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From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Where are returned values stored? (follow up to yesterday's
 question)
Date: Thu, 27 May 2004 09:51:52 +0200
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Date: 2004-05-27T09:51:52+02:00
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On 26 May 2004 21:44:04 +0100, Simon Wright <simon@pushface.org>
wrote:

>James Alan Farrell <anonymous@anonymous.com> writes:
>
>> Now its my understanding that when variables are declared like this,
>> they are added onto the stack, then when the function returns, they
>> are popped off again, along with the regular stack frame.  So the
>> stuff returned goes away.
>> 
>> But, the calling function will need use of that data.  So it cannot go
>> away.  
>> 
>> So how does that work?  Where is the data placed so that it does not
>> go away, and how long will it be there so it (or rather pointers to
>> it) can be passed around to other functions?
>
>The simple answer is that it returns a copy of the data, just like
>simple variables ..
>
>   int foo()
>   {
>     // stuff
>     {
>       int res;
>       // stuff
>       return res;
>     }
>     return 3;
>   }

There is an exception for limited types. They can be returned as well!
So the following is legal:

   type X is limited record ...
   A : X;
   
   function Foo return X is
   begin
      return A;
   end Foo;

Here no copy happens upon return.

>There may be a more complicated, implementation-dependent answer
>involving secondary stacks, but I see no reason in the language why
>you need the concept of pointer in this context. And, however it
>works, it will be stack-based, so the data will go away at some point
>as the stack is popped.

--
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de