DelphiBasics
Index
Directive
Principally defines indexed class data properties
  1.Property Name : Property-type Index Constant Read Getter Write Setter;
 
  2.Property Name[Index : Index type] : Property-type Read Getter Write Setter;
 
  3.Function|Procedure Index Constant;
 
  4.Function|Procedure header; External DllName Index Constant;
Description
The Index directive is principally used to declare indexed class data properties.
 
These principally work against array or list types of data.
 
Version 1
 
Retrieves or writes an indexed value by specifying a Constant index number.
 
Version 2
 
Retrieves or writes an indexed value by specifying an index parameter. As can be seen, this does not actually use the Index directive - it is included here for completeness.
 
See the example or the Property keyword for information about coding the routines associated with these properties.
 
Version 3
 
Defines a unique index for routine when exporting.
 
Version 4
 
Defines the unique index of an external routine being imported.
Related commands
Function Defines a subroutine that returns a value
Procedure Defines a subroutine that does not return a value
Property Defines controlled access to class fields
 Author links

 Buy Website Traffic at
 Buywebsitetrafficexperts.com

 Buy Proxies at
 Buyproxies.io
 
 Download this web site as a Windows program.




 
Example code : Illustrate both varieties of Indexed properties
// 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
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  // Class with Indexed properties
  TRectangle = class
  private
    fCoords: array[0..3] of Longint;
    function  GetCoord(Index: Integer): Longint;
    procedure SetCoord(Index: Integer; Value: Longint);
  public
    property Left   : Longint Index 0 read GetCoord write SetCoord;
    property Top    : Longint Index 1 read GetCoord write SetCoord;

    property Right  : Longint Index 2 read GetCoord write SetCoord;
    property Bottom : Longint Index 3 read GetCoord write SetCoord;
    property Coords[Index: Integer] : Longint read GetCoord write SetCoord;
  end;

  // The form class itself
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

// TRectangle property 'Getter' routine
function TRectangle.GetCoord(Index: Integer): Longint;
begin
  // Only allow valid index values
  if (Index >= 0) and (Index <= 3)
  then Result := fCoords[Index]
  else Result := -1;
end;

// TRectangle property 'Setter' routine
procedure TRectangle.SetCoord(Index, Value: Integer);
begin
  // Only allow valid index values
  if (Index >= 0) and (Index <= 3)
  then fCoords[Index] := Value;
end;

// Main line code
procedure TForm1.FormCreate(Sender: TObject);
var
  myRect : TRectangle;
  i : Integer;

begin
  // Create my little rectangle
  myRect := TRectangle.Create;

  // And set the corner coordinates
  myRect.Left := 22;       // Left using direct method
  myRect.Top  := 33;
  myRect.SetCoord(2,44);   // Right using indexed method
  myRect.SetCoord(3,55);

  // And ask for these values
  for i:= 0 to 3 do
    ShowMessage('myRect coord '+IntToStr(i)+' = '+intToStr(myRect.GetCoord(i)));
end;

end.
   myRect coord 0 = 22
   myRect coord 1 = 33
   myRect coord 2 = 44
   myRect coord 3 = 55
 
Delphi Programming Neil Moffatt 2002 - 2017. All rights reserved.  |  Home Page