On Tue, Sep 28, 2010 at 9:14 PM, David Abrahams <dave@boostpro.com> wrote:
At Tue, 28 Sep 2010 15:36:30 -0400, Daniel Walker wrote:
Was there something unsatisfying about the solution I posted in http://permalink.gmane.org/gmane.comp.lib.boost.devel/209012 ?
Nope, I just missed it.
Oh wait! I think I have another example that's more truthy. :) I'll use Boost.ConceptCheck to flesh it out. Let's define the concept of an AdditivePair to represent the arguments to binary addition. To model the concept a type needs to support first(x), second(x) and add(x) for an object x where all three functions have integer values.
<snip>
We can now adapt std::pair<int,int> to model the AdditivePair concept.
<snip>
And we can also adapt int to model the AdditivePair concept; i.e. int is an additive pair where the second member is 0.
<snip>
That seems pretty natural to me.
Do you honestly think it's as truthy as the solution I offered?
Oh, I didn't mean to compare my example to yours. I only meant mine was more truthy than the previous example in the thread that used first(x) and second(x). Describing the concept in terms of pairs is more natural when you consider that an addition operation always has a first and second argument. Your solution comes at it from a different angle, which is also plenty truthy: the pairs are being reduced to int values, so in that sense IntValued naturally arises as a concept.
IMO it's a slight improvement over the list's previous offering, but making int model AdditivePair is still an odd contortion. AdditivePair still doesn't represent a real abstraction that you'd use anywhere else. By contrast, you could make int model ComplexNumber; that would make real sense and be useful outside of this toy problem.
Are you just exploring, or is there another reason we keep heading down this path?
Just exploring. :) It's a good toy problem to see how concepts can express the type requirements of a function, even when the types are seemingly unrelated like int and pair. It works in this case, because of the semantics of addition, so it also illustrates how semantics are important to concepts, as you have stressed. Daniel Walker