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

From: David Abrahams (dave_at_[hidden])
Date: 2003-04-27 12:26:44

Mike Conley <conley.141_at_[hidden]> writes:

> David Abrahams <dave_at_[hidden]> wrote in
> news:uznmc0wpl.fsf_at_[hidden]: 
>
>>> and_<is_const<T>, is_convertible<T,int> >.  
>
> OK, this is wrong.  What I wanted was a boolean test: is_const<T>::value && 
> is_convertible<T,int>.  That's not what and_ does.  Duh.

Yes it does.

>> It doesn't matter if it's a template parameter.  It's an ODR violation
>> to have:
>
> <snip>
>  
>> in the same program, which is essentially the effect if is_whatever
>> uses a context-dependent is_convertible in its body.
>
> Which is why you pass it as a boolean constant template parameter.
> Then you can safely use that parameter in the body.  You get the
> context, and there's no ODR violation.

So will it be illegal to build a trait which tests is_convertible on a
dependent type?  If so, is_convertible is fragile and dangerous.  If
not, then you have to compute is_convertible on a non-dependent type,
which makes is_convertible almost useless.

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