cast(immutable) vs extra variable

[Namespace]

On Thursday, 19 September 2013 at 16:47:13 UTC, Daniel Davidson 
wrote:
> Multi-part question:
>
> 1) Why does the last line fail? If cast to immutable how is it 
> different than z? I know it is related to the ref. I'm using 
> ref because I think it is likely more efficient - so assume the 
> char[16] were really char[1024].
>
> 2) If I got rid of the ref, how many copies of the data would 
> be made? And without looking at assembly what is a good way to 
> answer this question? I've tried to add this(this) to other 
> Stuff type structures to print when they are being called, but 
> that does not work because no logic can be in the default 
> constructor.
>
> 3) Also, is storing immutable(STUFF) in a struct in the general 
> case (as opposed to just this one) useful or silly?
>
> Thanks
> Dan
>
>     import std.stdio;
>
>     alias char[16] Stuff;
>
>     struct T
>     {
>       immutable(Stuff) myData;
>       this(ref immutable(Stuff) data) {
>         myData = data;
>       }
>     }
>
>     void main() {
>       immutable(Stuff) iData = "1234567890123456";
>       Stuff data = "1234567890123456";
>       writeln(T(iData));
>       auto iData2 = cast(immutable)data;
>       writeln(T(iData2));
>       // WHY DOES THIS FAIL WHEN T(iData2) works?
>       // writeln(T(cast(immutable)data));
>     }

cast(immutable)data) is not an lvalue, it's a rvalue. ref accepts 
only lvalues.