On Mon, May 19, 2008 at 4:26 PM, Daniel Walker <daniel.j.walker@gmail.com> wrote:
a placeholder. Placeholders indicate a template parameter in the function's argument list (not an unspecified function object outside the argument list). Actually, come to think of it, _1(_1, _1) would probably not be possible because it doesn't encode the type of a polymorphic function object, so there's no way for the compiler to use that type's set of overloaded operator()s later on at the call site. _1(_1,_1) is type erasure again and that breaks polymorphic function objects.
Well, the first _1 is intended to be the return type, not the polymorphic type. It's _1(_1, _1) because plus and minus return T: T plus(T, T), T minus(T, T) So it's bool (T) because is_positive and is_negative return a bool. Actually I think there is no way to create a polymorphic functor wrapper, but anyway it's the logical extension of function signatures in the polymorphic world because function signatures do not imply _any_ algorithm or functionality apart from arguments and return type. So a poly signature should define return type and arguments ONLY. Because here we talk about templates, i.e. family of types and not a single type we can only specify argument arity and when argument types should be the same, as in plus, or different as _3(_1, _2) I would think overload_set is based on the concept of wrapping a function (call it type erasure), but when you assign plus(_1,_1) there is nothing to wrap, just use plus instantiated by compiler according to the calling sites arguments. If you need a generic fallback it is better to let overload_set derive directly from plus() and you have what you need. Comments? Thanks Marco