On 03/16/2011 10:28 AM, Steven Schveighoffer wrote:
On Wed, 16 Mar 2011 11:06:40 -0400, Ary Manzana <a...@esperanto.org.ar>
wrote:
On 3/15/11 4:06 PM, Steven Schveighoffer wrote:
On Tue, 15 Mar 2011 14:54:13 -0400, Jens <j...@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
Not to mention that alias this offers the ability of a runtime hook; you
don't need structural conformance as you can produce the conversion on
the fly.
Andrei
