[boost] Design of a compatible policy-based template

From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2007-06-25 13:22:39

The main problem with a policy-based type, such as a string type with 
many options (flex_string for example), is that different versions of 
the template are different types, and as such are incompatible.

An obvious solution would be a type that can hold any of the template 
instantiations, and dispatch the member functions to the appropriate 
type through virtual calls, that could eventually use the stack like 
boost.variant.

However, this should have quite of a performance penalty, and some of 
the advantages of the policies (such as Small String Optimization) would 
become either useless (dynamic allocation) or annoying when disabled 
(stack allocation).

An alternative would be to simply use typedef to choose the type you 
want to use throughout the program, which would somehow give source 
compatibility.

How do people feel about types with lots of policies? How could those 
issues be addressed, for example in the case of a string type, where 
there is quite some demand for policies?