class - 使用 RTTI 混淆继承

Question

如果问题不符合规则，请原谅我，但出于好奇，我只想知道为什么我的对象 classtype 让我感到困惑。

请看下面的代码:

 type 
 TFirstClass = class
 end;

  TSecondClass = class(TFirstClass)
  end;

  TThirdClass = Class(TSecondClass)
  End;

以上是我的类结构。

在一种方法中，我只想检查类对象的类类型。

    var
      obj1: TFirstClass;
      obj2: TSecondClass;
      obj3 : TThirdClass;
      str: string;
    begin
      obj1 := TSecondClass.Create;
      obj2 := TThirdClass.Create;
      try
    if obj1 is TFirstClass then
        str  := str + ' obj1: first ||';

    if obj1 is TSecondClass then
        str  := str + 'obj1 : Second ||';

    if obj2 is TSecondClass then
        str  := str + 'obj2 : Second ||';

    if obj2 is TThirdClass then
        str  := str + 'obj2 : Third ||';

    ShowMessage(str);
   finally
        FreeandNil(Obj1);
        FreeandNil(Obj2);
      end;

    end;

但是 str 中的结果是obj1:第一个 ||obj1:第二个 ||obj2:第二个 ||obj2:第三个 ||

为什么 is 关键字对所有 if 语句都返回 true？

Answer 1

documentation有答案:

The is operator, which performs dynamic type checking, is used to verify the actual runtime class of an object. The expression:

object is class

returns True if object is an instance of the class denoted by class or one of its descendants, and False otherwise.

他们的关键词是“或其后代之一”。