Interfaces allow member definitions?

[Frustrated]

On Thursday, 30 January 2014 at 21:16:05 UTC, Steven
Schveighoffer wrote:
> On Thu, 30 Jan 2014 15:57:06 -0500, Frustrated 
> <c1514843 at drdrb.com> wrote:
>
>> On Thursday, 30 January 2014 at 20:17:23 UTC, Steven
>> Schveighoffer wrote:
>>> But it's important to note that A does not define an instance 
>>> of A_vtbl, just the layout! You still need a concrete class 
>>> to get a vtable instance to point at.
>>
>> But what I think you are failing to realize is that regardless
>> that foo is defined in an interface, since foo does not use 
>> this,
>> it does not depend on the object, so it does not need an
>> object(for all practical purposes it is a static function but 
>> of
>> course must take an object as the first parameter so it is
>> compatible as a member function).
>
> But what if it does need an object?
>
> interface A
> {
>    void foo() { bar();} // need 'this' to call bar
>    void bar();
> }
>

I've said many times that the functions could not use this. If
you are going to go that far then why not allow interfaces to
have fields? In this case they would not be any different from
classes.

>> Also, regardless that it is an interface, doesn't mean it can't
>> have a concrete vtable to work with.
>>
>> The fact is that a vtable is more complex because it has the
>> ability to call functions defined in a base class. In this 
>> case a
>> "base" interface.
>>
>> Just because something doesn't exist there now doesn't mean it
>> can't exist.
>
> Keep in mind that an interface vtable exists as part of an 
> object. The compiler knows this, and performs thunks when 
> necessary.
>
>> Answer me this
>>
>> class A { void foo() { } }
>> class B : A {  }
>>
>> B b = new B;
>> b.foo();
>>
>> are you telling me there are actually two foo functions? or 
>> does
>> b actually call A's foo passing it the object b e.g., the call
>> made is actually foo(b);
>
> There is one function, but two vtables, one for A, and one for 
> B. Both point at the same function.

Yes, and they could point to the function defined in the
interface. When they call it, they would pass themselves as this.
The methods in the interface do not use this, so it doesn't
matter (they could potentially use it but it would require that
it always be valid)

>> If so, then what if A is an interface? (by interface, I am
>> talking about one that has a vtable created for it's members)
>>
>> interface A { void foo() { } }
>> class B : A {  }
>>
>> B b = new B;
>> b.foo();
>>
>> Whats the difference? Absolutely nothing but your 
>> interpretation
>> of what an interface is.
>
> The difference is, now there is only one vtable, and one 
> interface vtable inside B. A has no vtables. If you do this:
>
> A a = new B;
>
> a now points at the interface struct *inside B's object*. When 
> you call a.foo, it does this:
>
> 1. It looks up in the interface vtable for A that's specific 
> for B (accessed via the interface struct in the object itself) 
> to get the function to call.
> 2. Included in the interface vtable struct is an offset to add 
> so the 'this' pointer actually points at the object itself 
> instead of the interface struct.
>

Who says A doesn't have a vtable? That's something that you are
forcing on it. You have to get off that if we are ever to make
any headway.



>> This is all about semantics. If you want to think of an 
>> interface
>> as some idealized abstract compile time object that doesn't 
>> exist
>> at run time and has no vtable then so be it. But that doesn't
>> mean it has to be and I'm just saying it is limiting.
>
> An interface instance and an object instance are two VERY 
> different things, and are handled differently by the compiler.
>
> -Steve

Again, you have to get off of what has been defined. You have the
mentality exactly the same as those that thought the earth was
flat, imaginary numbers were nonsense/useless, man couldn't go to
the moon.

If you define your knowledge on what you already know what is the
point? You just run around in circles.... nothing changes....
you'll just continue believing the earth is flat...