[boost] Re: is_base_and_derived question

From: Gennaro Prota (gennaro_prota_at_[hidden])
Date: 2003-01-28 07:16:50

On Tue, 28 Jan 2003 03:06:35 -0800, "Andrei Alexandrescu"
<andrewalex_at_[hidden]> wrote:

>"Thomas Witt" <witt_at_[hidden]> wrote in message
>news:3E3590ED.8030100_at_ive.uni-hannover.de...
>> IIUC is_based_and_derived<T,T> evaluates to true as well. Is a class T
>> strictly speaking a base class of itself?
>
>Yes

That's a convention of is_base_and_derived though. To the standard a
class is not a base of itself, so this convention should be
documented. In other words, you have to specify whether the ordering
is strict or not.

Incidentally, I've noticed that boost's implementation of
is_base_and_derived has the same access-checking problems as
is_convertible. That could be easily fixed, as said in the thread
about is convertible, by using function templates:

    typedef char (&no_type)[1];
    typedef char (&yes_type)[2];

    template <typename T>
    struct identity { typedef T type; };

    template <typename To>
    no_type is_convertible(...);

    template <typename To>
    yes_type is_convertible(typename identity<To>::type);

Genny.