Smalltalk and Object Orientation: an Introduction - Free

They are more efficient for storage and certain logical operations (such as =) and so may be used
instead of strings if their values will not be altered and substring operations are not required.
7.4 Assignments
A variable name can refer to different objects at different times; assignments can be made to variable
names. The “:=” symbol is used in Smalltalk to indicate assignment . It is often read as “become equal
to” although some do read it as “colon equals”. Some examples are:
currentEmployeeIndex := 1.
newIndex := oldIndex.
myName := 'John Hunt'.
Assignments return values (like other expressions), so that several assignments can be made
together:
nextObject := newObject := oldObject.
The above example also illustrates a feature of Smalltalk style - the specification of variable names
which indicate what they contain. This technique is often used where a more meaningful name (such as
currentEmployeeIndex) is not available, i.e. where temp might be used in other languages.
It is worth reiterating the point that variables in Smalltalk are not strongly typed as in languages such
as Pascal and ADA. Instead, Smalltalk is dynamically typed. That is, a variable i s not un-typed, rather
its type is determined by its current contents. Thus, it is possible to determine the type contained in a
variable by sending it a message asking for its type. Of course, by type we really mean class. It is also
possible that a varia ble which is currently holding a string, may then be assigned an integer. This is
quite legitimate and often happens, for example:
myVariable := 'John'.
myVariable := 1.
myVariable := #(1 2 3 4).
An important point to note is that assignment is by reference. This means that in the following
example nextIndex, newIndex and oldIndex all refer to the same object.
new := old := (Bag new).
next := new.
The effect of these assignments is illustrated in Figure 7.1.
next
new
old
aBag
Figure 7.1: The result of a multiple assignment
As all three variables point to an instance of a container class (in this case
made to the contents of any one of the variables, it would be made for all three!
Bag), if an update was
7.5 Variables
7.5.1 Types of variable
There are a number of different types of variable available in the Smalltalk language. Some of these
have already been discussed. The following provides a review of some of the different types:
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