On Sunday, 10 September 2017 at 14:42:42 UTC, Moritz Maxeiner
wrote:
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.
With a final class reference, we always know what function to
call at compile time (since it can't be inherited). Therefore we
don't need a vtable.
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
