Re: Inheritance versus Generics

[stt@houdini.camb.inmet.com (Tucker Taft)]

Craig Smith (cdsmith0@flash.net) wrote:

: Generics were a very power feature of Ada-83.  With the OO features
: introduced into Ada-95 (in particuliar, the tagged record and ability to
: inherit), are Generics obsolute?  I would guess since several features
: were added to generics (like generic formal package parameters), the
: answer is no.  Does anyone have any thoughts on this?

This one should get a lot of responses...

Inheritance is good for building an extensible abstraction,
whereas a generic is good for building a parameterized abstraction.

The distinction between extension and parameterization is fundamental
in my view, and both are very useful.

It is interesting to look at Java, which supports only extension --
no parameterization.  You can't build a compile-time-type-safe container 
abstraction in Java, such as a list of T, or a dictionary mapping
T1 to T2, without writing a separate one for each possible T, or each
possible T1/T2 combination.  In Java, you end up using the univeral
"java.lang.Object" class for many things, allowing you to create
heterogenous "bags" of anything, but similarly providing no compile-time
guarantee that a given "bag" has only what you want in it.

By the way, our Ada 95 => Java byte code compiler supports both
generics and extension, since generics are basically a compile-time
concept, and we are only using the Java run-time model, not its
compile-time model.  I might say, when I am feeling a bit unfair, 
that Java has an elegant run-time model dressed in an impoverished 
surface syntax.

: Craig D. Smith

-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Burlington, MA  USA