Entry

Delphi: How to use the 'this' command (='self') in Delphi?

Dec 12th, 2003 15:38
Knud van Eeden,

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--- Knud van Eeden --- 29 April 2002 - 01:58 am ---------------------

Delphi: How to use the 'this' command (='self') in Delphi?

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The 'this' keyword in C++ maps onto the 'Self' keyword in Object 
Pascal.

[book: source: Entsminger, Gary - the way of Delphi - Prentice Hall - 
ISBN 0-13-455271-7 - p. xxxii 'Miscellaneous']

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All classes have a hidden field called Self.

Self is a pointer to the instance of the class in memory.

[book: source: - Teach Yourself Borland Delphi 4 in 21 Days - Kent 
Reisdorph - SAMS - ISBN 0672312867 - 
http://www.amazon.com/exec/obidos/tg/detail/-
/0672312867/qid=1064501349/sr=1-1/ref=sr_1_1/103-6292513-4946208?
v=glance&s=books#product-details]

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The keyword 'Self' is used by you to let the compiler distinguish
between different variables with the *same* name inside a class.

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So it lets the compiler to distinguish between:

 which *same* variable names correspond to class members

 and

 which *same* variable names correspond to method parameters,

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To distinguish, you precede that class member variable names with the
'self' keyword, as shown here:

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// save as Unit1.pas [kn, ri, su, 28-04-2002 18:00:34]
//
unit Unit1;
//
interface
//
uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, 
Dialogs;
//
type
  TForm1 = class(TForm)
  private
    { Private declarations }
  public
    { Public declarations }
  end;
//
TYPE
CLASSEmployee = CLASS( TObject )
 PRIVATE
  nameS: STRING[255];
  employeeidI: INTEGER;
  officenumberI: INTEGER;
  salaryD: REAL;
 //
 PUBLIC
 //
 CONSTRUCTOR Create( nameS: STRING ); overload;
 CONSTRUCTOR Create( nameS: STRING; employeeidI: INTEGER; 
officenumberI: INTEGER; salaryD: REAL ); overload;
 PROCEDURE PROCEmployeeShowRecord; overload;
 PROCEDURE PROCEmployeeShowRecord( xI: INTEGER; yI: INTEGER ); 
overload;
END;
//
VAR
 Form1: TForm1;
 defaultidI: INTEGER = 300;
 defaultofficeI: INTEGER = 200;
 defaultsalaryD: REAL = 10000.0;
 //
 bossO : CLASSEmployee;
 topworkerO : CLASSEmployee;
 worker1O : CLASSEmployee;
IMPLEMENTATION
//
{$R *.DFM}
//
// constructor function overload
CONSTRUCTOR CLASSEmployee.Create( nameS: STRING; employeeidI: INTEGER; 
officenumberI: INTEGER; salaryD: REAL );
BEGIN
 SELF.nameS := nameS;
 SELF.employeeidI := employeeidI;
 SELF.officenumberI := officenumberI;
 SELF.salaryD := salaryD;
END;
// constructor function overload
CONSTRUCTOR CLASSEmployee.Create( nameS: STRING );
BEGIN
 SELF.nameS := nameS;
 employeeidI := defaultidI;
 defaultidI := defaultidI + 1;
 officenumberI := defaultofficeI;
 defaultofficeI := defaultofficeI + 1;
 salaryD := defaultsalaryD;
END;
// procedure overload
PROCEDURE CLASSEmployee.PROCEmployeeShowRecord( xI: INTEGER; yI: 
INTEGER );
BEGIN
 ShowMessage( 'Name = ' + nameS + #13 +
              'Office = ' + IntToStr( officenumberI ) + #13 +
              'Employee Id = ' + IntToStr( employeeidI ) + #13 +
              'Salary = $' + FloatToStr( salaryD ) );
END;
// procedure overload
PROCEDURE CLASSEmployee.PROCEmployeeShowRecord;
BEGIN
 ShowMessage( 'Name = ' + nameS );
 ShowMessage( 'Office = ' + IntToStr( officenumberI ) );
 ShowMessage( 'Employee Id = ' + IntToStr( employeeidI ) );
 ShowMessage( 'Salary = $' + FloatToStr( salaryD ) );
END;
//
BEGIN
 //
 bossO := CLASSEmployee.Create( 'Suzanne' );
 topworkerO := CLASSEmployee.Create( 'Vanessa', 101, 124, 25000.0 );
 worker1O := CLASSEmployee.Create( 'Nathalie' );
 bossO.PROCEmployeeShowRecord( 1, 1 );
 topworkerO.PROCEmployeeShowRecord( 1, 1 );
 worker1O.PROCEmployeeShowRecord( 1, 1 );
 //
 bossO.Free;
 topworkerO.Free;
 worker1O.Free;
END.
//
// if you run it, you will see:
//
// Name = Suzanne
// Office = 200
// Employee Id = 300
// Salary = $10000
//
// Name = Vanesssa
// Office = 124
// Employee Id = 101
// Salary = $25000
//
// Name = Nathalie
// Office = 201
// Employee Id = 301
// Salary = $10000
//
//

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Internet: see also:

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Delphi: Class: Create: Simple: How to create a class in Delphi?
http://www.faqts.com/knowledge_base/view.phtml/aid/24789/fid/175

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C#: How to use the 'this' command in C#?
http://www.faqts.com/knowledge_base/view.phtml/aid/24776/fid/791

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C++: How to use the 'this' command in C++?
http://www.faqts.com/knowledge_base/view.phtml/aid/16106

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Java: How to use the 'this' command in Java?
http://www.faqts.com/knowledge_base/view.phtml/aid/16104

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Visual Basic: How to use the 'this' command (='Me') in Visual 
Basic.NET?
http://www.faqts.com/knowledge_base/view.phtml/aid/27372/fid/1610

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