Why can't structs be derived from?

[Daniel Gibson]

Am 15.03.2011 20:40, schrieb Jens:
> Daniel Gibson wrote:
>> Am 15.03.2011 20:24, schrieb Jens:
>>> Daniel Gibson wrote:
>>>> Am 15.03.2011 19:48, schrieb Jens:
>>>>> Andrei Alexandrescu wrote:
>>>>>> On 3/15/11 12:55 PM, Jens wrote:
>>>>>>> Steven Schveighoffer wrote:
>>>>>>>> That's all there is.  Structs do not have inheritance, only
>>>>>>>> alias this.
>>>>>>>
>>>>>>> Why don't they though? Inheritance does not have to mean
>>>>>>> polymorphic. It can mean composition, like in C++. I don't
>>>>>>> understand the reason for such ugly syntax.
>>>>>>
>>>>>> Using inheritance for composition is frowned upon in C++ for good
>>>>>> reasons. If you want composition, the best is to use composition.
>>>>>
>>>>> It was frowned upon early on because the compiler implementers
>>>>> didn't have their acts together and the resulting objects layout
>>>>> could not be relied upon. The example I gave came from the STL so
>>>>> I think "frowned upon" is something you are picking up from long
>>>>> ago.
>>>>>
>>>>> Composition means access through the members rather than direct
>>>>> access:
>>>>>
>>>>> struct point
>>>>> {
>>>>>     int x;
>>>>>     int y;
>>>>> };
>>>>>
>>>>> struct point3d
>>>>> {
>>>>>     point pt;
>>>>>     int z;
>>>>> };
>>>>>
>>>>> ...
>>>>>
>>>>> point3d mypoint;
>>>>> mypoint.pt.x = 3; // ugly
>>>>>
>>>>
>>>> This is why you add "alias pt this;" to point3d. So you can write
>>>>  mypoint.x = 3;
>>>>
>>>
>>> Still ugly though.
>>>
>>
>> I don't think so. It makes obvious what happens: a composition -
>> *not* an inheritance - with syntactic sugar that allows one to omit
>> the .pt in mypoint.(pt.)x - as long as mypoint doesn't have a member
>> x itself.
>>
> 
> YMMV. To me it's ugly enough to not take the language seriously.
> 
>> Allowing inheritance syntax on structs would only lead to confusion -
>> especially for people coming from C++.
>>
> 
> C++ does it that way, so what confusion are you talking about?
> 

The confusion that in C++ classes and structs are basically the same (only the
default visibility of members is different) while in D they're not.
So allowing "inheritance" on structs would give the false impression that
structs in D are also classes of some kind.

Furthermore I find C++'s class handling quite unfortunate.. only having
polymorphism when explicitly using pointers really sucks.
e.g. you have a base class Foo and a class Bar derived from Foo.. now you wanna
put Objects of type Foo and Bar in a list.. what do you do?
list<Foo>? won't work for Bar Objects. So you gotta use list<Foo*>.
Using list<Foo*> really sucks, especially with iterators.. you end up using
something like
  list<Foo*>::iterator it = ... ;
  (*it)->x = 3;
  int bla = (*it)->myFun(42);

Now *that* is ugly.