On Mon, 2006-12-04 at 23:10 -0800, Erick Tryzelaar wrote:
> So I was looking through your implementation of the felix STL wrapper,
> and I wanted to see if I could implement the new iterator concepts from
> boost:
>
> http://www.boost.org/libs/iterator/doc/new-iter-concepts.html
>
> I ran into a couple issues though. The spec says that readable iterators
> should reference should deref into the element type (eg: "fun deref: it
> -> t") and lvalue iterators should deref into a reference of the element
> type ("fun deref: it -> lvalue[t]").
Obvious crap. Lvalueness of an iterator only affects whether it
can be ++ incremented or not.
An iterator which when dereferenced returns an lvalue is a different
type to an iterator which when dereferenced returns merely an rvalue.
In addition in C++ a const lvalue is distinct from a non-const one.
To make matters worse, am STL map has a value which is a struct
type with one const and one non-const element.
> difference_type. Since we can't yet embed a type in a typeclass, I'm not
> sure how to model this.
Yes, at the moment the best you can do is add it as a parameter
to the typeclass.
> // value access
>
> typeclass Readable_iterator[it,t] {
> virtual fun deref: it -> t;
> }
>
> typeclass Writable_iterator[it,t] {
> virtual proc store: &it -> t;
> }
This is just wrong, the correct signature is
virtual proc store: it * t;
Whether or not a particular iterator supports storing
will determine whether to instantiate it under this
typeclass.
> typeclass Swappable_iterator[it,t] {
> virtual proc iter_swap: &it -> ⁢
> }
Not sure I understand this: iterators are always swappable.
Whether the value they point at are swappable depends on whether
the iterator types are Writable_iterator's (in your classification).
You can always implement swap using a temporary in that case ..
so swap method should probably be added to Writable_iterator,
in case there is a more efficient way to do it.
--
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net
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