On Wednesday, July 04, 2018 14:07:35 Timoses via Digitalmars-d-learn wrote:
> How can I return inferred storage class from interface functions?
> I can't use auto as return value in interface. Neither can I use
> "inout" as I don't pass a parameter.
>
> // Ref Type
> interface IRef
> {
> Ref opIndex(size_t idx) const;
> }
> class CRef : IRef
> {
> Ref[] a;
> this() immutable
> { this.a = [new Ref()]; }
> Ref opIndex(size_t idx) const
> { return a[idx]; } // Error: cannot implicitly convert
> expression this.a[idx] of type const(Ref) to app.Ref
> }
> class Ref{}
>
> void main()
> {
> auto a = new immutable(CRef)();
> auto s = a[3];
> }
>
> For value types it works, I presume since they are passed by
> value, so the instance returned is an actual copy of the stored
> value.
>
> // Value Type
> interface IValue
> {
> Value opIndex(size_t idx) const;
> }
> class CValue : IValue
> {
> this() immutable { i = [Value()]; }
> Value[] i;
> Value opIndex(size_t idx) const
> { return i[idx]; }
> }
> struct Value{}
>
> However, for ref types this doesn't work.
>
> Do I have to define two `opIndex` in the interface? One mutable,
> one for immutable type instances?
You can make opIndex inout instead of const. That way, inout applies to the
invisible this reference, just like it would with const.
- Jonathan M Davis
