DelphiBasics
Private
Directive
Starts the section of private data and methods in a class
  type Class declaration
  Private
    Data | Method declaration;
  {...}
  end;
Description
The Private directive starts the section of data (fields) and routines (methods) of a class that are private (internal) to that class.
 
It is a vital part of the Object Oriented concept that a class be treated like a black box - how it operates internally is not relevant to the external use. This is where the Private section comes in - it hides data and routines used solely by other routines in the class.
 
If private data needs to be accessed externally, a public (or published) property or routine is provided to give this access.
 
Private data and routines are not even accessible by a descendent class - you must use Protected to provide this access. Protected data and methods are externally invisible, but are accessible to all classes in a hierarchy.
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
Protected Starts a section of class private data accesible to sub-classes
Public Starts an externally accessible section of a class
Published Starts a published externally accessible section of a class
Type Defines a new category of variable or process
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Example code : A class with private data and a private routine
// 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
  
 
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