On Fri, May 15, 2009 at 12:09 PM, Eric Niebler <eric@boostpro.com> wrote:
Mathias Gaunard wrote:
...
The concepts are of critical importance, and these don't seem right to me. My C++0x concept-foo is weak, and I'd like to involve many more people in this discussion.
In C++0x concept-foo, I would say associated the validity invariant with a non-auto concept (i.e. the model has to specify it implements the concept, unlike auto concepts which are implicitly structurally matched to models), so as to distinguish ranges that aim at maintaining that invariants from ranges that don't.
Whoa, we're nowhere near talking about auto vs. non-auto C++0x concepts. We don't even have Unicode algorithms to work from, so we don't know what form the concepts will take. You're jumping to the end. I repeat: you don't know what the concepts are and you won't know until you write some concrete algorithms and try to lift generic implementations from them.
Please have a look at: http://www.generic-programming.org/
Strongly agree.
The purpose of the concepts are to allow algorithms to be implemented generically in terms of the operations provided by the concepts. So, what algorithms do we need, and how can we express them generically in terms of concepts? Without that most critical step, we'll get the concepts all wrong.
Concepts are not just about operations, but also about semantics. A single pass range and a forward range provide the same operations, but they've got different semantics. A single pass range can be traversed once, a forward range, which refines it, can be traversed any number of times.
Ahem. I know.
Refining the range concepts to create a new concept meaning a range that satisfies a given predicate doesn't seem that different in spirit.
The problem is that it is not possible to ensure that predicate programmatically with range adapters, so we have to fallback to design by contract.
Show me the (concrete) code.
Exactly.
Now, if it is believed this is a bad idea to model invariants as a concept, I simply will remove the concept and just deal with raw ranges directly, without any concept(=invariant) checking.
Now we're talking. Ditch the concepts. Write the algorithms. Then try to make them generic and *find* the concepts.
Amen. --Beman