Re: Ada requires too much typing!

["Martin Krischik" <krischik@users.sourceforge.net>]

Am 06.06.2010, 12:13 Uhr, schrieb Simon Wright <simon@pushface.org>:

> "Martin Krischik" <krischik@users.sourceforge.net> writes:
>
>> One should just
>> make a habit of defining classes like this:
>>
>> virtual Base
>>   {
>>   public:
>>     virtual ~Base ()
>>       {
>>       …
>>       }
>>   }
>>
>> And suddenly all your virtual inheritance needs work flawlessly.
>
> I suppose there must be some cost to doing this? or the language
> definition would require it to be always so.

Yes indeed, you have virtual dispatch not only on the methods but on the  
instance variables as well. Sounds worth then it is. It means the internal  
structure of a child type changes from (Ada pseudo code):

record Child_Type is
     Parent_Data : Parent_Type;
end record

to

record Child_Type is
     Hidden_Parent_Data : Parent_Type;
     Parent_Data : constant access Parent_Type := Hidden_Parent_Data'Access;
end record

The compiler will ensure that Hidden_Parent_Data exist only once - but  
there may be any amount of Parent_Data which all point to the same  
Hidden_Parent_Data. And the funny part: The technology used to archive  
this is the very same you use to implement interfaces. Once you have the  
scaffolding for Interfaces in place you can have virtual base classes as  
well.

Main problem with multiple inheritance was in  C++ itself where default is  
“fast” not ”save”. For the Eiffel users it was never a problem because  
there virtual base classes are default. And I just see I made a mistake in  
my original post. It should have been:


class Parent
    {
    public:
      virtual ~Parent ()
        {
        …
        }
    }

class Child
    virtual public Parent
    {
    …
    }

but main problem stay: not enough C++ programmer have known about virtual  
inheritance and the diamond problem only pops up at Grand_Child level and  
lower  (that means one has to plan ahead to avoid it). This in turn given  
multiple inheritance a bad name. But if you do it right it works just  
beautifully.

The real lesson learned: “doing it right” must be default and “make it  
fast” must be an optional extra to be taught in very advanced level  
courses only.

Martin
-- 
Martin Krischik
mailto://krischik@users.sourceforge.net
https://sourceforge.net/users/krischik