Cast delegate and functions.

[qznc]

On Thursday, 31 October 2013 at 13:12:31 UTC, Wolftein wrote:
> void delegate(Event)
> void delegate(T) Where T is a class that inherits Event.
>
> I'm trying to cast (void delegate(T)) to (void delegate(Event)) 
> to be able to store them in a map, but when i cast them i get 
> null exeception.
>
> Same thing for cast things like this
>
> TemplateClass!Plugin
> TemplateClass!OtherTypeOfPlugin -> Being OtherTypeOfPlugin 
> inherit Plugin
>
> Using C++ this is allowed.

It is not safe to allow this.

   auto f = delegate(T x) { x.methodOfT(); }
   auto g = cast(void delegate(Event)) f;
   g(new Event());

This would call methodOfT on an Event object, which does not have 
that method.

Co- and Contravariance for such stuff is tricky. Here is a 
discussion on how C# 4.0 approaches it:

http://stackoverflow.com/questions/245607/how-is-generic-covariance-contra-variance-implemented-in-c-sharp-4-0