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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-03-04 09:35:21

David Abrahams wrote:
> "Peter Dimov" <pdimov_at_[hidden]> writes:
>
>> David Abrahams wrote:
>>
>>> ADL might not be the best customization solution.
>>
>> You keep saying that, but I keep disagreeing. ADL is the best
>> customization solution. Not flawless, just better than any of the
>> alternatives.
>
> I guess I haven't seen enough be definitively convinced one way or the
> other.  I have heard stories from people who switch to dispatching via
> specialization and report how much better things get.  Furthermore,
> Daveed Vandevoorde has convinced me that it is far easier to reason
> about.  The rules for matching partial specializations are trivial
> compared to the mess you have to consider when there is overloading.
> The inconvenience of specialization is ugly, but it seems like a
> one-time cost that may be justified for what it buys us.

That's the thing. It doesn't actually buy us anything. It only claims to 
avoid potential problems.

* Uglier syntax for defining a specialization.

* Coupling; you need to include a primary template.

* Does not allow a base-specific version to be picked up automatically for 
derived classes.

* Does not allow non-exact matches.

* If four libraries need the same operation, you need to specialize four 
primary templates.

In practice people tend to respect de-facto standards; a + b is an addition, 
and swap(a, b) swaps a and b. The main fight is over who gets to keep the 
primary template. But this is not really a problem with ADL customization 
points, because in their purest form, there is no primary template. The 
primary template is usually a compatibility workaround for types that do not 
support the ADL interface yet.