change object class

[Vitaliy Fadeev]

Hi!
I want to change a method ```Draw``` on a custom object when the 
```MouseIn``` event occurs.
This is known as "Change State" of the object: ```Init``` -> 
```Hovered```.

I want to change the state of an object by changing its class, 
like this:

```d

this.__vptr = typeid(CLS).vtbl.ptr;

```

I have read the ABI and am confident in replacing ```__vptr``` as 
long as the classes contain the same fields and the same 
interfaces.

Example:
```d
// O
//   to!state
// State_Init    : O
//   Draw
// State_Hovered : O
//   Draw
//
// o.to!State_Hovered
// o.to!State_Init

class O
{
   void to(CLS)()
   {
     // if (same fields && same interfaces && same instance size)
     this.__vptr =
       cast(immutable(void*)*)typeid(CLS).vtbl.ptr;
   }
}

State_Init : O
   void Draw() { /* ... */ }

State_Hovered : O
   void Draw() { /* ... */ }
```

when MouseIn:

```d
   ...
   o.to!State_Hovered();
   ...
```

when MouseOut:
```d
   ...
   o.to!State_Init();
   ...
```

It works! But I want to ask how to make this 100% the best of the 
best?
What should I consider before changing ```__vptr``` ?