Delphi 2010：令人困惑的泛型类型 TList 场景？按值传递还是按引用传递？

Question

几天前，我在项目中使用 Generic TList 时遇到了一个问题。我在一个简单的测试项目中对其进行了测试，并遇到了同样的问题。这是确切的代码：

type
  TMyPoint = record
    x: Integer;
    y: Integer;
  end;

  TShape = record
    Corners: TList<TMyPoint>;
  end;

  TForm1 = class(TForm)
    Button1: TButton;
    Label1: TLabel;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    Shape_List: TList<TShape>;
  public
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  Shape: TShape;
  FirstPoint: TMyPoint;
  SecondPoint: TMyPoint;
  Temp: TMyPoint;
begin
  // Create the corners list
  Shape.Corners := TList<TMyPoint>.Create;
  // Add the first point to corners
  FirstPoint.x := 10;
  FirstPoint.y := 20;
  Shape.Corners.Add(FirstPoint);
  // Add the shape to the Shape_List
  Shape_List.Add(Shape);

  // Clear the shape corners
  Shape.Corners.Clear;

  // Add another point to corners
  SecondPoint.x := 100;
  SecondPoint.y := 200;
  Shape.Corners.Add(SecondPoint);

  // Show the x of the first point of the first shape
  Label1.Caption := IntToStr(Shape_List[0].Corners[0].x);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Shape_List := TList<TShape>.Create;
end;

Label1.Caption 将是 100 而不是 10 ！为什么会这样呢？我认为 TList.Add(const value) 是按值传递而不是按引用传递！

Answer 1

在您的程序中添加了一些注释，以准确指出“错误”所在。

procedure TForm1.Button1Click(Sender: TObject);
var
  Shape: TShape;
  FirstPoint: TMyPoint;
  SecondPoint: TMyPoint;
  Temp: TMyPoint;
begin
  // Create the corners list
  Shape.Corners := TList<TMyPoint>.Create; // We create a new TList<TMyPOint> OBJECT. Shape.Corners is a reference
  // Add the first point to corners
  FirstPoint.x := 10;
  FirstPoint.y := 20;
  Shape.Corners.Add(FirstPoint); // Add FirstPoint to the list we created at step 1.
  // Add the shape to the Shape_List
  Shape_List.Add(Shape); // Add a copy of the Shape record to the Shape_List

  // Clear the shape corners
  Shape.Corners.Clear; // Clear the Shape.Corners reference. This effectively clears the list of corners
                       // in the Shape you just added to Shape_List because it contains the same
                       // reference.

  // Add another point to corners
  SecondPoint.x := 100;
  SecondPoint.y := 200;
  Shape.Corners.Add(SecondPoint); // Add a new point to the Corners list. Remamber, Corners is actually
                                  // a reference. The first Shape you added to Shape_List contains a copy
                                  // of this exact same reference, so you're effectively adding a first point
                                  // to both Shape.Corners and Shape_List[0].Corners.

  // Show the x of the first point of the first shape
  Label1.Caption := IntToStr(Shape_List[0].Corners[0].x); // Yup, you just added that point, so you get 100
end;

Answer 2

添加 FirstPoint 后，您清除 Corners（之后，列表为空）。然后添加 SecondPoint ，它成为第一个元素（索引 0）。

编辑：为了说明：

var
  Shape1, Shape2: TShape;
begin
  Shape1.Corners := TList<TMyPoint>.Create;
  Shape2 := Shape1;
  OutputDebugString(PChar(Format('Shape1.Corners: $%.8x', [Integer(Shape1.Corners)])));
  OutputDebugString(PChar(Format('Shape2.Corners: $%.8x', [Integer(Shape2.Corners)])));
end;

Shape1.Corners 和 Shape2.Corners 指向同一列表。

Answer 3

不，它是通过引用传递的，因为它作为常量参数传递（请参阅：http://docwiki.embarcadero.com/VCL/en/Generics.Collections.TList.Add）。