[ Batavia (2009-05): ]

We observe there is a recent paper by Bjarne that overlaps this issue.

Alisdair continues to recommend NAD.

Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.

[ 2009-04-28 Alisdair adds: ]

I recommend NAD, although remain concerned about header organisation.

A user container will satisfy the MemberContainer concept, which IS auto. There is a concept_map for all MemberContainers to Container, and then a further concept_map for all Container to Range, so the stated problem is not actually true. User defined containers will automatically match the Range concept without explicitly declaring a concept_map.

The problem is that they should now provide an additional two headers, <iterator_concepts> and <container_concepts>. The only difference from making Range an auto concept would be this reduces to a single header, <iterator_concepts>.

I am strongly in favour of any resolution that tackles the issue of explicitly requiring concept headers to make these concept maps available.

[ 2009-04-26 Herb adds: ]

Here's a common example: We have many ISV customers who have built lots of in-house STL-like containers. Imagine that, for the past ten years, the user has been happily using his XYZCorpContainer<T> that has begin() and end() and an iterator typedef, and indeed satisfies nearly all of Container, though maybe not quite all just like valarray. The user upgrades to a range-enabled version of a library, and now lib_algo( xyz.begin(), xyz.end()); no longer works -- compiler error.

Even though XYZCorpContainer matches the pre-conceptized version of the algorithm, and has been working for years, it appears the user has to write at least this:

template<class T> concept_map Range<XYZCorpContainer<T>> {};

template<class T> concept_map Range<const XYZCorpContainer<T>> {};

Is that correct?

But he may actually have to write this as we do for initializer list:

template<class T>
concept_map Range<XYZCorpContainer<T>> {
   typedef T* iterator;
   iterator begin(XYZCorpContainer<T> c) { return c.begin(); }
   iterator end(XYZCorpContainer<T> c) { return c.end(); }
};

template<class T>
concept_map Range<const XYZCorpContainer<T>> {
   typedef T* iterator;
   iterator begin(XYZCorpContainer<T> c) { return c.begin(); }
   iterator end(XYZCorpContainer<T> c) { return c.end(); }
};