Name
Packed Keyword
Syntax
typeName= packed record ... end; typeName= packed class ... end; typeName= packed object ... end; typeName= packed array[...] of ...;
Description
Delphi aligns record and object fields on natural boundaries to
improve performance. If you use the packed
directive, Delphi does not insert any padding to align fields within
the object or
record.
An aligned record is also padded so its size is a multiple of 4 bytes. A packed record does not have any extra padding, but Delphi aligns and pads variables and dynamically allocated memory on 4-byte boundaries, so the size of a packed record is meaningful only if you are creating an array of records or using records as fields in other structured types.
The unpacked alignment for a field depends on the size of the field:
Byte-sized fields and sets of any size are aligned on byte boundaries.Word-sized fields are aligned on word (2-byte) boundaries.LongWord-sized fields (includingSingle) are aligned on long word (4-byte) boundaries.Other types (
Comp,Currency,Double,Extended,Int64,Real48,Variant) are aligned on 8-byte boundaries.Arrays are aligned according to the alignment of the array’s base type.
Records are aligned according to the largest alignment used by a member in the record.
Tips and Tricks
Accessing a packed field is slower than accessing an aligned field. Use the
packedkeyword only when the need to conserve memory outweighs the performance penalty.Delphi does not pack
Booleanfields into single bits, the way some Pascal compilers do.Arrays are always packed. You can supply the
packedkeyword in an array declaration for compatibility with standard Pascal, but it has no effect.
Example
type
Big = record // SizeOf(Big) = 16
B1: Byte;
W: Word; // Align W on a word boundary by inserting a pad byte
B2: Byte;
L: LongWord; // Align L on a 4-byte boundary by inserting 3 bytes
B3: Byte; // Record size is aligned to a long word boundary
end;
Small = packed record // SizeOf(Small) = 9
B1: Byte;
W: Word; // Align W on a byte boundary: no padding
B2: Byte;
L: LongWord;
B3: Byte; // Record size is not padded.
end;