From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,984d3d7860d7c8c X-Google-Attributes: gid103376,public Path: controlnews3.google.com!news1.google.com!news.glorb.com!cyclone.bc.net!news-in.mts.net!nf1.bellglobal.com!nf2.bellglobal.com!news20.bellglobal.com.POSTED!not-for-mail From: "Warren W. Gay VE3WWG" User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.4) Gecko/20030624 Netscape/7.1 (ax) X-Accept-Language: en-us, en MIME-Version: 1.0 Newsgroups: comp.lang.ada Subject: Re: Where are returned values stored? References: <75s9b0pgo91ctvlm5op2rcql82t9ip4me2@4ax.com> In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Message-ID: <1dptc.49822$tb4.1731604@news20.bellglobal.com> Date: Thu, 27 May 2004 13:05:06 -0400 NNTP-Posting-Host: 198.96.223.163 X-Complaints-To: abuse@sympatico.ca X-Trace: news20.bellglobal.com 1085677501 198.96.223.163 (Thu, 27 May 2004 13:05:01 EDT) NNTP-Posting-Date: Thu, 27 May 2004 13:05:01 EDT Organization: Bell Sympatico Xref: controlnews3.google.com comp.lang.ada:883 Date: 2004-05-27T13:05:06-04:00 List-Id: Marius Amado Alves wrote: >> It has a declarative part in the middle of the code, in which a >>variable (Result) is declared. The stuff in that variable is >>returned. > > This is a common idiom. In many languages. Don't think in machine terms, > think in language terms. *Values* are being passed around, not objects. You > are already thinking value (stuff in that variable), so that's good. When > you call the function, you provide a place, usually a variable, to hold the > value. > > Object := Function_Call; > > So no worries. The value might have travelled to Japan and back, but that's > the machine business, not the programmer's. Now it's held in your Object, > and that's all you need. This from a function user standpoint. From a > function writer standpoint, well, think value again. When you write > > return Result; > > you're passing the value. Under the language contract, the user we'll catch > it. (In Ada he has to catch it, in C he can drop it on the spot.) If you're worried about the inefficiency of copying data around, then I would suggest that the above advice is good. Leave that up to the compiler/machine team. If you really must know more than that, then you'll need to look at the assembler output, but maybe the following explanation might save you from bothering/worrying about it: Conceptually, there is _no_ reason a compiler _must_ copy a return value. Consider a function like: function Foo return String is I : Integer := 23; S : String(1..3) := "Bar"; X : Natural := 0; begin ... return S; end; To return S, all the compiler needs to do is to extend the caller's stack frame to claim all the storage up to and including S (I and S assuming sequential assignment), at the point when Foo returns (return effectively does reclaim the storage used for X however). Then the calling code can reference S as if it were a local variable. The space used for I is wasted at this point, but temporarily, who cares? Once the compiler knows that the caller's use of the returned value S is no longer required, the stack frame can be shrunk back to the original size that it had prior to calling Foo. Is this done? Is it done by GNAT? I have no idea, but I suspect that this is done in some places, some of the time, by some compilers. Perhaps, someone in this group can confirm/debunk this idea. -- Warren W. Gay VE3WWG http://home.cogeco.ca/~ve3wwg