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Subject: Re: [Boost-users] enable_if and multiple partial specializations
From: Joel Falcou (joel.falcou_at_[hidden])
Date: 2011-04-09 02:23:45

On 09/04/11 05:14, Gennadiy Rozental wrote:
> This has at least 2 problems:
> 1. It does not work. I need "scalar" level specialization to be applied if enum
> level specialization is not present. In your case it will not even compile. Or
> it will choose incorrect (generic) option if you non specialized version for
> that. We can try to follow STL's approach of iterator tag dispatching based on
> hierarhy, but that really cumbersome and require extra runtime wrappers.
I can be doen using compile-time stuff, no need for runtime.
> 2. It's intrusive. Once I introduce new subgroup of types I need to go in common
> header and update tag selection (and potentially something else to handle
> relation with other groups)

We faced this exact same problem in NT2 and we ended up having a whole
dedicated to complex, multiple arguments, hierarchical traits
dispatching. This is a non
trivial problem and it gets out of hand fast once you want to add new
types familly and still
be extensible. Our solution rely on having a "hierarchy" traits defined
for types and use
operator overload detection meta-function to select the proper hierarchy
from : the actual
type and the actual existence of an overload.

We use this exact system in our SIMD library, so it may be of interest
to make it a proper
reusable component.

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