"David Bergman" <davidb@home.se> wrote in message news:005301c6aeb6$576d2750$a0da1105@davidtp...
Beman wrote:
* Addition of a header boost/identifier.hpp. For years I've used a similar class to provide the type safety and freedom from unwanted conversions that is missing when a simple int is used for an identifier. This is logically separate from the system stuff, but for convenience is included in the system package since error_code.hpp uses it.
I like the idea of having such a (meta) construct in my programming portfolio, much like the "newtype" in Haskell, which introduces an isomorphic type.
A few comments:
1. You are right in that it is separate, and should definitely be separate from the error code stuff; actually, since it is a construct mapping types to types - i.e., a meta function - it might even find its proper home in MPL :-)
Yes - I just included it with the error code stuff because it was needed there, and I wanted to see if there was any interest in it as a separate submission.
2. As someone else pointed out, even though this functor establishing homorphic types is useful for the use case of having identifiers, it should not stop there. Thus, its name should reflect that wider scope, such as "homorphic_type." Ok, more pragmatic then: "embed_type." [hope that going from a adjective to a verb did not tick anyone off]
3. I see a few distinct types of homomorphisms here, which should correspond to policies: (i) order (ii) equivalence (iii) assignability (iv) conversion (to source)
I implemented them as individual policy links that are then chained together explicitly. Will use a hierarchy generator, such as Loki's, or create a chaining construct with the help of MPL and upload it to the vault.
I toyed with the idea of either a policy-base or generative-programming approach, and decided they would add a lot of complexity for very little gain when compared to just telling the user to code the members, which are very simple, when needed. Simplicty has its virtues. That said, I encourage you to keep refining your ideas. You may be able to craft a homomorphic type generater that has enough benefits to be compelling. --Beman