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

[Andrei Alexandrescu]

Bill Baxter wrote:
> 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

These thoughts are very much aligned with mine. Thanks Bill.

Andrei