关于结构体与类型转换的一点小技巧

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm1 = class(TForm)
Button1: TButton;
Button2: TButton;
Button3: TButton;
Button4: TButton;
Button5: TButton;
procedure Button1Click(Sender: TObject);
procedure Button2Click(Sender: TObject);
procedure Button3Click(Sender: TObject);
procedure Button4Click(Sender: TObject);
procedure Button5Click(Sender: TObject);
end;

var
Form1: TForm1;

implementation

{$R *.dfm}

type {下面两个结构体的大小都是 4 字节, 相当于一个 Cardinal 或 Integer 或 DWORD 的大小}
TMyByteStruct = record
b1: Byte;
b2: Byte;
b3: Byte;
b4: Byte;
end;

TMyWordStruct = record
w1: Word;
w2: Word;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
ByteStruct: TMyByteStruct;
num: Cardinal;
begin
{假如}
ByteStruct.b1 := $A1;
ByteStruct.b2 := $B2;
ByteStruct.b3 := $C3;
ByteStruct.b4 := $D4;
{那么}
num := Cardinal(ByteStruct);
ShowMessageFmt('$%x', [num]); //$D4C3B2A1
end;

procedure TForm1.Button2Click(Sender: TObject);
var
WordStruct: TMyWordStruct;
num: Cardinal;
begin
{假如}
WordStruct.w1 := $ABCD;
WordStruct.w2 := $1234;
{那么}
num := Cardinal(WordStruct);
ShowMessageFmt('$%x', [num]); //$1234ABCD
end;

procedure TForm1.Button3Click(Sender: TObject);
var
num: Cardinal;
ByteStruct: TMyByteStruct;
begin
num := $ABCD1234;
Cardinal(ByteStruct) := num;
//ByteStruct := TMyByteStruct(num); {这同上一句}

ShowMessageFmt('$%x', [ByteStruct.b1]); //$34
ShowMessageFmt('$%x', [ByteStruct.b2]); //$12
ShowMessageFmt('$%x', [ByteStruct.b3]); //$CD
ShowMessageFmt('$%x', [ByteStruct.b4]); //$AB
end;

procedure TForm1.Button4Click(Sender: TObject);
var
num: Cardinal;
WordStruct: TMyWordStruct;
begin
num := $ABCD1234;
Cardinal(WordStruct) := num;
//WordStruct := TMyWordStruct(num); {这同上一句}

ShowMessageFmt('$%x', [WordStruct.w1]); //$1234
ShowMessageFmt('$%x', [WordStruct.w2]); //$ABCD
end;

procedure TForm1.Button5Click(Sender: TObject);
var
ByteStruct: TMyByteStruct;
WordStruct: TMyWordStruct;
begin
WordStruct.w1 := $2211;
WordStruct.w2 := $4433;

Cardinal(ByteStruct) := Cardinal(WordStruct);
//ByteStruct := TMyByteStruct(WordStruct); {这同上一句}

ShowMessageFmt('$%x', [ByteStruct.b1]); //$11
ShowMessageFmt('$%x', [ByteStruct.b2]); //$22
ShowMessageFmt('$%x', [ByteStruct.b3]); //$33
ShowMessageFmt('$%x', [ByteStruct.b4]); //$44
end;

end.