c++ - 从派生**转换为基础**

Question

我正在阅读 this不幸的是，无法深入理解为什么编译器不允许从 Derived** 转换为 Base**。我也看过this它只提供了 parashift.com 链接的更多信息。

编辑:

让我们逐行分析这段代码:

   Car   car;
   Car*  carPtr = &car;
   Car** carPtrPtr = &carPtr;
   //MyComment: Until now there is no problem!

   Vehicle** vehiclePtrPtr = carPtrPtr;  // This is an error in C++
   //MyComment: Here compiler gives me an error! And I try to understand why. 
   //MyComment: Let us consider that it was allowed. So what?? Let's go ahead!

   NuclearSubmarine  sub;
   NuclearSubmarine* subPtr = ⊂
   //MyComment: this two line are OK too!

   *vehiclePtrPtr = subPtr;

   //MyComment: the important part comes here... *vehiclePtrPtr is a pointer to
   //MyComment: a vehicle, particularly in our case it points to a Car object.
   //MyComment: Now when I assign to the pointer to the Car object *vehiclePtrPtr,
   //MyComment: a pointer to NuclearSubmarine, then it should just point to the
   //MyComment: NuclearSubmarine object as it is indeed a pointer to a Vehicle,
   //MyComment: isn't it? Where is my fault? Where I am wrong?

   // This last line would have caused carPtr to point to sub!
   carPtr->openGasCap();  // This might call fireNuclearMissle()!

Answer 1

一碗香蕉不是一碗水果的原因基本相同。如果一碗香蕉是一碗水果，你可以把一个苹果放进碗里，它就不再是一碗香蕉了。

只要您只检查碗，转换是无害的。但是一旦你开始修改它，转换就变得不安全了。这是要牢记的关键点。 (这就是为什么不可变 Scala 集合实际上允许转换，但可变集合禁止转换的确切原因。)

与您的示例相同。如果有从 Derived** 到 Base** 的转换，您可以放置​​一个指向苹果的指针，而类型系统 promise 只存在指向香蕉的指针。轰隆隆!