Delphi : record v.s. "packed" record

簡單結論 record 會依據結構 ( record )中最長的變數做 Alignment ，然而 packed record 不會。結果就是 packed record 佔的記憶體空間可能比 record 來的少。

範例

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;

結果
   Default record size = 20
   Packed record size = 14

P.S. I didn't compile this code, I don't know why the size is 20 and 14. But the concept is reasonable.