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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Ideas for Ada 200X
Date: 06 Jun 2003 16:20:44 -0400
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18k11tm001@sneakemail.com (Russ) writes:

> Stephen Leake <Stephe.Leake@nasa.gov> wrote in message news:<uznkwflx8.fsf@nasa.gov>...
> 
> > procedure Matrix_Add
> >    (Target : in out Matrix_Type; 
> >     Addend : in Matrix_type);
> > procedure "+=" (...) renames Matrix_Add;
> > 
> > 
> > A += B;
> > 
> > the compiler actually does:
> > 
> > C = A;
> > Matrix_Add (C, B);
> > A = C;
> > 
> > I suppose you could change that rule, but that would be a bigger
> > change to the language, since "A += B" would no longer be identical to
> > "A := A + B".
> 
> You seem to be going around in circles here. The whole idea was that A
> += B can avoid the temporary matrix and the extra copy operation.
> That's how it works in C++. Without that benefit, yes you might as
> well just use A := A + B.

This is all speculation, since Ada does not *have* a +:= operator.
It seems to me that if it did, the semantics should be identical to
any other procedure call with two parameters (one 'in out', and one
'in').  It should *not* be similar to a *function* call, and should not
return a result.  If all that were true, then of course +:= would be
exactly as efficient as Matrix_Add.

Somebody has apparently suggested a different semantics for +:=,
which might have different efficiency properties.

- Bob