[boost] Re: is_base_and_derived

From: David Abrahams (dave_at_[hidden])
Date: 2003-10-27 15:06:05

Brian McNamara <lorgon_at_[hidden]> writes:

> On Mon, Oct 27, 2003 at 09:11:33AM -1000, David Abrahams wrote:
>> "Peter Dimov" <pdimov_at_[hidden]> writes:
>> > I've repeatedly suggested is_base_of<B, D> ("B is a base of D") and
>> > is_derived_from<D, B> ("D is derived from B") and repeatedly been
>> > ignored.  ;-)
>> 
>> If I ignored you it's probably because this ignores the currying
>> issue.
>> 
>>         is_base_of<D,B>
>> 
>> makes plenty of sense.  When the "D" gets bound visually to
>> is_base_of, we have a predicate on "B" which asks "is it a base of D"?
>
> All of a sudden I'm totally lost.
>
>
> First, do you mean
>
>    is_base_of<B,D>
>
> instead?  

No, that's my point.

> This is the order suggested by the name (IMO, anyway).

Yes, many people seem to think so.
>
> Second, what it the aforementioned "currying issue"?  

> Presumably MPL has
> some magical way to make
>
>    is_base_of<B,_>
> and
>    is_base_of<_,D>
>
> turn into the corresponding metafunctions that test if "T is a subtype
> of B" or "T is a supertype of D", respectively, yes?

Well, yes it does.  It's just what you quoted above about visual
binding:

        is_base_of<D,_>

Looks like a way of testing whether something is a base of  D.  How
would you name a metafunction which tests if something is a base of
some fixed class Foo?  Probably "is_base_of_Foo", right?

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com