Re: GCC 4.0 Ada.Containers Cursor danger.

["MMM" <terekhov@emc.com>]

Matthew Heaney wrote:

>
> There is no such thing as an "unordered" container (and here we're
> really talking about sets and maps), since then there would be no way to
> find anything without a linear search.  That's why we have both ordered
> and hashed containers.

You contradict yourself here. So, do an "unordered" containers exist or
do they not?

>
> The "position" of an element always relative to other elements in the

That depends on the semantic. For example the Hashed_Sets in your
terminology requires no "relative position" semantic. The fact that an
implementation will use some "relative position" semantic internaly is
an implementation detail. This internal relative position can be
different for the same set in different hash implementations.

> container.  For an "ordered" container (Ordered_Sets, Ordered_Maps), the
> position is determined by the "<" operator for elements.  For a "hashed"

This is a confusion. The "<" operator defines relative order, not
position. For an ordered container position is defined by both the "<"
operator and the container contents. Think of two vectors A(a,b,c,d)
and B(a,c,d) as an simple example. Suppose they have both the same "<"
operator. The position of the 'c' element in A is 3 and in B is 2.

> container (Hashed_Sets, Hashed_Maps), the position is determined from
> the hash value of the element (since that determines which bucket the
> element falls into).
>
> An iterator (either passive or active) simply enumerates elements in
> their order as specified by the type of container they're in (either

Exactly! Note also that the natural name for this is "iterator" not
"cursor". The main and natural purpose of this thing is to iterate over
the contained elements, not to provide a pointers to arbitrary
elements.  The iterator concept is the general way to go over all
elements or some subset of then of an any container.

> sort order or bucket order).

Not quite correct. Sort order has nothing to do with a vector or list
ordering in general. One can have a vector of integers filled in
randomly, for example. In case of Hashed_Sets bucket order is an
implementation detail which can differ for different implementations.
The semantic of Hashed_Sets simpli doesn't require any particular
order.

>
> Have you ever done any COM or DirectShow programming under Windows?  A
> cursor is the same as the EnumXXX class.  (If I remember correctly, I
> think Java also has something it refers to as an "enumerator".)


MS is not always the best example to follow.

Regards,
Mikhail