Re: Dynamic run-time

[Robert A Duff <bobduff@world.std.com>]

Tucker Taft <stt@averstar.com> writes:

> xaplos@my-deja.com wrote:
> > 
> > Does Ada provide an equivalent way of realizing the dynamic behavior
> > that SmallTalk or Objective-C provides? For instance is there a datatype
> > like Objective-C's id that allows variables and methods to be
> > dynamically bind at run-time?
> 
> Not really.  Using tagged types you can do the kind of dynamic binding
> that exists in C++ or Eiffel, but not the kind of "very dynamic" binding
> that you can do in SmallTalk.  The upside is that in Ada you will
> never get a "message not understood."  The downside is that you
> can't read in from a terminal or equivalent the name of a function
> or variable and somehow find and use the named entity.  

I once wrote a program that needed more dynamic dispatching than Ada's
tagged types provide -- more along the lines of Smalltalk.  So I wrote a
generic package that kept track of tables of access-to-subprogram
values.  The tables were initialized at elaboration time, and after that
everything was quite efficient.  It wasn't that hard -- a couple or
three hundred lines of generics, as I recall.

Normally, however, I wouldn't do that.  As Tucker pointed out,
the built-in dispatching calls using Ada's tagged types provide a huge
advantage in that you always dispatch to something that really does
exist.

- Bob