[Boost-users] Re: Using MPL to write is_derived_from_std_basic_ostream

From: Angus Leeming (angus.leeming_at_[hidden])
Date: 2004-09-03 03:04:38

Jonathan Turkanis wrote:

> 
> "Angus Leeming" <angus.leeming_at_[hidden]> wrote in message
> news:ch7h3d$466$1_at_sea.gmane.org...
>> I'm trying to ascertain whether a given class is, or is derived from,
>> std::basic_ostream, but I've ground to a halt.
> 
> I've attached a somewhat improved version inspired by your attempt.
> (Thanks!) 

Hey! ;-)

> It only works on compilers which support has_xxx, but it should
> handle the case of private inheritance now, by delegating to
> is_base_and_derived.

Hmmmmm. Looking at your solution, I can see that I was on the right track. 

What puzzles me is why my attempt fails and yours succeeds. Mine complains
that 'bool' has no char_traits member when I try
        is_ostream<bool>
Yours just works.

Ahhh. I think I get it. Yours is all wrapped up inside

template<typename T>                                                       
struct is_ostream
        : mpl::bool_<detail::is_ostream_impl<T>::value>
{};

Clever, clever, clever.

> Note that with the current cvs it should not be necessary to write
>     mpl::and_< mpl::bool_<has_char_type<U>::value>, ...
> You can simply say
>     mpl::and_< has_char_type<U>, ...

Oh, great. I didn't think to look in the aux_ directory and so never
spotted has_xxx.hpp.

Many, many thanks for all this, Jonathan.
Angus