Re: About private types and reusability?

[Ed Falis <efalis@mediaone.net>]

Didier Utheza wrote:

> There was an interesting comparison on the net of c++, Ada95 and Modula3.
> The only critic of Ada was about the inclusion of the description of the
> private type in the specification rather than the body of the module.
> Modula3 keeps everything private in the body. I know that it was question
> to create a third type of file that would have described the private type.
> The idea was abandoned because having to deal with three files for one
> module will not have simplify things. So my question, if anybody has an
> answer, is why this choice. The inclusion of the private type defenition
> in the body should keep things modular. The specifications describing to
> the user the method to access the object in a transparent way. By
> including the private type def. in the specifications, any following
> change means that you not only have to recompile the module but also the
> whole program since the specifications changed. Where is the advantage?
> Thanks for any answer.
>                                 Didier Utheza.

My understanding is that this is an efficiency tradeoff to some extent.  The
private part of the spec provides information needed by the compiler so that
private types would not need to always involve an extra indirection into the
body definition.  You can, of course, get around it (ie reduce recompilation)
by associating an access type explicitly to an incomplete type whose
defiinition is in the package body.  The presence of this "representation"
information in the private part to the compiler when it processes client code
also aids optimization.  I believe I've heard Tucker Taft use the analogy of
logical and physical interface for the public and private parts of a spec.
I like that analogy.

- Ed