Re: GCC 4.0 Ada.Containers Cursor danger.

[Mikhail Terekhov <terekhov@emc.com>]

Georg Bauhaus wrote:
> 
> Here is a silly example demonstrating a subprogram that is
> independent of any specific container. That is, you can choose
> any instance in place of My_Container.
> As arguments to the following function, use cursors
> returned by Floor and Ceiling (to mark a range of
> associations in a map.) Use First and Last of a set to express
> forall. Do the same using a vector.
> 
> 
> with My_Container;
>   -- some instance of some Ada.Container ("abstract" if you wish)
> 
> function gen_find
>  (first,
>   last: My_Container.Cursor;
>   item: My_Container.Cursor) return My_Container.Cursor
> 
> --  The first cursor of the sequence from `first` to `last` that designates
> -- an elemenent equal to the element designated by `item`.
> -- `No_Element` if not found.
> is
>   use My_Container;
> 
>   p: Cursor := first;
>   off: constant Cursor := Next(last);
> begin
>   loop
>      exit when p = off or else Element(p) = Element(item);
>      next(p);
>   end loop;
> 
>   if p = off then
>      return No_Element;
>   else
>      return p;
>   end if;
> 
> end gen_find;

The problem with your example is that it is based on the wrong
assumptions:
	- First is that all containers have an order.
	  Again, sets and hashes in general are *not ordered*.
	  To impose an order on them standard needs to put a
	  restriction on implementarion algorithms. You example
	  is completely legal generic function in the current
	  Ada.Containers framework. But if you decide at some
	  point to add unordered containers, then you generic
	  function became invalid because for unordered containers
	  "first" and "last" have no sence at all.
	- Second is that find/search operation makes sence only
	  for a mapping container i.e. vector or hash; in this
	  case it returns a key corresponding to the element
	  they search for (index for vectors is just a special
	  kind of key) if element has been found and some kind
	  of No_Element key or exception if there is no such
	  element. For a not mapping container i.e. set, list
	  or tree the find/search degenerates into membership test
	  because there is no sence to search for the element
	  if you already have it.
True generic operation for all container types can be membership
test not find/search.

Another problem with your example is that it is essentially a NOP.
If you already have a cursor for your element why to search for it?

Another problem with your example is that there is no collection
per se in the gen_find function signature. This is actually a
feature of the Ada.Containers itself. Some times cursors replace
collections completely! That kind of indirect interface is very
dangerous in my view.

Now to answer you question, just replace "My_Container" by
"My_Vector_Container", then "item: My_Vector_Container.Cursor" by
"item: My_Vector_Container.Element_Type" and then "Cursor" by "Index"
in that order and you will get a "vectorized" version of you
function ;)

Mikhail