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From: "Alex R. Mosteo" <devnull@mailinator.com>
Newsgroups: comp.lang.ada
Subject: Re: Amount of copying on returned constant objects
Date: Mon, 18 Jun 2007 20:23:49 +0300
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Date: 2007-06-18T20:23:49+03:00
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Georg Bauhaus wrote:

> Alex R. Mosteo wrote:
>> 
>> 
>> I wonder however about results of functions, that are not modified.
>> 
>> ... 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)?
> 
> OTOH, I might want a constant copy because the object in the
> container is going to be modified... Can a compiler detect this?

I don't think it can, at least easily. I guess that if you're keeping a
copy, constant or not, the optimization opportunity is lost. But what about
short-lived objects, like...

if Container.Element ("key").Is_Nice then -- Container for some tagged type
  ...
end if;

This is the kind of copies that I see interesting to optimize away. I don't
know enough about compilers to say if it is reasonable to expect one to
detect this situation or not.

If not, one possibility would be to have

function Element (Key : Key_Type) return Element_Type;

and

type Constant_Access is access constant Element_Type;
function Element (Key : Key_Type) return Constant_Access;

but I'm not sure about the amount of ambiguities one would get in that case.
In any case this does not exist in the standard 05 containers.