Delphi Language Learning 8-Class members (fields and methods)

1.Class Fields

Example 1 member variable
Type
   Tancestor = Class
      Value:integer;
   End
   Tdescendant = Class (Tancestor)
      value:string;    Hides the Inherited Value field
   End
Var
   Myobject:tancestor;
Begin
   MyObject: = tdescendant.create;
   Myobject.value: = ' hello! '     Error
  (MyObject as Tdescendant). Value: = ' hello! '   works!
End
Example 2 static field
Type
    Tmyclass = Class
      Public
        class Var        //Introduce a block of class static fields.
          Red:integer;
          Green:integer;
          Blue:integer;
        var              //Ends the class var block.
          Instancefield:integer;
    End
Static fields can be called individually
Tmyclass.red: = 1;
Tmyclass.green: = 2;
Tmyclass.blue: = 3;
Static fields can also be called through object instances
Var
    Myobject:tmyclass;
Myobject.red: = 1;
Myobject.green: = 2;
Myobject.blue: = 3;

2.Method Binding

Static Methods
Example 1
Type
Tfigure = Class
Procedure Draw;
End
Trectangle = Class (Tfigure)
Procedure Draw;
End
Var
Figure:tfigure;
Rectangle:trectangle;
Begin
Figure: = tfigure.create;
Figure.draw; Calls Tfigure.draw
Figure.destroy;
Figure: = trectangle.create;
Figure.draw; Calls Tfigure.draw
Trectangle (Figure).  Draw; Calls Trectangle.draw
Figure.destroy;
Rectangle: = trectangle.create;
Rectangle.draw; Calls Trectangle.draw
Rectangle.destroy;
End
Virtual methods and Dynamic methods
Type
Tfigure = Class
Procedure Draw; Virtual
End
Trectangle = Class (Tfigure)
Procedure Draw; Override
End
Tellipse = Class (Tfigure)
Procedure Draw; Override
End
Var
Figure:tfigure;
Begin
Figure: = trectangle.create;
Figure.draw; Calls Trectangle.draw
Figure.destroy;
Figure: = tellipse.create;
Figure.draw; Calls Tellipse.draw
Figure.destroy;
End
Example 2
Type
T1 = Class (TObject)
Procedure Act; Virtual
End
T2 = Class (T1)
Procedure Act; The ACT is redeclared and but not overridden
End
Var
SOMEOBJECT:T1;
Begin
Someobject: = T2. Create;
Someobject.act; Calls T1. Act
End
Classes Method Class Methods
Ordinary Class Methods
The definition of a class method must begin with the reserved word class. For example,
Type
Tfigure = Class
Public
class function Supports (operation:string): Boolean; Virtual
Class procedure GetInfo (Var info:tfigureinfo); Virtual
End
The defining declaration of Class A is must also begin with class. For example,
Class procedure Tfigure.getinfo (Var info:tfigureinfo);
Begin
End
Class Static Methods
Type
Tmyclass = Class
Strict private
Class Var
Fx:integer;
Strict protected
Note:accessors for class properties must is declared class static.
class function Getx:integer; Static
Class procedure SetX (Val:integer); Static
Public
Class Property X:integer read GetX write SetX;
Class procedure Statproc (s:string); Static
End
Call
Tmyclass.x: = 17;
Tmyclass.statproc (' Hello ');