opIndexDispatch?

[Jonathan M Davis via Digitalmars-d-learn]

On Wednesday, October 12, 2016 22:45:28 Yuxuan Shui via Digitalmars-d-learn 
wrote:
> On Monday, 10 October 2016 at 19:16:06 UTC, Jonathan M Davis
>
> wrote:
> > On Monday, October 10, 2016 19:01:19 Yuxuan Shui via
> >
> > Digitalmars-d-learn wrote:
> >> Hi,
> >>
> >> Why is there no opIndexDispatch for overloading a[x].func() ?
> >
> > There's opIndex for overloading a[x], and then you can call a
> > function on the return value. If you want some kind of
> > opDispatch on the return value, then the return type will need
> > to implement opDispatch.
> >
> > - Jonathan M Davis
>
> The opIndex* overloads are used for handling the case where the
> key is not already in the data structure, right?

opIndex is for overriding the subscript operator so that you can do stuff
like foo[bar]. What you actually make the function do is up to you. It could
require that the element already be there. It could add the element if it's
not there. It could even do something completely unrelated to indexing
(though that would generally be considered bad practice). But aside from
syntax, the requirements on it are largely just convention. It's good
practice to make indexing act similarly to what you'd expect from a built-in
type such as an array or associative array, but that's not actually
enforced.

opIndexUnary, opIndexAssign, and opIndexOpAssign exist to make it possible
to do some basic operations on the result of foo[bar] without having to have
opIndex return by ref, but assuming that you can return by ref, all of them
could be done by having opIndex return by ref. If you were just wrapping an
array or trying to mimic an array, then there's a decent chance that you'd
just make opIndex return by ref and not overload the others, whereas in the
case of something like an associative array, using opIndexAssign makes more
sense, because the only way for opIndex to return by ref when opIndex
creates the element is if it's default-initialized, whereas if opIndexAssign
is used, it can just be created directly with the new value.

If you're looking to be able to use opDispatch on the return value without
returning by ref and while still affecting something within the object being
indexed rather than just affecting the temporary that would be returned by
opIndex, then you'll have to do something like return a type that has a
pointer to the object being indexed and which implements opDispatch so that
foo[bar].baz() uses opDispatch and still is able to acess foo via the
pointer in the return value from foo[bar]. There is no opIndexDispatch to
combine opIndex and opDispatch.

- Jonathan M Davis