Re: [boost] Report from Berlin C++ Standards Committee meeting

From: Douglas Gregor (doug.gregor_at_[hidden])
Date: 2006-04-12 20:35:10

On Apr 12, 2006, at 5:36 PM, Paul Mensonides wrote:

>> -----Original Message-----
>> From: boost-bounces_at_[hidden]
>> [mailto:boost-bounces_at_[hidden]] On Behalf Of Doug Gregor
>>
>> When we initially type-check lower_bound, we need to
>> determine if there is an advance() that we can call. Both #1
>> and #2 have been seen, so we check them. Can we call #1?
>> Sure, because every ForwardIterator is also an InputIterator.
>> Can we call #2? Nope, our ForwardIterator is not guaranteed
>> to be a BidirectionalIterator.
>
> What happens if you have more than one seed?

The program is ill-formed and the compiler will report an ambiguity  
when type-checking the template definition.

> Is the name resolution related to the seed is implicitly added to  
> the concept
> requirements that are checked immediately prior to instantiation?   
> Or is the
> name resolution done during normal instantiation (with a restricted  
> set of
> possible bindings)?

The latter.

>> [*] Yes, instantiation can still fail if partial ordering of
>> function templates returns an ambiguity. We can synthesize it
>> all we want, but it has yet to actually bite us in practice.
>
> Well, it seems like an reasonable tradeoff--though I guarantee that  
> the above
> will bite eventually (which is no worse than what we have now).

Everything will bite you eventually, unless you give up  
specialization entirely. For more information on the issue, see:

Jaakko Järvi, Douglas Gregor, Jeremiah Willcock, Andrew Lumsdaine,  
and Jeremy Siek. Algorithm specialization in generic programming:  
Challenges of constrained generics in C++.
Note: Accepted for publication in PLDI, June 2006.

        Doug