把一个结构体当做属性后碰到的问题

当我把一个"结构体"在类中当做属性后, 在实用中可以直接读取结构体成员, 但不能直接写入...

下面是由此引发的小练习:

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unit Unit1;interfaceusesWinapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;typeTForm1 = class(TForm)Button1: TButton;Button2: TButton;Button3: TButton;Button4: TButton;Button5: TButton;procedure Button1Click(Sender: TObject);procedure Button2Click(Sender: TObject);procedure Button4Click(Sender: TObject);procedure Button3Click(Sender: TObject);procedure Button5Click(Sender: TObject);end;TMyClass = classstrict privateFPos: TPoint;procedure SetPos(const Value: TPoint);publicproperty Pos: TPoint read FPos write SetPos; //属性 Pos 对应一个点结构end;varForm1: TForm1;implementation{$R *.dfm}{ TMyClass }procedure TMyClass.SetPos(const Value: TPoint);
beginFPos := Value;
end;{测试}procedure TForm1.Button1Click(Sender: TObject);
varobj: TMyClass;
beginobj := TMyClass.Create;ShowMessageFmt('%d, %d', [obj.Pos.X, obj.Pos.Y]); //可以直接访问结构中的元素
//  obj.Pos.X := 11;  //但不能直接给结构中的元素赋值
//  obj.Pos.Y := 22;obj.Free;
end;//变通一
procedure TForm1.Button2Click(Sender: TObject);
varobj: TMyClass;
beginobj := TMyClass.Create;obj.Pos := Point(22,33); //ShowMessageFmt('%d, %d', [obj.Pos.X, obj.Pos.Y]);obj.Free;
end;//变通二
procedure TForm1.Button3Click(Sender: TObject);
varobj: TMyClass;pt: TPoint;
beginobj := TMyClass.Create;pt.X := 33;pt.Y := 44;obj.Pos := pt;ShowMessageFmt('%d, %d', [obj.Pos.X, obj.Pos.Y]);obj.Free;
end;//变通三(假如属性的 get 不是方法)
procedure TForm1.Button4Click(Sender: TObject);
varobj: TMyClass;p: PPoint;
beginobj := TMyClass.Create;p := Addr(obj.Pos);p.X := 44;p.Y := 55;ShowMessageFmt('%d, %d', [obj.Pos.X, obj.Pos.Y]);obj.Free;
end;//变通四(假如属性的 get 不是方法)
procedure TForm1.Button5Click(Sender: TObject);
varobj: TMyClass;
beginobj := TMyClass.Create;PPoint(Addr(obj.Pos)).X := 55;PPoint(Addr(obj.Pos)).Y := 66;ShowMessageFmt('%d, %d', [obj.Pos.X, obj.Pos.Y]);obj.Free;
end;end.

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练习二:

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unit Unit1;interfaceusesWinapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,Vcl.Controls, Vcl.Forms, Vcl.Dialogs;typeTForm1 = class(TForm)procedure FormCreate(Sender: TObject);end;TMyClass = classprivateFPos: TPoint;function GetPos: TPoint;procedure SetPos(const Value: TPoint);function GetXY(const Index: Integer): Integer;procedure SetXY(const Index, Value: Integer);publicproperty Pos: TPoint read GetPos write SetPos;property X: Integer index 0 read GetXY write SetXY;property Y: Integer index 1 read GetXY write SetXY;end;varForm1: TForm1;implementation{$R *.dfm}{ TMyClass }function TMyClass.GetPos: TPoint;
beginResult := FPos;
end;procedure TMyClass.SetPos(const Value: TPoint);
beginFPos := Value;
end;function TMyClass.GetXY(const Index: Integer): Integer;
beginResult := 0;case Index of0: Result := FPos.X;1: Result := FPos.Y;end;
end;procedure TMyClass.SetXY(const Index, Value: Integer);
begincase Index of0: FPos.X := Value;1: FPos.Y := Value;end;
end;{测试}
procedure TForm1.FormCreate(Sender: TObject);
varobj: TMyClass;
beginobj := TMyClass.Create;obj.X := 11;obj.Y := 22;ShowMessageFmt('%d, %d', [obj.Pos.X, obj.Pos.Y]);obj.Free;
end;end.

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