delphi 的结构体对齐关键字

Align fields (Delphi)

 

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Type	Switch
Syntax	{$A+}, {$A-}, {$A1}, {$A2}, {$A4}, {$A8} or {$A16} {$ALIGN ON}, {$ALIGN OFF}, {$ALIGN 1}, {$ALIGN 2}, {$ALIGN 4}, {$ALIGN 8} or {$ALIGN 16}
Default	{$A8} {$ALIGN 8}
Scope	Local

Remarks

The $A directive controls the maximum alignment used by the compiler when introducing padding to align a field.

In the {$A1} or {$A-} state, fields are never aligned. All record and class structures are packed.

In the {$A2} state, fields in record types that are declared without the packed modifier and fields in class structures are aligned on word boundaries.

In the {$A4} state, fields in record types that are declared without the packed modifier and fields in class structures are aligned on doubleword boundaries.

In the {$A8} or {$A+} state, fields in record types that are declared without the packed modifier and fields in class structures are aligned on quadword boundaries.

In the {$A16} state, fields in record types that are declared without the packed modifier and fields in class structures are aligned on double quadword boundaries.

Record type field alignment is described in the Delphi Language Guide at Internal Data Formats (Delphi)#Record_Types.

Regardless of the state of the $A directive, variables and typed constants are always aligned for optimal access. In the {$A8} state, execution will be faster.

For OS X applications, stack alignment must be on 16-byte boundaries. For details, see Eli Boling's blog at http://blogs.embarcadero.com/eboling/2009/05/20/5607.