Smalltalk and Object Orientation: an Introduction - Free

1. instance variables - discussed above
2. class variables - will be discussed later in the book
3. class instance variables - will also be discussed later in the book
4. temporary variables - see below
5. global variables- see below
7.5.1.1 Temporary variables
These exist only for the duration of some activity (e.g. the execution of a method). They are denoted by
being placed between two bars, e.g. | x y z | indicates that the variables x, y and z are temporary
variables. Try typing the following into the Workspace window:
| x y z|
x := 5.
y := 6.
z := x + y.
Transcript show: z printString.
Now select all the text you typed in. Now use the right mouse button (middle if you have a three
button mouse) and select the do it option on the menu. You should now see the value 11 printed in your
Transcript window.
7.5.1.2 Global variables
These are shared by all instances of all classes. These can be useful during testing if you want to keep
hold of a particular object. They always start with a capital letter and can be deleted by looking at the
Smalltalk system dictionary. This can be accessed from the global variable Smalltalk (you can use the
inspect message to examine the contents of the variable Smalltalk). To delete a global variable from the
current system, use:
Smalltalk removeKey: #.
It is interesting to note that class names start with a capital letter and are therefore actually global
variables!
7.5.2 Pseudo variables
There are a number of pseudo variable s in Smalltalk which can be referenced within a method. A
pseudo variable is a special variable, whose value is changed by the system but whi ch cannot be
changed by the programmer. That is, the value of these variables is determined by the current context
within which the code around it is executing. There are two such pseudo variables; they are self and
super:
self This refers to the receiver of a message itself. When a message is received, the search for the
corresponding method starts in the class of the receiver. It is the way in which a method in one
object can initiate the execution of another method in that same object.
super Also refers to the message receiver, but the method search starts in the superclass of the class in
which super is used. This is often used if the functionality of a method is to be extended rather
than overwritten. For example:
myMethod: anObject
new code before super.
super myMethod.
new code after super.
Don’t worry about the syntax or the meaning of this at the moment just make sure you get the idea
of things.
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