const ref in opAssign

[Era Scarecrow]

On Tuesday, 26 June 2012 at 12:44:46 UTC, monarch_dodra wrote:
> I just finished reading the chapters on user defined types and, 
> (as a C++ dev), this one line struc out to me as very odd:
>
> "ref Widget opAssign(ref Widget rhs) {..}"
>
> This means that during an assignment, I could potentially 
> change "Other" (!). This seems like a blatant violation of the 
> expected behavior of =. What's more, it prevents the assignment 
> from a const object...
>
> I am very tempted to change the call to "const ref". Is this 
> un-advised?
>
> Mr. Alexandrescu goes on to mention a "pass by value" in case 
> you wanted to assign from a temporary, mentioning:
> w = Widget(50); // Error!
> // Cannot bind an rvalue of type Widget to ref Widget!
>
> The only problem is that if I do this, all of my calls are then 
> re-routed to pass by value, and none to the pass-by-const-ref.
>
> ----
>
> Is there any way to enforce const correctness, while still 
> keeping advantage of both calls?
>
> PS: using Mr. Alexandrescu's design, it is not possible to 
> assign from a const Widget, I get:
> hello.d(50): Error: function hello.Widget.opAssign (Widget rhs) 
> is not callable using argument types (const(Widget))
> hello.d(50): Error: cannot implicitly convert expression (w2) 
> of type const(Widget) to Widget
>
> const can't be passed by value...?

  I actually remember having this issue (I'll look up the 
reference to it later). The issue was the non-const non-ref 
version was a better fit; Now obviously it's wrong, but you can't 
convince the compiler of that. If the assign never modifies the 
incoming object, then why should you make two versions?

  Anyways. Try making 2 versions of opAssign with ref and try it 
again: ie a const-ref & non const-ref, and finally your pass by 
value.