On Monday, 8 June 2020 at 06:13:36 UTC, mw wrote:
Hi,
I have this program:
----------------------------------------------------
import std.stdio;
void f(ref int[] arr) {
arr ~= 3;
}
void main() {
int[][] arrs;
int[] arr;
foreach (i; 0 .. 3) {
arr = new int[0];
arrs ~= arr; //(a) [[], [], []]
f(arr);
// arrs ~= arr; //(b) [[3], [3], [3]]
}
writeln(arrs);
}
----------------------------------------------------
This program will print out [[], [], []].
If I comment out (a), and use (b), it will print out [[3], [3],
[3]]
So based on this behavior, looks like "~=" will append a copy
of `arr`; but 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 have to say this semantics surprised me.
I tried to change arrs' decl to:
(ref (int[]))[] arrs; // the intended semantics I want
But I got compiler error out: "found ( when expecting function
literal following ref".
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?
Arrays (technically, slices) in D are essentially this struct:
struct Array(T) {
T* ptr;
size_t length;
// operator overloads
}
So when you have int[][], each element of the outer array is an
Array!int. These, as simple structs, are copied about, so that
changing one does not change another.
The simple solution here is to call f not on arr, but on
arrs[$-1] (the last element of arrs). If that is not possible you
will need arrs to be an int[]*[].
--
Simen
