What is the purpose of Vector.Query_Element?

What is the purpose of Vector.Query_Element?

[EGarrulo]

The package that defines Vector provides this procedure:

procedure Query_Element
  (Container : in Vector;
   Index : in Index_Type;
   Process : not null access procedure (Element : in Element_Type));

Why is this procedure provided?  Isn't `Query_Element (Vector, Index, Process'Access)` the same as `Process (Element (Vector, Index))`?

Thank you.

Re: What is the purpose of Vector.Query_Element?

[Georg Bauhaus]

On 27.07.15 00:09, EGarrulo wrote:
> The package that defines Vector provides this procedure:
>
> procedure Query_Element
>    (Container : in Vector;
>     Index : in Index_Type;
>     Process : not null access procedure (Element : in Element_Type));
>
> Why is this procedure provided?

To avoid copying.
"These procedures provide in situ access to an element."
  -- Ada Rationale 2005 Ch 8.2.
http://www.adaic.org/resources/add_content/standards/05rat/html/Rat-8-2.html

>  Isn't `Query_Element (Vector, Index, Process'Access)` the same as `Process (Element (Vector, Index))`?

Maybe the difference is a little more apparent when
considering Update_Element.

Re: What is the purpose of Vector.Query_Element?

[Randy Brukardt]

"EGarrulo" <egarrulo@gmail.com> wrote in message 
news:ca22434f-8216-4cbd-a4c0-46fea8e6cc98@googlegroups.com...
> The package that defines Vector provides this procedure:
>
> procedure Query_Element
>  (Container : in Vector;
>   Index : in Index_Type;
>   Process : not null access procedure (Element : in Element_Type));
>
> Why is this procedure provided?  Isn't `Query_Element (Vector, Index, 
> Process'Access)` the same as `Process (Element (Vector, Index))`?

Sure, if you don't care about performance they're the same. But a function 
result is a copy of the element ("Element" is a function), whereas 
Query_Element can pass the element by reference (and will for most types 
that aren't by-copy); in that case it is directly referenced in place. For 
large elements (say, like other containers), the overhead of copying could 
be considerable. OTOH, if the element is a by-copy type, there is no 
difference. So Query_Element is a specialized need. Besides, for Ada 2012, 
you probably ought to be using the indexing syntax and not use either of 
these.

                                Randy.