I have a new version of my counter-proposal (or suggestion, I just want to take FULL advantage of the power in his idea...) to Beman's 'identifier' notion. My proposal was a more generic 'embed_type' with various policies. The old version (a few hours old now...) used a set of fixed flags in order to enable or disable certain homomorphisms. The new version handles that much better, via a vector of policy types. I also have a (somewhat associated) proposal - that I just created - to create a hierarchy from a vector of types. Actually, two different kinds of hierarchies: 1. A spine-like hierarchy, where the types are attached as maximal elements. struct A { void a() {} }; struct B { void b() {} }; struct AB : generate_spine<mpl::vector<A, B> > {}; AB ab; ab.a(); ab.b(); 2. A chain, where the types have "holes" template<class Base> struct A : Base { void a() { b(); }; struct B : Base { void b() {}; struct AB : generate_chain<mpl::vector<quote_raw3<A>, quote_raw3<B>
{};
I have gathered those two proposals in files 'embed_type.hpp' and 'generate_hierarchy.hpp', which I will upload as soon as I get access to the vault. Till then, I talk about the hierarchy generation here: http://davber.blogspot.com/2006/07/hierarchy-generation-in-c.html. Back to the 'embed_type' proposal, an example - which happens to be Beman's 'identifier' looks like: template <typename T, typename D> struct identifier : embed_type<T, D, mpl::vector<policy<embed_order>, policy<embed_equivalence> > > {}; A more concrete example (yes, I know it will fail on GCC 3.4, due to the base class being mentioned in the constructor without the type parameters; ugh...): class CustomerId : public embed_type<int, CustomerId, mpl::remove<embed_policies_isomorphism, policy<embed_conversion> >::type> { CustomerId(int num) : embed_type(num) {} } CustomerId id = 42; int idNum = id; // Will fail, since we removed the conversion policy! In this case we preserved all operations minus conversion to int, which we explicitly removed from the type vector. /David