| Description |
The Abstract directive defines a class method as being implemented only in derived classes. It is abstract in the sense that it is a placeholder - it has no implementation in the current class, but must be implemented in any derived classes.
It is used where the base class is always treated as a skeleton class. Where such a class is never directly used - only based classes are ever instantiated into objects.
For example, a TAnimal class may have an abstract method for how the animal moves. Only when creating, say, a TCat class based in TAnimal will you implement the method. In this instance, the cat moves by walking.
An Abstract class must be used to qualify a virtual class, since we are not implementing the class (see Virtual for more details).
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| | Notes | If you create an instance of a class that has an Abstract method, then delphi warns you that it contains an uncallable method.
If you then try to call this method, Delphi will try to call AbstractErrorProc. If not found, it will throw an EAbstractError exception.
| | | Related commands | | AbstractErrorProc | | Defines a proc called when an abstract method is called | | Function | | Defines a subroutine that returns a value | | Inherited | | Used to call the parent class constructor or destructor method | | 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 | | Virtual | | Allows a class method to be overriden in derived classes | | Dynamic | | Allows a class method to be overriden in derived classes |
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| Example code : Polygon skeleton class with triangle and square based classes | // 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 triangle
shapeArea := (sideLength * sideLength*0.5) / 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 = 43.3
Square area = 100.0
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