Why can't structs be derived from?

[Steven Schveighoffer]

On Wed, 16 Mar 2011 11:06:40 -0400, Ary Manzana <ary at esperanto.org.ar>  
wrote:

> On 3/15/11 4:06 PM, Steven Schveighoffer wrote:
>> On Tue, 15 Mar 2011 14:54:13 -0400, Jens <jne at somewhere.org> wrote:
>>
>>> I didn't ask how to do composition in D. I asked why composition cannot
>>> be done via derivation, i.e., the reasoning behind the language design
>>> choice. A design faux paus IMO.
>>
>> Because composition by inheritance can be *completely* implemented using
>> alias this -- a feature that also provides other niceties. Why provide
>> another mechanism to do the exact same thing, just because you are used
>> to it?
>>
>> Note that "inheritance" is actually done exactly this way in C++, by
>> putting the derived type at the front of the "derived" type, and
>> aliasing all the methods/fields into the derived namespace.
>>
>> Show me what composition by derivation provides that alias this does  
>> not.
>
> Oh, come on, why use alias this when you can implement it in assembler?

There is a huge difference.  We don't use the same syntax for polymorphic  
inheritance for struct inheritance because we do not want to confuse the  
developer.

Essentially, the syntax looks better, but it looks exactly like  
polymorphic class/interface inheritance.  The two types of extension are  
drastically different, and I think the syntax should reflect that.

Besides, we are not talking the difference between assembler and  
high-level code here.  We're talking the difference between X : Y and X {  
Y y; alias y this;}

-Steve