Re: Default value for a record component

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

Maciej Sobczak <see.my.homepage@gmail.com> writes:

> On 22 Lip, 10:08, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
>
>> > I would like extend it and do this:
>>
>> > package P is
>> >    type T is record
>> >       V : Integer := Get_Default_Value;
>> >    end record;
>> > private
>> >    function Get_Default_Value return Integer;
>> > end P;
>>
>> > without losing any of the properties of the original solution and
>> > without "leaking" any implementation detail to the public view.
>
>> Yes:
>>
>> package P is
>>    function Get_Default_Value return Integer;
>>    type T is record
>>       V : Integer := Get_Default_Value;
>>    end record;
>> end P;
>
> No, this leaks the implementation detail (the Get_Default function) to
> the public view and pollutes it. I don't want my users to even see it.

I don't see why that's important.  You "leak" the name
Get_Default_Value, but why is that a problem?

You're already leaking the component V.
The fact that clients can say X.V := ...
seems much worse than the fact that they can say
Mumble : Integer := Get_Default_Value.

Why do you want to hide the default value, but expose the component
V itself?

If you want to hide both, then make the type private, and then you can
hide Get_Default_Value also.  Usually a good idea.

> Ada needs real constructors, not just initial values.

I don't get it.  Please explain.

- Bob