how to append (ref) int[] to int[][]?

[Dukc]

On Monday, 8 June 2020 at 06:13:36 UTC, mw wrote:
>
> what I really want in (a) is append `ref arr` and output [[3], 
> [3], [3]], i.e. the real `arr` be appended instead of its copy.
>
> I tried to change arrs' decl to:
>
>     (ref (int[]))[] arrs;  // the intended semantics I want
>
> 1) I'm wondering how to achieve what I want? and
> 2) why "~=" here will append a copy rather than the real `arr` 
> itself to arrs?
>
>
> Thanks.

```
import std.stdio;

void f(ref int[] arr) {
         arr ~= 3;
}

void main() {
	import std.algorithm;
	
	int[]*[] arrs;
	int[]* arr;
	foreach (i; 0 .. 3) {
		//stack allocated array pointer&length would expire on end of 
scope, so must allocate the them on heap since we're keeping 
refeerences on them.
		auto arrayAllocation = new int[][1];
		arr = &arrayAllocation[0];
		*arr = new int[0];
		arrs ~= arr;
		f(*arr);
	}
	
	//will print an array of addresses otherwise
	arrs.map!(ptr => *ptr).writeln;
}
```

But I must say I would generally prefer just calling `f()` on 
`arrs[i]` directly after appending, or calling the function 
before appending. One reason is that needless heap usage is 
generally less efficient than using the stack.