| Description |
The AbstractErrorProc variable defines to Delphi a procedure that it should call when your code erroneously calls an abstract method of a class.
Abstract methods are placeholder methods - only to be implemented by derived classes. The parent class is really a skeleton class, and therefore should not be instantiated into an object. delphi will warn you if you do. If you call an abstract method, it will call AbstractErrorProc before terminating with an error. If not found, it will throw EAbstractError exception instead.
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| | Notes | You should normally avoid calling abstract methods.
| | | Related commands | | Abstract | | Defines a class method only implemented in subclasses | | Addr | | Gives the address of a variable, function or procedure | | Procedure | | Defines a subroutine that does not return a value |
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| Example code : A polygon class with an abstract method called | // 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; // 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 a square descendent
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 square
procedure TSquare.setArea;
begin
// Calculate and save the area of the square
shapeArea := sideLength * sideLength;
end;
procedure ReportAbstractError;
begin
// Report the error
ShowMessage('ERROR : Abstract method called.');
end;
// Main line code
procedure TForm1.FormCreate(Sender: TObject);
var
square : TSquare;
polygon : TPolygon;
begin
// Set up a procedure to catch calls to abstract errors
AbstractErrorProc := Addr(ReportAbstractError);
// Create a square
square := TSquare.Create(4, 10);
// Show the area of the square
ShowMessageFmt('Square area = %f',[square.area]);
// Try to create a polygon (internally calls abstract method)
polygon := TPolygon.Create(2, 10); // Meaningless shape!
// Show the area of the polygon
ShowMessageFmt('Polygon area = %f',[polygon.area]);
end;
end.
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Square area = 100.0
ERROR : Abstract method called.
The program then terminates with an error dialog:
Runtime error 210 at 00452208
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