And I think the same. IMO any example of trait/policy mixure in the same class would be an example of bad design.
I don't know; it seems to me that the
template <class T, class Policy = xxx_traits<T> > class Foo;
idiom might not be a bad one for some uses if xxx_traits is only supplying associated types. It allows customization of the meaning of Foo<X> for any X by specializing xxx_traits<X>, while also allowing users to explicitly choose Foo<X, Y> when they don't like the default for X.
And you get basic_string design as a result. If user do not like default value for the trait. It's value should be set *externally* to the component definition. That is the major point I am trying to prove in regards to numeric_cast design and in thic generic discussion. Gennadiy.