Hi Guys, On Sep 29, 2006, at 10:25 AM, David Abrahams wrote:
Joel de Guzman <joel@boost-consulting.com> writes:
To me it's like, generalizing that something with a wing, a propeller, and all things that comprise an airplane-- is an airplane. No, not necessarily. The presence of these elements do not complete the picture. It can be, say, *just a collection of parts*. Hence, pair<iterator,iterator> is just a collection of 2 iterators, nothing more.
External, non-intrusive, adaptation of a 3rd party type to make it model a particular concept that it doesn't model "inherently" is a fundamental generic programming maneuver. I don't see anything particularly wrong with making a pair of iterators model Range.
Yes. I also agree with Joel's position on airplanes (I misunderstood what he was saying at first... but yes, a pile of airplane parts is not an airplane), which is why I've always been in favor of explicit concept conformance. I see the concept_map declaration as the final touch that turns the parts into an airplane.
Jeremy Siek has described cases to me wherein one type can play two different roles with the *same* library. For that you need two different concept maps, to map the type into its different roles. Thus his recent "scoped concept_map" proposal.
Think of a variant of the accumulate function that instead of having parameters for an initial object and function object, requires the value_type of the iterators to be a model of Monoid... which just requires a binary operator and an initial object that is an identity with respect to the binary operator. The type "int" could model Monoid many ways. For example, it could use "+" for the binary operator and "0" for the initial object. It could also use "*" for the binary operator and "1" for the initial object.
All that being said, I don't care too much either way. It's not a concern of Fusion, AFAICT. Unless, you are asking for std::pair to be removed from the list of supported Fusion sequences?
Nope. Just raising the point for discussion.
I think the main thing to take away from this is that scoped concept maps are good. They allow you to have it both ways, in different parts of your program. And yes, we could solve the problem by requiring wrappers, but wrapping stuff is a pain. Cheers, Jeremy __________________________________ Jeremy Siek <siek@cs.colorado.edu> http://www.cs.colorado.edu/~siek/ Visiting Assistant Professor Department of Computer Science University of Colorado at Boulder