Re: Combining Generative Programming and Policy-Based Design

From: timatdvc (timw_at_[hidden])
Date: 2002-02-18 13:30:40

--- In boost_at_y..., Beman Dawes <bdawes_at_a...> wrote:
> Tim,
> 
> You might also read Mat Marcus' 
> http://groups.yahoo.com/group/boost/message/23654 if you haven't 
done so 
> already.
> 
> The techniques you, Dave, Jeremy, and Mat are exploring are on the 
research 
> frontier of generic and generative programming.  Very few people 
understand 
> them fully.  I know I don't.
> 
> For these techniques to migrate into the main stream, there is a 
real need 
> for descriptive material that is easier to understand.  Andrei 
Alexandrescu 
> did a great job in his book at making policies easy enough to 
understand 
> that many of us could start using them in everyday programming.  
Someone 
> needs to do something similar for combining generative programming, 
named 
> template parameters, and policy-based design into a unified, 
> understandable, technique.
> 
> --Beman

Thanks for the insight.  I have read Mat's post, and if his 
experience was anything like mine, I found GenVoca to be rather 
cumbersome.  It wasn't clear to me how to divide the architectural 
layers.  That difficulty is partly why I've explored using policy-
based design as a "back-end" to a generator, rather than something 
like GenVoca.  Policies are just easier to understand.

I definitely agree that these concepts can be quite difficult and I 
second the motion that we need some sort of unified description of 
these techniques.  Up to this point, I've seen a lot of work being 
done in the areas of policy-based design and generative programming 
being done independently.  I see policies as being part of the 
solution domain.  That is, in a policy based-design, each policy 
explicitly represents a part of the solution.  They are, by their 
nature, good at solving fine-grained problems.  Generative 
programming, on the other hand, addresses a much larger granularity, 
where understanding the solution domain may be too much to ask of a 
client.

To continue in the car factory analogy used by C&E, I would equate 
policies as being the interchangable parts of the vehicle.  This 
allows for great flexibility.  However, expecting a user to know 
about and understand all of the parts may be too much to ask.  By 
using a generator, the user only needs to make high-level decisions 
using the language of the *problem domain*, not the solution domain.  
So using policy-based design makes a flexible library easier to 
develop, while a generator would make a library easier to use:  the 
best of both worlds!