Const ref and rvalues again...

[martin]

On Wednesday, 7 November 2012 at 03:35:19 UTC, Era Scarecrow 
wrote:
> Still that zlib entry is coming to mind. But more importantly
> is that you still need code duplication to get both accessible.
>
> //many possible combinations thereof
> int func(const ref x);
>
> int func(int x) {
>   return func(cast(const int) x);
> }

int func(in ref int x);
int func(int x) { return func(x); }

The latter overload is for rvalues (no need to cast to const) and 
wouldn't be required if the first one took rvalues directly 
(having a _const_ ref parameter). That's not an example for 
mutable references though, it's basically a shortcut so as to not 
having to declare all rvalues for x manually.

> enum ZlibEnum {}
> ZlibEnum compress(void[] output, void[] input,
>   &ref Zlib state) nothrow pure;
>
> string input = "something long";
> ubyte[1000] output;
> ubyte[5] output2;
> Zlib state;
>
> compress(output, input, null);
> compress(output2, input, state);

Where's the rvalue? You're talking about an optional mutable 
reference here, which is only doable via a pointer (references 
cannot be null, not in C++ and not in D):

ZlibEnum compress(void[] output, void[] input, Zlib* state = 
null);
...
compress(output, input);
compress(output2, input, &state);

That would be correct. But you're completely missing the point 
here. Let's assume you wanted to require a state to be passed:

ZlibEnum compress(void[] output, void[] input, ref Zlib state);

You could now use:

Zlib state; // lvalue
compress(output, input, state);

but not:

compress(output, input, Zlib()); // rvalue

And that is most likely a good thing, since your state would be 
lost after the compress() call. If you don't want it, you don't 
pass it, and as I said, the only way to do that is to pass a 
nullable pointer. So this is off-topic I'm afraid.