Re: Parameter Modes, In In Out and Out

[John English <je@brighton.ac.uk>]

Robert Dewar wrote:
> 
> In article <7Cx56.90736$A06.3322588@news1.frmt1.sfba.home.com>,
>   tmoran@acm.org wrote:
> > > An out parameter is still taken in.
> >     No, actually, it is not.
> 
> This is false information. In many instances, outlined clearly
> in the RM, the questioner is right, an IN OUT parameter and
> an OUT parameter are identical.

Ouch. The implication of this statement, read loosely, is that "out"
parameters are redundant and can be replaced by "in out" parameters.
It may be strictly false information, but I think it's more confusing
to assert that "out" means the same as "in out" (since many readers will
forget to factor in the leading "in many instances" and will get lost in
a maze of twisty little grammar rules, all alike).

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): "out" parameters are initially uninitialised (if that
makes sense to you ;-) and the final value is copied out on return
from the procedure, whereas "in out" parameters are copied in on entry
and so are guaranteed to have valid initial values.

Side effects due to aliasing complicate this (you then have to know
which
types are passed by reference and which aren't) but I think that
assuming
by-reference parameter passing as the default makes understanding what's
going on (as in this question of "out" vs. "in out") more complicated,
not
less; in the absence of side effects, copy-in-and-out is equivalent to
by-reference and distingusiahes clearly between the various cases. And
anyone who uses aliasing for side effects deserves to sow what they
reap,
IMHO... ;-)

YMMV, especially if your name is "Robert"; language lawyers and language
users are different audiences for such debates IMHO ;-)

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