[Boost-users] Re: [Foreach]TroubleiteratingoverBoost.MultiIndexcontainers

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2005-03-05 13:31:26

David Abrahams wrote:

> Ian McCulloch <ianmcc_at_[hidden]> writes:
> 
>> David Abrahams wrote:
>>
>>> Ian McCulloch <ianmcc_at_[hidden]> writes:
>>> 
>>>>> * Coupling; you need to include a primary template.
>>>>
>>>> Ok, but what does that primary template do?  Surely nothing, or some
>>>> very conservative default.
>>> 
>>> Actually you don't need to include a primary template.  All you need
>>> is a declaration with no body.  You can do that without even pulling
>>> in a header.
>>> 
>>
>> I think, an empty body is better than a bare declaration.  At least, with
>>
>> template <typename T>
>> struct my_algo_impl {};
>>
>> you can do SFINAE on whether my_algo_impl<T> is actually defined or not,
>> for some T (ie. using T::result_type, or probably better
>> boost::result_of).
> 
> That's a useful technique, but I don't see how it relates to coupling.
> In order for a user to provide a specialization of some library
> template for her type, she need only provide the bare declaration.
> Bare declarations and empty definitions aren't alternatives, they're
> apples and oranges.
> 

True.  I had forgotten that you might want to define a specialization but
only make use of it elsewhere.  But what happens if the primary template
has some additional paramters with default values (eg, Enable = void)? 
Would you regard that as bad style ?  Actually, I tend to avoid that
anyway, and instead if the primary template has a default then forward it
to another template that does the enable_if tricks/hacks.  But thats mostly
because I usually end up very confused when I try to combine partial
specialization and enable_if ;-)

Cheers,
Ian