Multiple subtyping with alias this and nested classes

[Andrei Alexandrescu]

I just realized that nested classes work so well with alias this, you'd 
think it was an evil plot all along. It wasn't, but I'm happy about the 
coincidence.

Here's how to effect multiple subtyping in D very effectively:

import std.stdio;

class Base1 {
     void fun() { writeln("Base.fun"); }
}

class Base2 {
     void gun() { writeln("Base.fun"); }
}

class Multiple : Base1 {
     // Override method in Base1
     override void fun() { writeln("Multiple.fun"); }
     // Override method in Base2
     class MyBase2 : Base2 {
         override void gun() { writeln("Multiple.gun"); }
     }
     // Effect multiple subtyping
     Base2 _base2;
     alias _base2 this;
     this() {
         _base2 = new MyBase2;
     }
}

void main()
{
     auto obj = new Multiple;
     Base1 obj1 = obj;
     obj1.fun();
     Base2 obj2 = obj;
     obj2.gun();
}

The program above segfaults because somehow obj2 is null. That is a bug 
I just reported. For now, you can replace obj2.gun() with obj.gun() to 
make things work.

When we first introduced alias this, I knew multiple subtyping was 
possible. I didn't expect it to dovetail so nicely with nested classes.


Andrei