RE: [boost] Re: move_ptr

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-01-23 21:24:04

Howard Hinnant wrote:
>On Jan 23, 2004, at 7:21 PM, Jonathan Turkanis wrote:
>>Also, I'm a little nervous about relying on is_convertible, because of
>>problems like:
>>
>>      is_convertible<noncopyable,noncopyable>
>
><nod> has me worried too.  This is a bigger problem than just move_ptr.  It 
>is effecting other projects too.  I'm not sure of the answer.

I'm not sure of how is_convertable is implemented, but can't noncopyable be 
added as a special case, like generating a compile-time error when used as a 
parameter to it, e.g.
   error: cannot instantiate disallowed_conversion< noncopyable >
   from is_convertable< noncopyable, T >::type
or something similar.

This should be a case of adding some MPL code to detect is_same< 
noncopyable, T >. The only problem I forsee would be in compile time, or am 
I missing something?

e.g. (pseudocode style):

is_convertable_ex< T1, T2 >::type =
   if_
   <
      or_< is_same< T1, noncopyable >, is_same< T2, noncopyable > >,
      disallowed_conversion< noncopyable >,
      is_convertable< T1, T2 >
   >::type

Regards,
Reece

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