Re: Choice of OO primitives in Ada95

[Ray Toal <rtoal@eecs.lmu.edu>]

donh@syd.csa.com.au (Don Harrison) wrote:

>1) Why should you have to specify that a type is 'tagged'? Can't the compiler work
>that out for itself? eg. by seeing whether the type is extended elsewhere. The
>developer is forced to worry about what should be an implementation issue.

Requiring 'tagged' is a GOOD THING!  Inheritance weakens encapsulation.
The default case (no tagged) is that you design a type, and you
provide all and only those operations that work on the type and you
don't make any details available to anyone else - in short you
fully control the type.  Now if your intent is to ALLOW derivation
then you must mark it tagged.  This alerts the reader that this
type may be derived from.  Imagine a language in which you could
inherit from any type you wanted to! :-)  Whether or not a type
should be tagged is a DESIGN decision; I totally disagree that it
should be an implementation decision.  By the way a compiler can not
in general determine if the "type is extended elsewhere" since in
Ada extensions can appear in other compilation units.


>2) Similarly, why should the developer have to specify that an operation dispatches
>(classwide operations)? Presumably, you're aiming for quicker execution, but
>compilers could perform a certain degree of optimisation eg. If it knows the type
>is not extended anywhere, there is no need to dispatch. There would also be
>situations where the specific variant of an inherited type is known eg. following
>an explicit assignment from an entity of that type.

The reason you need to specify whether or not an operation dispatches
is that you can write code like

  procedure P (X: T) is begin R(X); end P;
  procedure Q (X: T'Class) is begin R(X); end Q;

which behave differently.

Ray Toal