Zitat von Alexander Lamaison <awl03@doc.ic.ac.uk>:
I would like to dispatch messages to a class based simply on the *presence* of a message handler method.
Currently, the classes maintain an MPL vector of the messages they handle and must implement a on(message<MESSAGE_ID>) method for each one. The dispatcher uses this compile-time list to build the dispatching code. This means the information is maintained twice and may fall out of sync (e.g. adding the handler but forgetting to update the message vector.
Are there any template metaprogramming tricks I can employ to dispatch the message to a handler method if it exists and the default handler otherwise? All this information is available at compile time. The question is are templates are powerful enough to make use of it?
templates are not, and there is no portable way to detect the presence of a member function. you can however use ADL for this type of thing, if you don't need the information whether the message handler is present for anything other than calling it: class A{ friend void message(A &this_,message msg){ ... } }; namespace detail{ template<class T,class Msg> void message(T &this_,Msg msg){ //default handler } template<class T,class Msg> void dispatch(T &this_,Msg msg){ message(this_,msg); } } A a; B b; detail::dispatch(a,msg); //calls message handler of A detail::dispatch(b,msg); //calls default handler