Newbie initial comments on D language - scope

[Janice Caron]

On 06/02/2008, Janice Caron <caron800 at googlemail.com> wrote:
>
> You're right. That was a mistake. I meant
>
> >             atomic { if (--pa->refCount == 0) delete pa; }
> >             atomic { ++pb->refCount; }
>

And just to be really clear, real code would need extra checks, above
and beyond the simplified version I wrote above. In fact, it would be
more like

        if (pa != null && pa->refCount != 0)
        {
            if (pb != null && pb->refCount != 0)
            {
                atomic { ++pb->refCount; }
                atomic { if (--pa->refCount == 0) delete pa; }

etc. It's important to do the increment before the decrement because
otherwise you run the risk of crashing if (pa == pb). And if you store
refCount in too small a variable, you also would need to be concerned
about ++pb->refCount wrapping. Microsoft VC++'s implementation of
std::string, for example, contains a one-byte reference count, and
they have some complicated code in there which interprets 0xFF
differently so that things don't fall over.

But that's using your suggested convention that (refCount == 0)
implies "not reference counted". In my real life implementation,
ref-countedness was a compile-time property, not a runtime property,
so I didn't have to do all of those tests. I think compile-time
ref-countedness is a very good idea, and has many fantastic
advantages. I just think it's a very bad idea to make it a runtime
property.