Re: tagged record child: override constructor?

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Jeffrey Carter <spam@spam.com> writes:

> sean.gilbertson@gmail.com wrote:
> > I have a tagged record that is declared private, along with a
> > constructor function which returns an instance.  I do this to enforce
> > the assignment of several required fields in the record, so if you know
> > how to do this another way, please let me know!
> 
> It depends on what you mean by "enforce the assignment of several
> required fields". It is impossible to force the client to call your
> function, though you can arrange things so that the type is useless to
> the client if the client doesn't first call your function.
[... lots of good possibilities snipped...]

There's another way to "enforce...":

    package P is
        type T(<>) is private;
        function Make_T(...) return T;
    private
        type T is record...
    end P;

    use P;

Now clients can't create uninitialized objects of type T:

    X: T; -- illegal
    type T_Ptr is access T;
    Y: T_Ptr := new T; -- illegal

    type R is
        record
            Component: T; -- illegal
        end record;

    type A is array(...) of T; -- illegal

The client is forced to call Make_T:

    X: T := Make_T(...); -- OK
    Y: T_Ptr := new T'(Make_T(...)); -- OK

etc.

- Bob