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From: "Thierry Lelegard" <thierry.lelegard@canal-plus.fr>
Newsgroups: comp.lang.ada
Subject: Re: Parameter Modes, In In Out and Out
Date: Tue, 9 Jan 2001 16:15:54 +0100
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> procedure P (X : out Some_Type);
> 
> By looking at the specification (NOT the body), I can tell that the
> procedure cannot rely on the previous value of the parameter,
> and therefore that it is safe to call it to initialize an
> unitialized variable, whose value can be invalid. I could not assume this
> with an in out parameter.

This is quite true, especially with floating point types for which
some binary patterns are NaN (Not a Number), ie. invalid values.
On some architectures (I remember the old time of the VAX),
passing an uninitialized variable as "in out" may raise an
exception _even_if_the_procedure_never_reads_the_input_value_.
The by-copy semantic generates a MOVF ("move F_float") instruction
for "in" and this instruction generates a trap if the binary pattern
is an invalid floating point value.

So, Jean-Pierre Rosen is right, never use "in out" for "out" when you
cannot be sure that the actual parameter is properly initialized.

____________________________________________________________________________

Thierry Lelegard, "The Jazzing Troll", Email: thierry.lelegard@canal-plus.fr
CANAL+ Technologies, 34 place Raoul Dautry, 75516 Paris Cedex 15, France
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