| Description |
The Override directive defines a class method as overriding the same named method in a parent class.
For example, you might want to override (replace) the operation of the constructor to take into account the class changes introduced by your class.
You can only override classes defined as virtual or dynamic (the latter are beyond the scope of Delphi basics).
Only those methods that can be sensibly overriden by a derived class are normally allowed to do so.
If a method is marked as abstract as well as virtual then you must override it and implement it for it to be useable by a user of your class.
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| | Related commands | | Abstract | | Defines a class method only implemented in subclasses | | Function | | Defines a subroutine that returns a value | | Overload | | Allows 2 or more routines to have the same name | | Procedure | | Defines a subroutine that does not return a value |
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| Example code : Overriding abstract methods | // Full Unit code.
// -----------------------------------------------------------
// You must store this code in a unit called Unit1 with a form
// called Form1 that has an OnCreate event called FormCreate.
unit Unit1;
interface
uses
Forms, Dialogs, Classes, Controls, StdCtrls, SysUtils;
type
// Define a base TPolygon class :
// This class is a triangle if 3 sides, square if 4 sides ...
TPolygon = class
private
sideCount : Integer; // How many sides?
sideLength : Integer; // How long each side?
shapeArea : Double; // Area of the polygon
protected
procedure setArea; Virtual; Abstract; // Cannot code until sides known
published
property count : Integer read sideCount;
property length : Integer read sideLength;
property area : Double read shapeArea;
constructor Create(sides, length : Integer);
end;
// Define triangle and square descendents
TTriangle = class(TPolygon)
protected
procedure setArea; Override; // Override the abstract method
end;
TSquare = class(TPolygon)
protected
procedure setArea; Override; // Override the abstract method
end;
// Define the form class used by this unit
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.dfm} // Include form definitions
// Create the TPolygon object
constructor TPolygon.Create(sides, length : Integer);
begin
// Save the number and length of the sides
sideCount := sides;
sideLength := length;
// Set the area using the abstract setArea method :
// This call will be satisfied only by a subclass
setArea;
end;
// Implement the abstract setArea parent method for the triangle
procedure TTriangle.setArea;
begin
// Calculate and save the area of the square
shapeArea := (sideLength * sideLength) / 2;
end;
// Implement the abstract setArea parent method for the square
procedure TSquare.setArea;
begin
// Calculate and save the area of the square
shapeArea := sideLength * sideLength;
end;
// Main line code
procedure TForm1.FormCreate(Sender: TObject);
var
triangle : TTriangle;
square : TSquare;
begin
// Create a triangle and a square
triangle := TTriangle.Create(3, 10);
square := TSquare.Create(4, 10);
// Show the areas of our polygons:
ShowMessageFmt('Triangle area = %f',[triangle.area]);
ShowMessageFmt('Square area = %f',[square.area]);
end;
end.
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Triangle area = 50.0
Square area = 100.0
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