Re: Parameter Modes, In In Out and Out

[Robert Dewar <robert_dewar@my-deja.com>]

In article <3A57CD7F.2228BFD5@brighton.ac.uk>,
  John English <je@brighton.ac.uk> wrote:

> I prefer to consider parameter passing by copy-and-return
> rather than by-reference as the norm (even though it applies
> to fewer situations in practice)

But unfortunately, this model is just wrong for by-reference
types. Especially in object oriented programming, where all
tagged types are passed by reference, it is quite wrong to
regard the input as uninitialized, since this does not
correspond with the semantics of the language.

For example, one can imagine a situation in which some field
of an object serves merely as a kind of logical "tag", and it
makes sense to read it, just as it makes sense to read the real
tag, and bounds information. You might decide that logically it
is reasonable to consider a parameter to be be an OUT parameter
if the routine only reads this logical tag field, but
initializes all the other fields.

The treatment in Ada 95 is different from Ada 83 in this area,
you need to readjust thinking a bit :-)


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