Why is opIndexAssign replaced by opSlice here?
Paul Backus
snarwin at gmail.com
Mon Oct 18 03:42:35 UTC 2021
On Sunday, 17 October 2021 at 22:52:27 UTC, Elmar wrote:
> Hello Dear community.
>
> I'd like to overload `opIndexAssign` for a struct which wraps
> around a generic array (so that it can't support `opIndex` due
> to unknown return type).
>
> Broken down as much as possible this is the code:
>
> ```
> import std.stdio : writeln;
> import std.range : ElementType;
>
> struct S {
> void opIndexAssign(X, RANGE)(X x, RANGE range)
> if (is(ElementType!RANGE : size_t))
> {
> writeln(__FUNCTION__);
> }
>
> auto opSlice(size_t start, size_t end) {
> import std.range : iota;
> return iota(start, end);
> }
> }
>
> void main()
> {
> auto arr = new int[7];
>
> S s;
> s.opIndexAssign(arr, s.opSlice(1,4)); // works
> s[0..3] = arr[1..4]; // does not work, compiles to
> `s.opSlice(0,3) = arr[1..4]`
> }
> ```
>
> I'm clueless about why it wouldn't compile the last statement
> to `s.opIndexAssign(arr[1..4], s.opSlice(0,3))`.
>
> Help appreciated :-)
What happens here is, the compiler first tries the D2-style
rewrite:
```d
s.opIndexAssign(arr[1..4], s.opSlice!0(0, 3))
```
However, that rewrite fails to compile, because your `opSlice`
does not take a template argument specifying the dimension along
which to slice, as specified in the language spec's section on
["Slice Operator Overloading".][1]
Since the above rewrite fails to compile, it falls back to
rewriting the expression using D1-style operator overloads:
> For backward compatibility, `a[]` and `a[i..j]` can also be
> overloaded by implementing `opSlice()` with no arguments and
> `opSlice(i, j)` with two arguments, respectively. This only
> applies for one-dimensional slicing, and dates from when D did
> not have full support for multidimensional arrays. This usage
> of opSlice is discouraged.
...which results in the following rewritten statement:
```d
s.opSlice(0, 3) = arr[1..4];
```
My guess is that you got into this situation by trying to follow
the example in the spec's section on ["Slice Assignment Operator
Overloading"][2]. Unfortunately, that example is incorrect.
Here is a fixed version of your example on run.dlang.io:
https://run.dlang.io/is/dtfT5y
[1]: https://dlang.org/spec/operatoroverloading.html#slice
[2]:
https://dlang.org/spec/operatoroverloading.html#slice_assignment_operator
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