-betterC and extern(C++) classes

[Moritz Maxeiner via Digitalmars-d]

On Sunday, 10 September 2017 at 14:04:20 UTC, rikki cattermole 
wrote:
> On 10/09/2017 2:19 PM, Moritz Maxeiner wrote:
>> If TypeInfo for extern(C++) classes is removed, couldn't final 
>> extern(C++) classes without base class and which don't 
>> implement any interfaces omit the vtable so that the following 
>> assert holds:
>> 
>> ---
>> final extern(C++) class Foo {}
>> static assert (__traits(classInstanceSize, Foo) == 0LU);
>> ---
>> 
>> The reason I ask is that fairly often I have an abstraction 
>> that's better suited as a reference type than a value type, 
>> but doesn't need any runtime polymorphy (or the monitor 
>> standard classes have). Structs + pointers are the only way I 
>> know of to avoid the (in this special case) unneeded vtable 
>> overhead, but it always ends up looking worse to read.
>
> We can do it for any class if its final.

Even final classes can always inherit (potentially already 
overridden) virtual methods from their parent classes and since 
all normal D classes inherit from core.object : Object [1], which 
defines virtual methods (`toString`, `toHash`, `opCmp, and 
`opEquals`), I don't see how this can be true.

> The problem isn't generating the vtable's. But the information 
> required for casting.

This applies to normal D classes, but D doesn't support (dynamic) 
casts for extern(C++) classes, anyway, so this shouldn't be an 
issue for them.

[1] 
https://github.com/somzzz/druntime/blob/74882c8a48dd8a827181e3b89c4f0f205c881ac5/src/object.d#L50