Abstract
Belongs to : DirectiveDescription
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).
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.
Example code
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// 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. { Triangle area = 43.3 Square area = 100.0 } |