Re: Amount of copying on returned constant objects

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 15 Jun 2007 19:19:00 +0300, Alex R. Mosteo wrote:

> I know that some "in" arguments may be passed as copies or as references, at
> compiler discretion (this is one of these things that "programmers
> shouldn't care about", many times quoted).
> 
> I wonder however about results of functions, that are not modified. Look for
> example at the Element function of the new Ada.Containers. They return the
> stored item, that may well be a quite large controlled tagged type, for
> example.
> 
> Now, many times I want to query an element just for read-only purposes. I'm
> faced with two options:
> 
> 1) Just call Element on the container Key/Index, and be done with it.
> 
> 2) Do a Find+Query_Element, which requires defining an extra procedure and
> somewhat breaks the flow of control, but ensures no copying.
> 
> I tend to go with 1) because of laziness and the "no premature optimization"
> rule. In C++ I could use constant references. Now, I wonder if 
> 
> a) is there something in the ARM that prevents an equivalent transparent
> optimization in the Ada side (returning the reference when it is detected
> that the returned object is not modified)?

No [*], but a way to help the compiler to know what you wanted is:

   declare
      Item : Element renames Get (Collection, Key);
                 -- Please, don't make new objects, if you can
   begin
      ... -- Use Item

> Thanks in advance, have a nice week-end.

Same to you.

---------------
* In Ada 95 there were limited types returned strictly by reference, but
this semantics was crapped in Ada 2005.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de