Dan Ingalls wrote:

> Folks -
> I'm preparing a talk (for tomorrow ;-), and would really like to show the very simplest class description, such as a stack, and the couple of lines that would instantiate it, push 1, 2, 3, and then print three pops.
> While I'd like it as simple as possible, I'd like it to be complete with any necessary type declarations and, eg, how the array buffer gets allocated.  

> Any help will be greatly appreciated.  I thought it would be a snap to find some class notes on the Internet, but I failed in my first attempt.
> Thanks in advance
> 	- Dan


Here is an example taken from
 
"Programming Language Concepts"
Ghezzi - Jazaeri 2nd edition.
Wiley ISBN 0-471-82173-X

Chapter 4.5 Abstract Data Types (pp. 144-145); since there is a third edition of 
this book, at least, page numbers may not coincide, or miss this section.

// these are my comments, taken from the text, to clarify the program

--------------------------
....

Suppose we want to define the abstraction "stack of elements"

[....]

We first describe the class of items that can be stacked.

class stack_member;
begin ref (stack_member) next_member,
	next_member:- none
	// :- is the reference assignement symbol and read "denotes"
end

This specifies that the only property shared by all stackable objects is the 
existence of an attribute that is a reference to the next item in the stack.

[...]

class stack
begin 
ref (stack_member) first;
	ref (stack_member) procedure top;

		top:- first;

	procedure pop;
		if -, empty then first :- first.next_member;
                 // -, (an half arrow?) means NOT
	procedure push(e); ref (stack_member) e;
	begin if first =/= none 		// =/= means NOT EQUAL
		then e.next_member:- first;
		first:- e
	end push;
	boolean procedure empty;
		empty := first==none;
	first:- none;
end stack

...we can now create stackable objects of a particualr type, for example, 
comples, by prefixing class complex

stack_member class complex (...);
	...
	end complex

specifies that the objects generated by

	new complex

have all the atributes of stack_member, as well as the atribute of complex.
In other words, thay are complex numbers that can be stacked.

If s is declared to be of type ref (stack), we can create a stack of complex 
numbers by doing

	s:- new stack;

Stack s courrently contains no elements, sto that s.empty return true.
If we have several complex objects, c1, c2, c3, we may insert them in the stack 
in the following way.

s.push (c1);
s.push(c2);
s.push(c3).

We can look at the top element by doing s.top, which returns a reference; we can 
remove the top element by s.pop.

....

---------------------------

I hope this helps

Regards,

	Sergio

-- 
______________________________________________________________________
Sergio Ruocco   CS PhD student    mailto:sergio.ruocco at disco.unimib.it
Università Statale di Milano-Bicocca        http://www.disco.unimib.it
DISCo - Software Architecture Lab.      http://www.sal.disco.unimib.it
phone: ++39 02 6448 7874                        cell: ++39 347 2519828