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

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2005-03-04 22:34:56

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). 
This is what I meant by 'doing nothing'; although that was probably not a
good way to express it;)

By 'conservative default' I meant something like (in the boost::range
example) enable_if on whether T does indeed have an end() member function
that returns a known iterator type.  The last thing you want is a
specialization to be viable, but does the wrong thing.

Cheers,
Ian