Re: Ada OOP question

[Jeff Carter <jrcarter@acm.org>]

mark_lundquist@my-deja.com wrote:
> Baloney!  How is mixing public and private any more EVIL than having
> all components public, which Ada makes no more difficult than any other
> language?  Arguably, if one feels that some component of an abstraction
> must be public, it's still better to have the rest private than to make
> all public, wouldn't you say?  I think one could make a case that
> public components in general are about the same level of evilness as
> goto, i.e. easy to abuse, but also easy enough not to abuse, and the
> abuse is of a kind that is more a consequence of bad design than a
> cause.

You have presented no evidence to support your claim of baloney. Salami,
perhaps.

Seriously, I was employing hyperbole in describing it as EVIL. However,
I think that this practice should be discouraged. The problem is that
such a type creates an interface that is difficult to understand. What
is the difference between

Op (Object, Param1, Param2);

and

Object.Public_Component := Some_Value;
Op (Object, Param1, Param2);

? They could be the same; Op could perform differently in the 2nd case,
but this be considered valid; or Op could be invalid as a result of the
assignment.

This doesn't bother C++ people, who expect the client to read the
implementation to learn how to use a type. Frequently such components
are used both to shorten parameter lists and store state between calls.
In general, visible components in an abstraction are a bad idea,
including when they are all visible: you cannot change the
implementation without changing the interface.

-- 
Jeff Carter
"I blow my nose on you."
Monty Python & the Holy Grail