On Thu, 15 Oct 2009 22:55:07 -0400, Andrei Alexandrescu
<seewebsiteforem...@erdani.org> wrote:
I talked to Walter about T[new] today and it seems we are having a
disagreement.
The problem is that I believe T[new] is a container, whereas Walter
believes T[new] is nothing but a slice with a couple of extra operations.
Paradoxically this seems to be conducive to subtle efficiency issues.
For example, consider:
int[new] a;
...
a = [1, 2, 3];
What should that do?
Walter: T[new] is a slice with benefits, assignment for slices rebinds
the slice, therefore the assignment must do the same. In this case, the
assignments allocate a new array and make a refer to that array.
Whatever old array a referred to will continue to live wherever it was.
Me: T[new] is a container, therefore the assignment must resize the
container from whatever size it had to 3 and then write 1, 2, 3 to its
three slots.
I guess each of us has a point, but this is a setup for an increasingly
unpleasant situation. Here's the dialog as it happened.
A: Ok, then how do I say the common operation "I want to overwrite
whatever the array had with 1, 2, 3"? I can only presume there must be
an obvious and simple way to do so, and I thought a = [1, 2, 3] was the
obvious syntax to achieve that.
W: No, you must write
a[] = [1, 2, 3];
A: But that only works if the container already had length 3. So what I
need to do is this:
a.length = 3;
a[] = [1, 2, 3];
A: But that is inefficient if the array had length less than 3 because
it means double assignment
W: Nobody complained about it with slices.
A: So if I do want something that does the obvious operation "Whatever
that array had, make it now have 1, 2, 3 as it contents" at a reasonable
cost I need to call an elaborate function that is highly nontrivial to
write?
W: Looks like so.
A: So then the first "Effective D" standard would have Item #1: "Avoid
assignment to arrays. Call the assign() function"?
W: Nobody complained about it with slices.
===============
This goes into something more interesting that I thought of after the
conversation. Consider:
T[new] a;
T[] b;
...
a = b;
What should that do?
Andrei
I like (and have used) the opSliceAssign syntax to represent by value/copy
assignment as opposed to opAssign's by reference syntax. You could always
define T[new] auto-resize in the case of a[] = b, but then you'd have to
decide if that behavior should be extended to slices.
