[boost] Re: Re: Policy or Trait?

From: David B. Held (dheld_at_[hidden])
Date: 2004-04-02 02:31:32

"Gennadiy Rozental" <gennadiy.rozental_at_[hidden]> wrote in message
news:c4ivdk$hgl$1_at_sea.gmane.org...
> [...]
> Trait is specific to the type
> Policy is orthogonal to the type.
> [...]

Where "the type", I presume, is the [template] class using the
policy | trait?  That sounds like a good distinction to me.  May I
formalize it by proposing that:

    1. A trait is a metafunction of the client type.

     2. A client type is a metafunction of a policy.

So definition 1 means that a trait is a metafunction which maps a
client type X to a specific trait type trait<X>.  Definition 2 means
that a policy client X is a metafunction which maps the client type
to X<policy>.  This makes it clear why a policy *class* need not be
a template class.  It also shows why a traits *client* need not be a
template class.  But I would welcome examples of non-template
traits classes or non-template policy clients.

Now, the definitions I give are necessary but not sufficient
conditions for defining traits and policies.  You also need to add
fluff about modifying behavior, yada yada yada.

Dave

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