Re: [boost] [Accumulators] Passing construction arguments to dependent statistics

From: Eric Niebler (eric_at_[hidden])
Date: 2007-08-01 16:09:39

Hugo Duncan wrote:
>>> Ideally, though,  I was looking for a solution that allowed me to reuse
>>> the delay buffer without modifying it.  That way it could be used as a
>>> lower layer for any algorithm that needed recent time history.  Maybe I  
>>> am
>>> being over ambitious...
>> The coupling of the cache_size and the coefficients parameters has to be
>> expressed in code somewhere. Where do you suggest?
> 
> It would be nice to do it in the filter code.  The coefficents parameter  
> is a parameter of the filter after all, and the use of the delay can be  
> looked at as an implementation detail.  If the filter constructor (body)  
> could access the delay buffer, then the delay buffer could expose a method  
> to allow resizing.  Is that possible?

Ah, ok. Yes, that's possible. In the version you have, that would be 
done with a post-constructor. In the post-review version (which I 
haven't made available yet), post-constructors are dead, accumulators 
within the set are guaranteed to be constructed only after the 
accumulators on which they depend are constructed, and the entire 
accumulator_set is available in the argument pack.

You'd basically do something like:

   template<class Args>
   filter_impl(Args const &args)
     : ...
   {
     // filter depends on delay_buffer, so delay_buffer is
     // guaranteed to be constructed already.
     find_accumulator<tag::delay_buffer>(args[accumulator]).
         resize(boost::size(args[coefficients]));
   }

I'll try to package up the latest version of accumulators Real Soon. If 
you can't wait, just put the same code in a member function called 
post_construct and it should do the right thing. Either way, you'll need 
to write your delay_buffer_impl so that it works when cache_size is not 
specified.

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