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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: About conversions
Date: 2000/11/20
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In article <RaaS5.5488$6W1.468063@news.flash.net>,
  "Ken Garlington" <Ken.Garlington@computer.org> wrote:


<<much general and probably irrelevant material on type
conversions removed>>

> In your example, you have a
> procedure P that expects values of type Integer, you convert
> your Typ value
> to an Integer value before calling P, so everything is
> consistent.

No, that's quite wrong. Ken misread the example.
This is indeed the "weird" case where
a type conversion is used on an OUT parameter. Even someone who
understands all the stuff about type conversions in general
can be puzzled at this peculiar use.

The deal here is that the conversion happens AFTER the call
in the OPPOSITE direction from what it looks like. So if the
call says

  p (10,20,integer(c));

the actual effect is that the function computes something of
type Integer and puts it in the output parameter. This value is
then converted FROM integer TO type "typ" and stored in C.

How's that for strange? The conversion is exactly "backwards".
In the case of an IN OUT parameter, you get two conversions, one
the way that it looks (and to which Ken was referring
incorrectly above, not having noticed the OUT keyword), and one
the peculiar opposite direction.

Why is this feature in the language?

It's really there to provide an easy explanation for what
happens in the case of derived types. I think it's actually
misguided, because although it makes the explanation of derived
types easier, it introduces a bogus misleading feature into the
language that is of very little practical use and which causes a
lot of confusion, even among people who know Ada well!

Robert Dewar


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