Overriding iteration

[Jonathan M Davis]

On Friday, March 04, 2011 09:13:34 spir wrote:
> On 03/04/2011 05:43 PM, Steven Schveighoffer wrote:
> > On Fri, 04 Mar 2011 11:29:08 -0500, Magnus Lie Hetland <magnus at hetland.org> 
wrote:
> >> From what I understand, when you override iteration, you can either
> >> implement the basic range primitives, permitting foreach to
> >> destructively iterate over your object, or you can implement a custom
> >> method that's called, and that must perform the iteration. The
> >> destructiveness of the first option can, of course, be mitigated if you
> >> use a struct rather than a class, and make sure that anything that
> >> would be destroyed by popFront() is copied.
> >> 
> >> What I'm wondering is whether there is a way to do what Python does --
> >> to construct/return an iterator (or, in this case, a range) that is
> >> used during the iteration, rather than the object itself?
> > 
> > That's exactly how to do it.
> > 
> >> I'm thinking about when you iterate directly over the object here. As
> >> far as I can see, the solution used in the std.container is to use
> >> opSlice() for this functionality. In other words, in order to iterate
> >> over container foo, you need to use foreach(e; foo[])? Is there no way
> >> to get this functionality directly (i.e., for foreach(e; foo))?
> > 
> > I believe someone has filed a bug for this, because TDPL has said this
> > should be possible.
> > 
> > But with the current compiler, you can use opApply to achieve that
> > behavior.
> 
> opApply should work but it is supposed to be slower.
> Defining range primitives directly on the object/container cannot work as
> of now, unfortunately, because of a pair of bugs (conflicts in formatValue
> template definitions between struct/class on one hand and ranges on the
> other).

You don't _want_ range primitives directly on the container. That would mean 
that everything in your container goes away when you process it. Every 
popFront() call would be removing an element from your container. So, for 
insteance, you try and call find() on your container and everything before what 
you were looking isn't in the container anymore - and if it isn't there at all, 
you have an empty container. You _want_ to have a separate type which is a slice 
of our container and has the range primitives.


Now, it could very well be that

foreach(v; container)

should be calling opSlice on the container, allowing you to feed the container 
to foreach directly instead of having to slice it yourself

foreach(v; container[])

but that's just syntactic sugar. You don't want to actually treat your container 
like a range. Ranges should be slices of containers, not containers themselves.

- Jonathan M Davis