DelphiBasics
Packed
Keyword
Compacts complex data types into minimal storage
  type Name = Packed array[...] of ...;
  type Name = Packed class  ... end;
  type Name = Packed object ... end;
  type Name = Packed record ... end;
Description
The Packed keyword tells Delphi to minimise the storage taken up by the defined object.
 
Normally, complex data types, such as records have their elements aligned to 2, 4 or 8 byte boundaries, as appropriate to the data type. For example, a Word field would be aligned to a 4 byte boundary.
 
Records are also padded to ensure that they end on a 4 byte boundary.
 
These alignments ensure optimal access performance.
 
The Packed overrides this, compressing the data into the smallest storage, albeit with consequential reduced access performance.
Notes
Examples of unpacked alignments :

Word     = 2 bytes
LongWord = 4 bytes
Single   = 4 bytes
Double   = 8 bytes

Related commands
$Align Determines whether data is aligned or packed
Array A data type holding indexable collections of data
Class Starts the declaration of a type of object class
Object Allows a subroutine data type to refer to an object method
Record A structured data type - holding fields of data
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Example code : Packing a record to reduce storage
// 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
  SysUtils,
  Forms, Dialogs;
 
type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  end;
 
var
  
Form1: TForm1;
 
implementation
{$R *.dfm} // Include form definitions
 
procedure TForm1.FormCreate(Sender: TObject);

type
  // Declare an unpacked record
  TDefaultRecord = Record
    name1   : string[4];
    floater : single;
    name2   : char;
    int     : Integer;
  end;

  // Declare a packed record
  TPackedRecord = Packed Record
    name1   : string[4];
    floater : single;
    name2   : char;
    int     : Integer;
  end;

var
  defaultRec : TDefaultRecord;
  packedRec  : TPackedRecord;

begin
  ShowMessage('Default record size = '+IntToStr(SizeOf(defaultRec)));
  ShowMessage('Packed record size = '+IntToStr(SizeOf(packedRec)));
end;
 
end.
Hide full unit code
   Default record size = 20
   Packed record size = 14
 
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