Re: Cumbersome Polymorphism

[Dave Gibson <dgibson@cis.ohio-state.edu>]

> > However, it seems that in order to make a very simple use of
> > polymorphism, I have been obliged to introduce access types.
> 
> Yes, this is generally the case in Ada 95.

Right.  Access types do seem necessary for inheritance-style inclusion 
polymorphism required for dynamic binding in Ada.  However, access types 
are not necessary to achieve generics-based parametric polymorhphism.  While
generics without exposed use of access types only provide static binding
in Ada, this approach can have a number of advantages (assuming a good
generics-savy compiler).  

> > And now by bringing in access types I have imported the problems
> > that go with them.
> 
> You are using an access type in a very restricted and disciplined way.
> I'm not sure what you mean by "the problems that go with them", but the
> fact that undisciplined use of access types may cause problems is
> irrelevant.  Your solution does not involve any such problems.

Well, even if you have a strong discipline that avoids aliasing-related
reasoning problems which access types can introduce, I personally think
that the code is just more difficult to read and understand when access
types are used as much as dynamic dispatching in Ada seems to require.

> > However, as is well known, the problem with requiring the user
> > of a type to perform explicit deallocation is that he may forget
> > to do so somewhere in the code, resulting in a memory leak.
> 
> I don't understand the problem.  The need for the deallocation operation
> is encapsulated in the package.  The writer of the package ensures that
> there is never more than one object allocated.  The user of the type
> never has to know about it.  Indeed, the declaration of your access type
> ought to be moved out of the package declaration and into the package
> body.

Yep.  That's an important part of a strong discipline for software
component engineering.

Dave
--
dgibson@cis.ohio-state.edu