OpenEdge Getting Started:<br />Object-oriented Programming Defining an object reference as a field in a temp-table

Temp-tables can define classes as fields. Classes are always represented by value, that is, the value of the object reference, not the object itself. You define a class field in a temp-table as a Progress.Lang.Object, the built-in class that is the implicit super class (root class) of all user-defined classes. When object instances are assigned to the field, they are implicitly cast to the root class, Progress.Lang.Object. When you want to use the object reference as a specific class type, you must cast the object reference to the specific type. For more information on casting, see the "Assignment and the CAST function" section.

As with class type parameters, a temp-table containing a class field cannot be passed to or from a remote application server.

Syntax
FIELD `field-name` AS [ CLASS ] Progress.Lang.Object

The following example defines a temp-table field to hold a class instance, by defining the field as Progress.Lang.Object. In this case, the purpose is to store different ShapeClass instances in order to calculate an area on each ShapeClass object in sequence, according to its subclass type (RectangleClass or CircleClass), as shown:

CLASS Main: DEFINE VARIABLE width AS DECIMAL NO-UNDO INITIAL 10.0. DEFINE VARIABLE length AS DECIMAL NO-UNDO INITIAL 5.0. DEFINE VARIABLE radius AS DECIMAL NO-UNDO INITIAL 100.0. `DEFINE TEMP-TABLE myTT NO-UNDO` `FIELD shapes AS CLASS Progress.Lang.Object` `FIELD area AS DECIMAL.` CONSTRUCTOR PUBLIC Main ( ): CREATE myTT. `myTT.shapes = NEW RectangleClass (INPUT width, INPUT length).` CREATE myTT. `myTT.shapes = NEW CircleClass (input radius).` FOR EACH myTT: `/* Cast field to common super class ShapeClass */` `myTT.area =` `CAST(myTT.shapes, "ShapeClass"):CalculateArea ( ).` MESSAGE myTT.area VIEW-AS ALERT-BOX. END. END CONSTRUCTOR. END CLASS.

CLASS Main: 
    DEFINE VARIABLE width AS DECIMAL NO-UNDO INITIAL 10.0. 
    DEFINE VARIABLE length AS DECIMAL NO-UNDO INITIAL 5.0. 
    DEFINE VARIABLE radius AS DECIMAL NO-UNDO INITIAL 100.0. 
    DEFINE TEMP-TABLE myTT NO-UNDO 
            FIELD shapes AS CLASS Progress.Lang.Object 
            FIELD area AS DECIMAL. 
    CONSTRUCTOR PUBLIC Main ( ): 
        CREATE myTT. 
        myTT.shapes = NEW RectangleClass (INPUT width, INPUT length). 
        CREATE myTT. 
        myTT.shapes = NEW CircleClass (input radius). 
        FOR EACH myTT: 
            /* Cast field to common super class ShapeClass */ 
                myTT.area =  
                    CAST(myTT.shapes, "ShapeClass"):CalculateArea ( ). 
            MESSAGE myTT.area VIEW-AS ALERT-BOX. 
        END. 
    END CONSTRUCTOR. 
END CLASS.

The previous example is based on the classes used to demonstrate polymorphism in a previous chapter (see the "Using polymorphism with classes" section). This version of the Main class thus shows how you might use temp-table fields to support polymorphic object references and use the CAST function to cast down to the common functionality of the polymorphic super class (ShapeClass).