Passing a reference to a returned reference

[Timoses]

On Friday, 6 July 2018 at 13:13:43 UTC, Michael wrote:
> static auto ref consensus( ... )

`auto ref` infers the return type from the return statement [1]. 
So it's not necessarily returning a ref type.

However, I don't think this matters if the only concern you have 
is that the setter function actually modifies the passed object.

>
> and the agent's setter method looks like the following:
>
> void beliefs(ref double[int] beliefs)
>
> Now obviously if I remove ref in the above signature, 
> everything works fine, but I am surprised I wasn't getting any 
> kind of warning about the reference because the setter function 
> just doesn't run at all. I tried checking if (beliefs is null) 
> but perhaps this isn't the correct way to check if an 
> associative array's reference is no longer reachable?
>
> Some explanation would be great, thanks!

This works for me:

     auto create()
     {
         string[int] dict;
         dict[2] = "hello";
         return dict;
     }

     void modifyNoRef(string[int] m)
     {
          writeln("Address not ref: ", &m);
          m[0] = "modified";
     }

     void modifyRef(ref string[int] m)
     {
          writeln("Address ref: ", &m);
          m[1] = "modified";
     }

     unittest
     {
         auto r = create();
         writeln("Address r: ", &r);
         assert(r.keys.length == 1);
         modifyNoRef(r);
         assert(r.keys.length == 2);	
         modifyRef(r);
         assert(r.keys.length == 3);	
     }

So either with ref or not as parameter storage class the assoc. 
array is modified. Note the address in the "ref" one is the same 
as "r" in the unittest.

[1]: https://dlang.org/spec/function.html#auto-ref-functions