Re: [boost] extending type_traits::rank ?

From: David Abrahams (dave_at_[hidden])
Date: 2006-05-05 13:36:07

"John Maddock" <john_at_[hidden]> writes:

>> The easiest way of course is to add a specialization for
>> boost::type_traits::rank in my project's sources but this is not a
>> very
>> clean solution because this would require that I add stuff to the
>> boost-namespace in my own project. Ideas?
>
> Unfortunately I think you need to define your own trait: the idea behind 
> type-traits is that they work with the core C++ type system only, and that 
> we don't extend them to classes that are "pretending" to be something else 
> (like an array for example).  Having just said that, I'm having trouble 
> thinking of a reason why extending boost::rank in this way would cause 
> problems in a way that extending is_pointer to smart-pointers would for 
> example.

Most of the type traits are designed to do something legal and
reasonable even when doing the right thing is impossible, e.g.

  remove_pointer<shared_ptr<T> >::type 

is designed to be shared_ptr<T>.  If rank does somthing similar, like
return zero when not passed a true array, then specializing it could
break libraries.  Besides which, it's just conceptually wrong ;-)

> So.... why not create your own traits class, call it rank if you
> want, place it in your projects own namespace, and have the primary
> template inherit from boost::rank.  Does that make sense?

That sounds sensible.

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