[boost] Re: enable_if formal review ?

From: David Abrahams (dave_at_[hidden])
Date: 2003-08-21 08:17:43

"Peter Dimov" <pdimov_at_[hidden]> writes:

> David Abrahams wrote:
>> Jaakko Jarvi <jajarvi_at_[hidden]> writes:
>>
>>> 4. Non-SFINAE compilers
>>>
>>> Dave has an implementation of enable_if which defaults to being
>>> always enabled for compilers that do not support SFINAE. We do not
>>> think this is the right approach, and believe that attemts to use
>>> enable_if on a compiler that does not support SFINAE, should cause
>>> an immediate error.
>>
>> Can you justify that choice a bit?  In the applications where I've
>> used it, no-op behavior provides good gradual degradation in
>> functionality (I don't have a strong opinion though).
>
> template<class X, class Y>
>   typename enable_if<is_expression<X>::value || is_expression<Y>::value,
> ...>::type
>   operator+(X const & x, Y const & y);

Yes, I have precisely such a situation (see
boost/python/object_operators.hpp).  I put those in an isolated
namespace with the "expression" template, which gets us 90% of the
way there.  The last 10% is given by enable_if, when available.

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