| 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.
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| | 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.
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myRect coord 0 = 22
myRect coord 1 = 33
myRect coord 2 = 44
myRect coord 3 = 55
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