Developing a plan for D2.0: Getting everything on the table

[Bill Baxter]

On Mon, Jul 27, 2009 at 9:54 AM, Bill Baxter<wbaxter at gmail.com> wrote:
> On Mon, Jul 27, 2009 at 7:48 AM, Andrei
> Alexandrescu<SeeWebsiteForEmail at erdani.org> wrote:
>> Leandro Lucarella wrote:
>>>
>>> Andrei Alexandrescu, el 27 de julio a las 07:59 me escribiste:
>>>>>
>>>>> For example, should the author of a container library prefer classes or
>>>>> structs?
>>>>
>>>> I've been struggling with this forever. I don't know. I don't even know
>>>> whether reference or value semantics are best for containers. I don't
>>>> know whether abstract container interfaces and container-independent
>>>> code are a net win; experience with STL seems to say "don't" and
>>>> experience with Java seems to say "ho-hum".
>>>
>>> About values vs. reference semantics, I think reference is the best.
>>> Containers usually are big, and you don't want to copy them arround.
>>> I find myself using lots of references to pass containers arround in C++
>>> and almost never use the copy() method of Python containers, so based on
>>> *my* experience, I'd say that reference as the default is the best
>>> approach.
>>
>> Sounds convincing.
>
> That's exactly right except for the times when it isn't and what you
> want is value semantics.
>
> I've just finished recently refactoring some C++ code that wasn't
> designed with copying in mind.  Changing all the "float* data; int
> data_length;" members into std::vectors did the trick.  The data they
> contained wasn't particularly large, and there was no real need to
> introduce the complexity that sharing it between copied instances
> would have created.

So if I didn't make it clear -- my opinion is that whether containers
should be by value or by ref depends on the circumstances and that
choice should be left to the programmer.

Thus the question becomes what's the easiest way to provide both?
Three options I see:

1) Write ref classes and wrap them in structs
2) Write value structs and wrap them in classes
3) Write template mixins and mix them into both struct and class shells.

I think given the existence of "alias this" for structs, 1 is actually
the best-supported paradigm.
I think with that you should be able to value-ize a by-ref container
class just by writing a post-blitter to copy the contents.

--bb