“As”受限通用类型的运算符 [英] "As" operator for constrained generic types

问题描述

考虑：

TTest <T : class, constructor> = class 
public
  function CreateMyObject : T;
end;
function TTest<T>.CreateMyObject : T;
var
  Obj : TObject;
begin
  Obj := T.Create; 
  Result := (Obj as T);
end;

为什么这不可能？编译器为 as 运算符产生不适用于此类型的运算符错误消息。 T被限制为一个类类型，所以这应该是正常的，不应该吗？

Why isn't this possible? Compiler yields an "Operator not applicable to this type" error message for the as operator. T is constrained to be a class type, so this should work, shouldn't it?

感谢您的帮助。

推荐答案

我遇到同样的问题，并通过在课堂中添加低级别的指针复制方法来解决这个问题：

I ran into the same problem and solved it by adding a low-level pointer-copy method to the class as a workaround:

  TTest <T : class, constructor> = class
  private
    function _ToGeneric(AItem: TObject): T; inline; //inline, so it's a little faster
  public
    function CreateMyObject : T;
  end;

  function TTest<T>.CreateMyObject : T;
  var
    Obj : TObject;
  begin
    Obj := T.Create;
    Result := _ToGeneric(Obj);
  end;

  function TTest<T>._ToGeneric(AItem: TObject): T;
  begin
    System.Move(AItem,Result,SizeOf(AItem))
  end;

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