arrays - 包含动态数组的通用记录列表

Question

我有一个通用的记录列表。这些记录包含一个动态数组，如下所示

Type
  TMyRec=record
MyArr:Array of Integer;
    Name: string;
    Completed: Boolean;
  end;

var
  MyList:TList<TMyRec>;
  MyRec:TMyRec;

然后我创建列表并设置数组长度，如下所示

MyList:=TList<TMyRec>.Create;
SetLength(MyRec.MyArr,5);
MyRec.MyArr[0]:=8;  // just for demonstration
MyRec.Name:='Record 1';
MyRec.Completed:=true;
MyList.Add(MyRec);

然后我更改 MyArr 中的数据，还更改 MyRec.Name 并向列表中添加另一个项目

MyRec.MyArr[0]:=5;  // just for demonstration
MyRec.Name:='Record 2';
MyRec.Completed:=false;
MyList.Add(MyRec);

当将第一项添加到列表后 MyRec.MyArr 发生变化时，存储到列表中的 MyArr 也会发生变化。但其他记录字段则不然。

我的问题是如何防止 MyRec.MyArr 中的更改反射(reflect)在已存储在列表项中的数组上。

我需要声明多条记录吗？

Answer 1

这个例子可以像这样简化，删除所有对泛型的引用:

{$APPTYPE CONSOLE}

var
  x, y: array of Integer;

begin
  SetLength(x, 1);
  x[0] := 42;
  y := x;
  Writeln(x[0]);
  y[0] := 666;
  Writeln(x[0]);
end.

输出为:

42666

The reason for this is that a dynamic array is a reference type. When you assign to a variable of dynamic array type, you are taking another reference and not making a copy.

You can resolve this by forcing a reference to be unique (that is have just a simple reference). There are a number of ways to achieve this. For instance, you can call SetLength on the array that you want to be unique.

{$APPTYPE CONSOLE}

var
  x, y: array of Integer;

begin
  SetLength(x, 1);
  x[0] := 42;
  y := x;
  SetLength(y, Length(y));
  Writeln(x[0]);
  y[0] := 666;
  Writeln(x[0]);
end.

输出:

4242

So, in your code you can write it like this:

MyList:=TList<TMyRec>.Create;

SetLength(MyRec.MyArr,5);
MyRec.MyArr[0]:=8;  // just for demonstration
MyRec.Name:='Record 1';
MyRec.Completed:=true;
MyList.Add(MyRec);

SetLength(MyRec.MyArr,5); // <-- make the array unique
MyRec.MyArr[0]:=5;  // just for demonstration
MyRec.Name:='Record 2';
MyRec.Completed:=false;
MyList.Add(MyRec);

您可以使用各种其他方式来强制唯一性，包括Finalize、分配nil、Copy等。

documentation 中详细介绍了此问题。 。以下是相关摘录:

If X and Y are variables of the same dynamic-array type, X := Y points X to the same array as Y. (There is no need to allocate memory for X before performing this operation.) Unlike strings and static arrays, copy-on-write is not employed for dynamic arrays, so they are not automatically copied before they are written to. For example, after this code executes:

 var
   A, B: array of Integer;
   begin
     SetLength(A, 1);
     A[0] := 1;
     B := A;
     B[0] := 2;
   end;

the value of A[0] is 2. (If A and B were static arrays, A[0] would still be 1.) Assigning to a dynamic-array index (for example, MyFlexibleArray[2] := 7) does not reallocate the array. Out-of-range indexes are not reported at compile time. In contrast, to make an independent copy of a dynamic array, you must use the global Copy function:

 var
   A, B: array of Integer;
 begin
   SetLength(A, 1);
   A[0] := 1;
   B := Copy(A);
   B[0] := 2; { B[0] <> A[0] }
 end;