无法为包含记录数组的对象赋值

Question

我正在写一个包含记录数组的简单对象.就像发票一样.(ID,日期,客户名称和有关物品的记录数组).

type
  Trows = record
  private
    Fcode: string;
    Qty: Double;
    cena: Currency;
    procedure Setcode(const value: string);
  public
    property code: string read Fcode write SetCode;
  end;

  Tcart = class(TObject)
  private
    Frow: array of Trows;
    function  Getrow(Index: Integer): Trows;
    procedure Setrow(Index: Integer; const Value: Trows);
  public
    ID: integer;
    CustName: string;
    Suma: currency;
    Payed: boolean;
    constructor Create(const Num: Integer);
    destructor Destroy;
    function carttostr: string;
    procedure setcode(Index: integer;val: string);
    property Row[Index: Integer]: Trows read Getrow write setrow;
  end;

一切似乎很好,因为我试图改变一条记录的价值.我找到了3种方法.首先,第二个工作正常,但我想简化修改此记录值的代码,如下所示:

cart.row[0].code:='333';

但它不起作用.

我错过了什么？

这是代码:

procedure TForm1.Button1Click(Sender: TObject);
var
  Arows: Trows;
begin
  Cart:=Tcart.Create(0);
  cart.custName:='Customer 1';
  cart.Suma:=5.55;
  cart.Payed:=false;
  Arows.code:='123';
  cart.setrow(0,Arows);  // this way working
  cart.setcode(0,'333');   // this way also working
  cart.row[0].code:='555';      //this way doesn''t change value. How to make it work?
  memo1.Lines.Text:=cart.carttostr;
end;

Answer 1

它不起作用,因为您的Row[]属性按值返回TRows记录,这意味着调用者会收到原始记录的副本.您对副本所做的任何修改都不会反映在原件中.

您需要将副本分配回属性以应用更改:

procedure TForm1.Button1Click(Sender: TObject);
var
  Arows: Trows;
begin
  ...
  Arows := cart.row[0];
  Arows.code:='555';
  cart.row[0] := Arows; // <-- equivalent to 'cart.setrow(0,Arows);'
  ...
end;