Arrays passed by almost reference?

[Andrei Alexandrescu]

Ali Cehreli wrote:
> Thanks for all the responses.
> 
> And yes, I know that 'ref' is what works for me here. I am trying to figure out whether I should develop a guideline like "always pass arrays with 'ref', or you may face surprises."
> 
> I understand it very well now and was able to figure out a way to cause some bugs. :)
> 
> What can be said about the output of the following program? Will main.a[0] be printed as 1 or 111?
> 
> import std.cstream;
> 
> void modify(int[] a)
> {
>     a[0] = 1;
> 
>     // ... more operations ...
> 
>     a[0] = 111;
> }
> 
> void main()
> {
>     int[] a;
>     a ~= 0;
>     modify(a);
> 
>     dout.writefln(a[0]);
> }
> 
> It depends on the operations in between the two assignments to a[0] in 'modify':
> 
> - if we leave the comment in place, main.a[0] is 111
> 
> - if we replace the comment with this code
> 
>     foreach (i; 0 .. 10) {
>         a ~= 2;
>     }
> 
> then main.a[0] is 1. In a sense, modify.a caused only "some" side effects in main.a. If we shorten the foreach, then main.a[0] is again 111. To me, this is at an unmanagable level. Unless we always pass with 'ref'.
> 
> I don't think that this is easy to explain to a learner; and I think that is a good indicator that there is a problem with these semantics.

The ball is in your court to define better semantics.

Andrei