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

From: David Abrahams (dave_at_[hidden])
Date: 2005-03-05 12:48:36

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.

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