std.collection - changing the collection while iterating

[Steven Schveighoffer via Digitalmars-d]

On 6/21/15 7:02 PM, Andrei Alexandrescu wrote:
> While I work on making std.allocator better, here's some food for
> thought regarding std.collection.
>
> Consider a traditional container with reference semantics, Java-style.
> Regarding changing the collection (e.g. adding/removing elements) while
> iterating, we have the following possibilities:
>
> 1. Leave it undefined, like the STL does. Probably this is too extreme.

I don't think it's undefined, it depends on the container:

http://www.cplusplus.com/reference/set/set/erase/

"Iterators, pointers and references referring to elements removed by the 
function are invalidated. All other iterators, pointers and references 
keep their validity."

I do the same for dcollections.

Note that I support a mechanism whereby the user can choose to remove 
*while* iterating (I call it purging) via the container itself.

One of the main reasons I wrote dcollections in the first place is 
because I had a requirement to remove while iterating. The use case was 
a map of processes I was keeping track of, looking for ones that were 
done so they could be removed. In effect, I would do a foreach on the 
container, and add keys of the elements I'd like to remove to an array. 
Then afterward, I would use the key array to remove all those elements 
(these were Tango containers). I didn't like that second step, nor the 
inefficiency of building another array that I was going to discard.

With dcollections, this becomes as simple as:

foreach(key, value, ref dopurge; container)
{
    dopurge = checkIfDone(key, value);
}

One nice thing, you get effectively O(1) removal from non-node 
containers like arrays, and you can remove while iterating containers 
that may normally be impossible to remove from while iterating.

-Steve