I'm trying to figure out the size difference between a final class and a class (which carries a vtable pointer).

import std.stdio;

class A { void print(){} }

final class B { void print(){} }

void main(){
        writeln(__traits(classInstanceSize, A));
        writeln(__traits(classInstanceSize, B));
}


Returns:
8
8

I'm not sure, why does a final class carry a vtable pointer?

Etienne Cimon:

> I'm not sure, why does a final class carry a vtable pointer?

In D all class instances contain a pointer to the class and a monitor pointer. The table is used for run-time reflection, and for standard virtual methods like toString, etc.

Bye,
bearophile

On 2014-10-23 20:12, bearophile wrote:
> In D all class instances contain a pointer to the class and a monitor
> pointer. The table is used for run-time reflection, and for standard
> virtual methods like toString, etc.
>
> Bye,
> bearophile

So what's the point of making a class or methods final? Does it only free some space and allow inline to take place?

Etienne Cimon:

> So what's the point of making a class or methods final?

It forbids subclassing. And final methods are not virtual, so they can be inlined.

Bye,
bearophile

On Friday, 24 October 2014 at 00:21:52 UTC, Etienne Cimon wrote:
> On 2014-10-23 20:12, bearophile wrote:
>> In D all class instances contain a pointer to the class and a monitor
>> pointer. The table is used for run-time reflection, and for standard
>> virtual methods like toString, etc.
>>
>> Bye,
>> bearophile
>
> So what's the point of making a class or methods final? Does it only free some space and allow inline to take place?

Like bearophile said the vtable is required for virtual methods. Consider this code:

import std.stdio : writeln;

class A { void foo() {writeln("A");} }
final class B : A { override void foo() {writeln("B");} }

void main() {
    A a = new B();
    a.foo();
}

In order for the call to foo to run the correct version of foo, B needs to have a vtable. Since all classes in D implicitly inherit from Object, which has some virtual methods, all classes need to have a vtable.

--
  Simen