| Description |
The Virtual directive allows a method in a class to be overriden (replaced) by a same named method in a derived class.
You would mark a function or procedure as Virtual when you happily allow a programmer who creates a class based on your class to replace its functionality.
For example, you might allow a base class to paint a canvas in white, but allow a derived class to paint a picture on the canvas instead. Here, the virtual directive is allowing the code to be extended, to be enriched.
Virtual may be followed by the Abstract directive. This modifies the effect of the virtual directive. It means that the current class must not code the method - it is there only as a placeholder to remind and ensure that derived classes implement it.
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| | Notes | Virtual is semantically equivalent to Dynamic. The former is optimised for speed, the latter for memory.
| | | Related commands | | Abstract | | Defines a class method only implemented in subclasses | | Class | | Starts the declaration of a type of object class | | Function | | Defines a subroutine that returns a value | | Overload | | Allows 2 or more routines to have the same name | | Override | | Defines a method that replaces a virtual parent class method | | Procedure | | Defines a subroutine that does not return a value | | Dynamic | | Allows a class method to be overriden in derived classes |
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| Example code : Implementing abstract virtual class 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 traingle 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; // Must be implemented in child
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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