使用类引用的多态性和继承(第2部分)？

Question

下面的控制台应用程序给出了"运行时错误"......

为什么会这样？非常感谢 !

PS:相关SO帖子

program Project2;

{$APPTYPE CONSOLE}

type
  TParent = class;
  TParentClass = class of TParent;

  TParent = class
  public
    procedure Work; virtual; abstract;
  end;

  TChild1 = class(TParent)
  public
    procedure Work; override;
  end;

  TChild2 = class(TParent)
  public
    procedure Work; override;
  end;

procedure TChild1.Work;
begin
  WriteLn('Child1 Work');
end;

procedure TChild2.Work;
begin
  WriteLn('Child2 Work');
end;

procedure Test(ImplClass: TParentClass);
var
  ImplInstance: TParent;
begin
  ImplInstance := ImplClass.Create;
  ImplInstance.Work;
  ImplInstance.Free;
end;

begin
  Test(TParent);
  Test(TChild1);
  Test(TChild2);
  Readln;
end.

Answer 1

该TParent.Work方法声明为abstract.该文件说:

您只能在已重写方法的类的类或实例中调用抽象方法.

当你打电话给TParent.Work你时打破那个规则,所以遇到运行时错误.