Re: Package Instances???

["Pat Rogers" <progers@classwide.com>]

"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C73F5F6.7050704@mail.com...
> Matthew Heaney wrote:
> > Yes.  An Ada package is analogous to a C++ namespace, not a class.
>
> Except that in Ada, a package can be used as a parameter for a
> generic instantiation, but in C++ a namespace cannot be used as
> a template argument. Given that C++ classes can have arbitrary
> things defined in them (types, static functions, constants, etc.)
> considering them as an analogue of Ada packages is not wrong.

Yes, I agree with that sentiment, but I don't agree that one can make the
analogies in isolation from other language elements.  I would express it thusly:
a C++ class declared is analogous to a tagged type declared within a package
visible part.  The package, the type, and the corresponding primitive operations
work together to form an abstraction.  This atomicity is illustrated, for
example, by the fact that subprograms are not primitive operations of the type
unless they are declared in the same package as the tagged type declaration
itself (in addition to the other requirement, namely, that they mention the type
in their parameter and (for functions) result type profile).  Subprograms that
meet the second requirement but not the first are not primitive operations of
the type and thus are not inherited and are not dynamically dispatched.

A package not exporting a tagged type with primitive operations could correspond
to a C++ namespace.  What's missing is that, like Java's packages, C++
namespaces can be extended dynamically -- they can be "re-opened".  An Ada
package is a module, with a definite end.  But then there are subsystems...

You might be interested in the Ada generated from UML by a product called "UML
Studio" (www.pragsoft.com).  It uses the specified namespace of the UML class,
if present, as the package name, and the UML class name for the tagged type name
within that package.  If the namespace is not specified, it uses the class name
as the package name and "Object" for the tagged type name.

---
Patrick Rogers                       Consulting and Training in:
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