Also I want to create a class instance from a class method,  but having
problems.

I thank anybody in advance for any help.

A frustrated OU student.

Hello John,

First of all, if these issues are frustrating for you as an OU student,
something is not right with the way you are doing this course. But I'll let
you deal with that for now and simply put you on the right track with your
questions, as best I can.

You can do many things in Smalltalk, but accessing methods is not one of
them. The basic things you can do are.

(0. Denote an object)
1. Send a message
2. Assign a value to a variable
3. Return a value from a method

If what you are seeking to do is invoke an instance method, the way to go
about it is...

1. Send a message.

When you send a message, a method will be invoked. Which one is decided by
the system based on the code. IOW, you don't get to invoke methods, you just
get to send messages, and the system will decide which method it invokes.

If you want an instance method to be invoked, what you will need to do is
send a message to an instance.

If you need more help with that question, come back here.

Let's assume you mean 'I want to create an instance of a class C from a
class method of C'.

In that case, your best strategy is to send the message #new to the class,
in that class method. Do you know how to do that or do you need more help?

Thank you for your help.

You are right my terminology is bad. Terminology has never been my strong
point.

Lets start again.

I have created a class method. Within that class method I want to do 2
things:

1. Send a message to a instance variable.
2. Create a new object of the class

In general I am finding the OU course interesting, but when you need
clarification on a point, it is terrible. There are a lot of unhappy
student!.

Thanks once again.

Best regards
John L. Aldridge

Number one sounds confusing to me, there are two essential problems with it:
1) instance variables are private to an instance, so you cannot reach
them (send messages to them) directly from the outside (unlike other
languages like Java where this is possible). You _can_ reach them
directly from instance methods of the object. To make an instance
variable accessable from the outside create a getter and setter method
for it.

2) a class can have many instances, the question is: the instance
variable of _which_ instance.

Since I don't understand what you want to do (yet) let me go on a
tangent and assume you want the following:

in a class method you want to create an instance and then set an
instance variable of that instance to some value. I further assume that
'some value' is a literal (constant) defined in that method. Here is a
straitforward implementation of such a class method:

myClassMethod
     "return a new instance with 'myInstVar' initialized"

     | instance |
     instance := self new.
     instance myInstVar: 'some value'.
     ^instance

note that on the instance side you will need to create the setter method
#myInstVar: before this will work.

here is a variation you'll see very often in smalltalk code:
If all instances of your class need to have the instance variable set to
a same value when they are instantiated we can move that knowledge to an
instance method. We can also make it hard to create instances that are
not initialized.
We do that by redefining (overriding) the #new method on the class:

new
     "return a new instance with 'myInstVar' initialized"

     | instance |
     instance := super new.  "'self new' will not work. why?"
     instance initialize.
     ^instance

and by defining the instance method #initialize like so:

initialize

     myInstVar := 'some value'

note that here you can access the instance variable directly, you don't
need a setter method.

For those that don't like newsgroups try the vwnc-list or the IRC
channel, details at:
   http://wiki.cs.uiuc.edu/VisualWorks/VisualWorks+Online+Communities

HTH,

Hi John. I'm doing M206 as well, so perhaps I can help.

(self new) message; message: arg

Believe me its bad. I thought there would be quite a free exchange of
information, but unfortunately its not so. The tutors seem to be always on
holiday!

Best regards
John

Regards,

Peter van Rooijen

Nigel Mercier ? wrote

First, you might say that you access a variable,
but you either "call" or, more properly, "invoke" a method.
Methods are invoked by sending a message.
Some folks think this is splitting hairs,
but I used to teach this stuff and the words matter.

You can only invoke an instance method on an instance.
You can do this in a class method if you have an instance.
You can probably get an instance by saying "MyClass new"
or "self new" if the class method is a method of the class you want.

It sounds to me like you haven't yet understood
that this OO stuff is not just a fancy way of organizing functions.
Please flush this if I'm off base.
I'll be happy to discuss it more with you if I'm not.

Best regards,

Steve
--
Steven T Abell
Software Designer
http://www.brising.com