[boost] [MPL] Potential use for multiple sequence types

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-08-01 11:26:00

[Usual disclaimer: this post should not be confused with my role as review 
manager]

At WCGP'02 last month, Michael Burton presented a paper titled "Static Data 
Structures: Reconciling Template Metaprogramming and Generic Programming". 
The most important part of their work, I think, was that they gave a very 
natural way to shift code from using dynamic data structures (STL sequences) 
to static data structures (type vectors), by appropriately overloading STL 
algorithms. The same line of code, e.g., 

  for_each(particles.begin(), particles.end(), DoSomething());

would call either the normal STL for_each loop for a dynamic data structure 
or, for a static data structure, would fully unroll the for_each loop in the 
typical manner. I think this would be a great syntactic addition for MPL, but 
that's not the point. Back to the "multiple sequence types" argument...

In their system, they used type vectors instead of type lists, because it is 
easy to specify the _dynamic_ contents of each element. For instance, they 
were working finite element analysis, so they might define the particles in 
the system like this:

template<int N> struct ParticleArray {};

template<> struct ParticleArray<0> { 
  Neutron particle; ParticleArray() : particle(...) {}
};

template<> struct ParticleArray<1> { 
  Proton particle; ParticleArray() : particle(...) {}
};

// etc etc etc for 10,000 particles 

Anyone want to figure out a typelist equivalent for this static data structure 
with dynamic elements? I can't find a good one, and I think that justifies 
the need for a type vector in MPL. I also think that typelists are 
well-justified, so conclude what you may.

The proceedings for WCGP'02 haven't been published yet, but I'll try to 
contact the author to get an electronic copy of the paper if anyone would 
like to read it.

        Doug