Mega Code Archive
$a - determines whether data is aligned or packed
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1 {$A-}
2 {$A+}
Description
The $A compiler directive determines whether Delphi aligns data, or whether it packs the data into the smallest space.
With $A+ (default), 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.
With $A+, the default value, you can override this setting with the packed option for complex data structures.
These alignments ensure optimal access performance.
$A- tells Delphi to ignore alignment, and thereby pack data.
Notes
Examples of unpacked alignments :
Word = 2 bytes
LongWord = 4 bytes
Single = 4 bytes
Double = 8 bytes
$Align is equivalent to $A.
This directive can be used multiple times within your code.
The default value is $A+
Related commands
$Align Determines whether data is aligned or packed
Packed Compacts complex data types into minimal storage
Example code : Packing a record to reduce storage
type
// Use the default setting : $A+
// Declare an unpacked, aligned record
TAlignedRecord = Record
name1 : string[4];
floater : single;
name2 : char;
int : Integer;
end;
// Declare a packed record
TPackedRecord = Packed Record
name1 : string[4];
floater : single;
name2 : char;
int : Integer;
end;
// Set alignment off
{$A-}
// Declare an unpacked record
// This will get treated as if packed was on
TUnPackedRecord = Record
name1 : string[4];
floater : single;
name2 : char;
int : Integer;
end;
var
alignedRec : TAlignedRecord;
packedRec : TPackedRecord;
unPackedRec : TUnPackedRecord;
begin
ShowMessage('Aligned record size = '+IntToStr(SizeOf(alignedRec)));
ShowMessage('Packed record size = '+IntToStr(SizeOf(packedRec)));
ShowMessage('UnPacked record size = '+IntToStr(SizeOf(unPackedRec)));
end;
Show full unit code
Aligned record size = 20
Packed record size = 14
UnPacked record size = 14
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