[boost] Generic object construction

From: Eric Niebler (eric_at_[hidden])
Date: 2006-09-25 14:00:19

Generic programming is fairly well understood. Concepts define 
requirements, types model concepts, generic code manipulates models via 
valid expressions for that concept, etc. Also pretty well understood is 
how to define customization points so that arbitrary types can be made 
to model a concept non-intrusively.

But these techniques seem to break down for object construction. Consider:

   template< class Inner >
   struct facade
   {
     facade( /* what here? */ )
       : inner_( /* and what here? */ )
     {}
   private:
     Inner inner_;
   };

How does facade know how to initialize inner_? The answer affects how 
facade itself should be initialized.

One possibility is that facade defines N constructors that take up to N 
arguments, and just forwards them to inner_. That might work in this 
narrow case, but what if facade has 2 inner objects of different types? 
How should constructor arguments get dispatched to the different 
sub-objects? In general, facade has no way of knowing. Is this a problem 
others have run into? What sorts of techniques are people using?

I have come up with a little generic object construction utility using 
Boost.Parameter that works for me. It allows you to non-intrusively 
associate named parameters with a type for the purpose of construction, 
and a way to forward a (possibly filtered) argument pack on to 
sub-objects. Is there general interest in such a utility?

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