Re: In-Out Parameters for functions

["Alexandre E. Kopilovitch" <aek@VB1162.spb.edu>]

Dmitry A. Kazakov wrote:

> >Therefore, IN OUT parameters in functions will not be worse than the ability
> >of functions to change global variables if we forbid use of such functions in
> >expressions, that is:
> >
> >  If at least one parameter of a function is in IN OUT mode then the function
> >  cannot be used in a compound expression. This function's call must always
> >  constitute the whole RHS of an assignment.
> >
> >or less restrictive rule:
> >
> >  If at least one parameter of a function is in IN OUT mode then in any
> >  expression in which this function is called all other calls may be for
> >  parameterless functions only.
>
> What about:
>
> F(X) and then F(X)

Naturally, forbidden. Perhaps the wording of my (above) "less restrictive rule"
was not good enough, but I meant that "all other calls" includes other calls
of the mentioned function (which has an IN OUT parameter), and that the meaning
is entirely static, not dynamic. So, probably at least one correction should
be made in this rule - replacing "calls" by "references":

  If at least one parameter of a function is in IN OUT mode then in any
  expression in which this function is called all other references may be
  for parameterless functions only.

Well, I leave the exact wording of the rule to native English speakers -;) .

> The rule, if any, should prevent using an object in IN OUT mode in an
> expression where the evaluation order is not specfied by the language.

Yes, certainly. And I'd forbid that even if that order IS specified but may be
non-obvious... actually I'd forbid that in all cases where that order can
matter at all - for anything.

> Looks much like the aliasing problem:
>
> procedure Foo (X, Y : in out Object) is
> begin
>   X := F(X) + F(Y); -- Is this legal?
>   Foo (X, X); -- And this?
> end Foo;

Perhaps legal, but awfully wrong -;)




Alexander Kopilovitch                      aek@vib.usr.pu.ru
Saint-Petersburg
Russia