Re: [boost] Re: is_convertible: rationale and wording

From: Gennaro Prota (gennaro_prota_at_[hidden])
Date: 2003-01-31 07:45:12

--- John Maddock <jm_at_[hidden]> wrote:
> 
> > The technique I know relies on detecting whether the template argument
> > is *derived* from some policy, and that's doable with expressions.
> > That's why I asked you what was the exact technique you were using.
> > Also, if you have any (other) example where "convertibility of a type"
> > is needed I would be glad to see it, because this is IMHO a crucial
> > point.
> 
> Yes, it's really a derived from test, so is_base_and_derived would do I
> guess.  Which brings us back to things like:
> 
> template <class I>
> struct is_bidirectional_iterator
> {
> private:
>    typedef typename std::iterator_traits<I>::iterator_category cat;
> public:
>    BOOST_STATIC_CONSTANT(bool, value = (::boost::is_convertible<cat*,
> std::bidirectional_iterator_tag*>::value));
> };

And I've already showed how to do this with expressions. With pointers you have
no problem at all: to see if T* "is convertible" to U* you see whether the
expression ((T*)(0)) is implictly convertible to U*.

> Perhaps my main argument is philosophical

Frankly, and with no offense intended, I think it's just clutching at straws
:-)

> I
> think that the current solution is well understood

Uh?? Well understood??

> and easy to use, and it
> *conceptually*

Conceptually? Or "formally"?

> fits in well with the other type traits.  IMO a free function
> is more like a code snippet that a packaged solution: but that is very much
> a personal preference.

:-)

Genny.

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