T[new] misgivings

[Robert Jacques]

On Thu, 15 Oct 2009 22:55:07 -0400, Andrei Alexandrescu  
<SeeWebsiteForEmail at 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.