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From: "Matthew Heaney" <mheaney@on2.com>
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 <mailman.304.1097670377.390.comp.lang.ada@ada-france.org>
Subject: Re: Memory leak - What the ...?
Date: Wed, 13 Oct 2004 10:45:38 -0400
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"Stephen Leake" <stephen_leake@acm.org> wrote in message
news:mailman.304.1097670377.390.comp.lang.ada@ada-france.org...
> Surely any halfway decent compiler would optimize these to be the
> same?
>
> Hmm. Perhaps declaring an object requires calling Initialize, Adjust,
> and Finalize, unless the compiler can _prove_ those subprograms have
> no side-effects, which it probably can't do. Is that your point?

Something like that.  If I have this:

function Op return CT is --some controlled type
   O : CT;
begin
   O.X := ...;
   return O;
end;

then local controlled object O must be Initialize'd when it is declared, and
Finalize'd when the scope of its declaration ends.

If you do it this way:

function Op return CT is
begin
   return CT'(Controlled with X => ...);
end;

then the compiler can probably build the return value in place:

declare
    O : CT := Op;
begin

This is similar to the return-value optimization in C++.

As another example, the AI-302 set functions Union, Intersection, etc, are
implemented this way:

function Union (L, R : Set) return Set is
   Tree : Tree_Type;  --ordered set uses red-black tree (a type which isn't
controlled)
begin
   ... -- build Tree
   return Set'(Controlled with Tree);
end;