Multiple Inheritance of Classes

[Steven Schveighoffer]

"superdan" wrote
> Steven Schveighoffer Wrote:
>
>> "superdan" wrote
>> > Lars Ivar Igesund Wrote:
>> >
>> >> Chris R. Miller wrote:
>> >>
>> >> > Understand, I'm NOT demanding ANYTHING.
>> >> >
>> >> > What is the current state of thought about Multiple Inheritance for
>> >> > classes in D?  I'd like to have that feature, since it'd make some
>> >> > stuff
>> >> > I want to do a bit easier.  Is it not there because it's not worth 
>> >> > the
>> >> > effort to implement?  Because it's evil and needs to die (I don't 
>> >> > know,
>> >> > some people could possibly be adamantly anti-MI)?
>> >>
>> >> This is actually the reason, not the adamantly anti-MI part, just that 
>> >> MI
>> >> is
>> >> evil and that is well acknowledged almost everywhere. You will find 
>> >> good
>> >> argumentation against it if you look, too.
>> >
>> > appeal to authority. appeal to ridicule. appeal to the majority. all in
>> > one sentence. wow. at least could you space out your fallacies a bit 
>> > more.
>> >
>> > the man has kindly asked a sensible question. he deserves a good 
>> > answer.
>> > if u can't give one just don't reply. this is just ignorance.
>>
>> This kind of bullying bullshit does nothing to further communication, or
>> help anyone in the least.  You've managed to call many of the brightest
>> developers for D idiots, usually based on useless crap like this (which 
>> has
>> no bearing on anything).  So shut the fuck up.
>
> or what, u gonna kick my ass. relax. you can always block me. (but hold 
> onto that a bit more. dee's plea is just too cool.) my post did further 
> communication. it exposed the hackneyed "mi is evil" shit... i mean poop. 
> (damn.) it may have helped someone. you know what i like about walter. 
> when he doesn't know something he is open in admitting it. for that alone 
> i'd wash his feet. i didn't call the poster any name. but that particular 
> post was bull... i mean crap and i just said it. the fact that the post 
> sucked bears nothing on the fact that he's good or anything. even worse, 
> if he's good then why would he use his goodwill to get away with 
> statements like that. they only reveal ignorance and attempt at continuing 
> ignorance because it puts a stigma on anyone investigating mi. it's silly 
> we need to still talk about it. now shall we just move on to something 
> technical.

I don't want to block you.  You have some good things to say (although 
colorful).  Just can it with the "what you said is stupid, so don't post 
here"  It makes tentative posters not want to post for fear of being 
ridiculed (not me BTW :) )  Some of them might have interesting things to 
say.  If you want to argue against someone's point, argue the point (which 
you did later, and I found it interesting, although my personal experience 
with MI (on C++) is that it sucks, and should never be used).

What Lars said basically is that many people don't like MI, and you can find 
proof of that (people don't like it) if you search online.  This is the 
reason Walter doesn't implement it, because he's in that camp.  I think that 
is a reasonable answer to the question given.

It's sort of like most the reasons Walter gives for everything new he comes 
out with: "X is fundamentally broken in C++".  Substitute fundamentally 
broken with evil, substitute threading, const, etc. for X.  Can't say I 
always disagree, but his proof is certainly lacking ;)

>> > interface Customer
>> > {
>> >    string ssn();
>> >    string name();
>> >    string uniqueName() { return name ~ "(ssn: " ~ ssn ~ ")"; }
>> > }
>> >
>> > so uniqueName formats a specific way. a descendant can choose to change
>> > that or just use the default. no idea why walt chose to disallow that.
>> > walt?
>>
>> What do you pass as the 'this' pointer?  When you call a function on an
>> interface, the compiler uses the offset of an interface to the 'this'
>> pointer to get to the object, but in this case, there is no object, so 
>> what
>> does the compiler do to call the ssn() and name() functions while
>> implementing this function?
>
> i'm unclear about this so maybe it ain't as easy as i thought.
> but i'm thinking the same problem goes for the global string 
> uniqueName(Customer c) { return c.name ~ "(ssn: " ~ c.ssn ~ ")"; }

Not the same as the above problem, because the vtable layout for Customer is 
always the same.  The name() function expects a pointer to the object, and 
the implementation knows everything about the object including the vtable. 
The problem with putting the implementation in the interface is that a 
member function gets a pointer to the *object* not the *interface*.  In your 
new example, the 'c' parameter is a pointer to the interface, so that 
problem doesn't exist.

> a pointer to this function should be put in the vtable if the object does 
> not implement it.

That is fine for calling the function, but not for what to do when compiling 
it.

>
>> If you pass the interface pointer as the 'this' pointer, then how do you
>> override it in an Object that implements the interface?  The function in 
>> the
>> concrete class can't be passed the interface pointer, so you can't really
>> override it.
>
> that pretty much kills what i wrote above eh. but thunking will take care 
> of it. if there's no impl in an object put a pointer to a thunk that 
> adjusts the pointer (from obj to interface) and calls the default impl 
> with the adjusted pointer. the latter is a direct call which makes it 
> fast.

Hm... I think this would work actually, as I think this is a runtime lookup. 
This sounds like a reasonable tradeoff, and having the final modifier if you 
want it to be quicker (I think the compiler can statically do the thunk if 
you know the concrete object type).  Actually, it would have to do a thunk 
for a derived interface, even if you put final on it, as it can't always 
know the offset between 2 interfaces in a particular object (or can it?).

Perhaps someone with better knowledge of the way interfaces work could tell 
if it would be possible?

-Steve