visitation
>   typedef mpl::vector<int, float> tags;
> 
>   // container of algorithms
>   Ctr<algorithms> ctr;
> 
>   Func<MyVisitor> ftor;
>   mpl::for_each<tags>(bind(ref(ftor),
>                            ::_1,              // [A]
>                            ctr[INDEX])));     // [B]
> 
> 
> 
> [A]:  dispatch the tag
> [B]:  pass through algorithm instance at INDEX.. I want to do INDEX++
> 
> 
> Of course the problem here is that I can't update INDEX in each
> mpl::for_each call, .. and thinking boost::fusion might be the
> machinery I need, but I seemingly run into other problems with that
> one too, there, since my tag-types are not of the same sequence as my
> container of algorithms.
> 
> 
> What I want is to pass in the appropriate tag, for each instance of
> algorithm from a container.
> 
> 
> Indeed, for those wondering, I have tried a typedef on each
> algorithm, ..was hoping I could access the nested type A::type in
> place of R in Func above, however, owing to the visitation mechanism,
> doing so gives problems of access to incomplete-types elsewhere in
> the chain.
> 
> 
> If anyone can suggest something, I'd be happy to hear it, ...as I
> said, what I have already works, but I have a hard time letting this
> one rest, knowing that there is a better alternative, but am not quite
> seeing it yet.
> 
> 
> Cheers,