refRange and @disable this(this);

[Jonathan M Davis via Digitalmars-d]

On Wednesday, September 14, 2016 12:39:16 Jerry via Digitalmars-d wrote:
> I got a range which disables copy construction and I want to loop
> the range within another loop using the same range.
> So I thought I can mark the struct range with @disable this(this)
> and then use refRange to initialize the loop.
>
> So with something like this:
>
> void main()
> {
>   auto valueRange = FooRange("123");
>   foreach(ch; refRange(&valueRange))
>       writeln(ch);
> }
>
>
> struct FooRange {
>   @disable this();
>   @disable this(this);
>   this(string str) {
>       this.str = str;
>   }
>
>   @property bool empty() { return str.empty; }
>   @property dchar front() { return str.front; }
>   void popFront() { str.popFront; }
>
> private:
>   string str;
> }
>
>
> But I get compile time errors messages saying:
> std/range/package.d(8155,23): Error: struct app.FooRange is not
> copyable because it is annotated with @disable
>
> It feels strange that refRange ever want to copy.
> Bug or feature?

It's opAssign does a copy. The rationale is given in its documentation:

http://dlang.org/phobos/std_range.html#.RefRange.opAssign

The result of that is that the range you give it needs to be copyable. But
the reality of the matter is that ranges in general expect to be copyable,
and range-based code is going to have a tendency to fall flat on its face if
you try to define a range that's non-copyable. I'm actually kind of
surprised that such a range passes isInputRange given that it tests this
line:

    R r = R.init;

But I guess that it works, because you didn't explicitly disable opAssign
(though at least sometimes, disabling the postblit constructor seems to
disable opAssign from what I recall).

If want to guarantee that a range's state is never copied, and you're
creating a range specifically for this, you might as well just make it a
class and force it to have reference semantics. RefRange was intended for
cases where you have a range that works normally, but you need to pass it to
some code and have the the state of the range be updated rather than having
a copy updated. It wasn't intended that it be used for a range that you
wanted to always be a reference type, since if you want that, you can just
declare a range that's a reference type.

- Jonathan M Davis