Array operations and 'Copy on Whatever'

[Serg Kovrov]

In my case it's more like 'copy on delete' then COW, but anyway... How 
array concatenation, assignment, etc. are handled? Is it implemented on 
'library level', or on 'language level'? I mean some operations silently 
copies array contents, but some are not (eg. referencing). And I do not 
see a pattern.

In particular, did array concatenation (~, ~=) make a copy of right operand?

Consider this example:
> char[][] array_of_strings;
> 
> for (int i = 0; i < 10; i++)
> {
> 	char[] tmp = format("this is %d", i);
> 	array_of_strings ~= tmp;
> 	delete tmp; // <-- here
> }
> 
> foreach (char[] str; array_of_strings)
> 	writefln("%s", str);

When I delete tmp, strings in array_of_strings being trashed.

But on the other hand, in more simple example:
> char[] s1 = "one";
> char[] s2 = "two";
> s1 ~= s2;
> delete s2;

Deleting s2 will not affect s1.

The first one is much simplified example reproducing a problem I have 
encountered some time ago. As I do not understand how array operations 
works and cant rely them, I have to .dup near every string involved. 
Which seems no very smart.

So, Copy on Write, what does it mean in context of D/Phobos implementation?

Thanks
--
serg.