Re: Ada.Containers.Vectors - querying multiple elements

[Duncan Sands <baldrick@free.fr>]

Hi Georg, thanks for replying.

> >    procedure Query_Elements
> >        (Container   : in Vector;
> >         First_Index : in Index_Type;
> >         Last_Index  : in Extended_Index;
> >         Process   : not null access procedure (Elements : in Element_Array));
> 
> 
> > It would be good to have for both
> > efficiency reasons and simple implementation of certain classes of algorithms.
> > In my case I have a "divide and conquer" recursive algorithm which while it
> > can be written using the current package, would be simpler to write if something
> > like Query_Elements existed.
> 
> Couldn't you use Cursors,
> 
>   here: Cursor := find_or_something(container, ...);
>   there: constant Cursor := find_or_something(container, ...);
> 
>   while here /= there loop
>      ...
>      next(here);
>   end loop;

I'm particularly concerned about:

(1) efficiency - suppose (for example) I want to transmit the contents
of my vector down a socket.  Then I would have to do an element-by-
element copy into a buffer (using code like you suggest), and then send
the buffer.  The element-by-element copy is inefficient, and with a
procedure like Query_Elements above simply wouldn't (necessarily) be
needed.  An element-by-element copy can't compete with memcpy (what
you'd probably get if you copied a slice directly, which would be
possible using Query_Elements), and certainly can't compete with no
copy at all.  By the way, this is not a real example, it's just the
first thing that came to mind.  In any case, it is a special case of
point (2):

(2) reuse of legacy code - I don't know about you, but I've a lot of
legacy code around that takes an array as a parameter.  I mean code
that doesn't change the length of the array (or anything like that) -
code that just operates on the array in-place.  Suppose I am using
a Vector, and want to have some legacy code operate on it, or on
part of it.  Right now I would have to copy the appropriate part of
the Vector into a buffer, and pass that to the legacy code, and then
copy it back.  Not very efficient - and not necessary if the Vectors
api was extended slightly.

Ciao,

D.