Introduction

Every class has a set of fields, each of which has a unique name. Some of the fields may be defined in the class itself, but other may be inherited from the class’s superclasses: see Inheritance.

The fields can be divided into four basic categories. Here is a class with one field of each sort.

class X()
   public a

   public static b

   public c(v)
        self.a := v
   end

   public static d()
   end
end

The field a is an instance variable. That is, every instance of X will have its own value of the field, accessed as follows :-

x1 := X()
x1.a := 100
x2 := X()
x2.a := 99   # Distinct from x1.a

The field b is a static variable. This means that there is only one value of the variable in existence, and it is accessed via the class itself, rather than via any instance :-

X.b := 101

The field c is an instance method. It doesn’t take up any space in the instance, but it is accessed in the same way as an instance variable :-

x1 := X()
x1.c(40)

An instance method has an implicit parameter self, which refers to the instance which was used to invoke the method (in the above example, x1). It is possible (and often very useful) to get a reference to an instance method without invoking it :-

x1 := X()
m := x1.c
...
m(40)

Here, m will have a value of type methp, short for “method pointer”. See this page for details.

Finally, the field d is a static method. It doesn’t have an implicit self parameter, and is invoked via the class, rather than an instance. Instance variables and methods cannot be accessed from inside a static method (an error on linking will be produced).

X.d()

Like an instance method, it is possible to get a reference to a static method without invoking it :-

m := X.d
...
m()

In this case m has the type “procedure”.