At Tue, 14 Dec 2010 07:41:17 -0500, Matt Calabrese wrote:
On Tue, Dec 14, 2010 at 7:11 AM, Matt Calabrese <rivorus@gmail.com> wrote:
Anyway, I've said more than I wanted to right now. I didn't expect this thread to be bumped -- I was hoping to post a new thread about all of this once everything was polished and up in the sandbox.
Actually, since this thread has started going again, I've hit some complicated issues regarding concept maps that I'd hope someone could help out with. In particular, there is one very troubling situation I've encountered. Imagine three concept types: a "base" concept,
concrete code helps: concept Base<typename T> { T operator*(T, T); };
a "left" concept which is a refinement of "base", and a "right" concept, which is also a refinement of "base".
concept Left<typename T> : Base<T> { T operator-(T, T); }; concept Right<typename T> : Base<T> { T operator+(T, T); };
A programmer creates a type called "foo" and wishes to make a concept map for "left" and also a concept map for "right".
struct foo {}; // This one would be an error, since foo doesn't have operator* concept_map Left<foo> { foo operator-(foo,foo) { return 0; } }; concept_map Right<foo> { foo operator+(foo,foo) { return 0; } }; So how are you going to supply operator*? The answer to your question depends on that.
Because "left" is a refinement of "base", making a concept map for "left" implicitly makes a concept map for "base". Similarly, since "right" is a refinement of "base", the same thing happens. So the issue is, if someone now writes a function that requires a "base", which concept map is to be used?
I don't see a problem. If you answer the question by giving foo an operator* of its own, the two implicitly-generated concept maps are the same concept map.
Should this be a compile-time error? If so, how could this possibly be safely resolved (does anyone know how this was handled in concept proposals)?
I need more information. Please try to write out a complete example (using ConceptC++ syntax, please).
Assuming it should be a compile-time error, would it be enough to just require a third concept map to be written, explicitly for base, or perhaps a way to specify explicitly that "right" should be used? If so, what happens when a function takes a "left" and that function references the parts of the concept map that are used to satisfy the requirements of "base"? I'd imagine that it would use the "left" concept map, but that means that a different concept map for "base" would be used depending on whether the function took a "base" or a "left" or a "right", which seems to be very wrong. Am I expressing this problem correctly? Does anyone see an obvious resolution that I've missed?
too many questions; too little code ;-) -- Dave Abrahams BoostPro Computing http://www.boostpro.com