Daniel Gibson wrote:
> 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.
Not a good reason to change the syntax. That's what the manual is for.
What you said was: "Someone is going to try to program in D without
reading the manual and then be surprised when it doesn't work like C++ so
we'll just remove the construct so that doesn't happen -- at the expense
of elegant syntax".
>
> Furthermore I find C++'s class handling quite unfortunate..
Me too, some of it.
> only
> having polymorphism when explicitly using pointers really sucks.
How is it different in D where all polymorphic objects are reference
types? Take have the design space away, make everything a glorified
pointer and things are better?
> 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?
We'll have to save container design for another day.
