Problem Statement

In object-oriented programming class is a set (a transitive closure) of types bound by the relation of inheritance. It is said that all types derived from some base type T and the type T itself form a class T. The first type T from the class T sometimes is called the root type of the class. A class of types itself, as a type, has the values and operations of its own. The operations of are usually called methods of the root type. Both operations and values are called polymorphic. A polymorphic operation (method) selects an implementation depending on the actual specific type of the polymorphic argument. The action of choice the type-specific implementation of a polymorphic operation is called dispatch. Correspondingly, polymorphic operations are often called dispatching or virtual. Operations with multiple arguments and/or the results of the class are called multi-methods. A further generalization of is the operation with arguments and/or results from different classes.

A polymorphic value has a type tag indicating its specific type from the class and the corresponding specific value of that type. This type is sometimes called the most specific type of a [polymorphic] value. The type tag of the value is used in order to resolve the dispatch. The set of polymorphic values of a class is a transitive closure of the sets of values of all types from that class. In many OO languages the type of the class of T and T itself are considered equivalent. In some languages they are distinct (like in Ada). When class T and T are equivalent, there is no way to distinguish polymorphic and specific values.

Create a basic class with a method, a constructor, an instance variable and how to instantiate it.

Let's start with the solution:

LOCAL o1 As MyClass, o2 As MyClass
*!* Instantiate o1
o1 = NEWOBJECT("MyClass")
o1.ShowInstance()
*!* Instantiate o2
o2 = CREATEOBJECT("MyClass", 2)
o2.ShowInstance()


DEFINE CLASS MyClass As Session
*!* Custom property (protected)
PROTECTED nInstance 
nInstance = 0

*!* Constructor
PROCEDURE Init(tnInstance As Integer)
IF VARTYPE(tnInstance) = "N"
    THIS.nInstance = tnInstance
ELSE
    THIS.nInstance = THIS.nInstance + 1 
ENDIF
ENDPROC

*!* Custom Method
PROCEDURE ShowInstance
? "Instance", THIS.nInstance 
ENDPROC
ENDDEFINE