Re: 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