On Fri, Nov 14, 2008 at 2:18 PM, Bruno Lalande <bruno.lalande@gmail.com> wrote:
The "specialized" typedef is a bit weird, as it forces the user to sort of say "yes, I've really specialized it". But the goal is reached I think: I have never explicitly specialized ellipse_traits<circle>. I have only said that if circle_traits<X> exists then ellipse_traits<X> exists and ellipse_traits<X>::width() is circle_traits<X>::radius(), except if a better specialization of ellipse_traits<X> exists. And the compiler correctly understood what I meant when I asked for ellipse_traits<circle>::width().
Does anybody know a way to know if a specialization exists without using this kind of trick?
I've done something similar, where the default template implementation has a member typedef of a particular type `not_specialized`. You can see this at: http://tinyurl.com/5dba48 (look at port_binary_operation_impl and are_port_binary_operable) In this case, the member typedef used is result_type but it can be anything. The mechanism basically checks whether a particular template instantiation has a result_type that is_same as not_specialized. If it has no result_type, or the result_type is something different, it knows that the template has been specialized. The method is not perfect as you can fool it into thinking that the template has not been specialized for certain arguments by setting the typedef used to not_specialized. I don't know whether there is a way to prevent that (perhaps make `not_specialized` a private nested struct of something?) Best, Stjepan