Delphi Basics : Packed command

DelphiBasics

Packed
Keyword

Compacts complex data types into minimal storage

keyword Packed(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

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;

Show full unit code

Default record size = 20
Packed record size = 14