c++ - Subclass Overwrite c++

Question

I'm trying to get my head around why I can't overwrite a subclass object with another subclass object if they share base class.

Say that Letter is the base class. A and B are subclasses. The following doesn't seem to work:

Letter* a_p = new A();
Letter* b_p = new B();

delete a_p;
*a_p = *b_p;

My ambition is to change what is located at a certain adress so that all pointers to the adress in question changes what they point to. In the above example, I'd like to somehow change the "content" of a_p to a copy of the "content" of b_p.

Is this possible somehow?

score 4 · Accepted Answer

如果您正在复制对象并且指针的类型是基类，那么您将丢失有关子类的所有信息。

而且由于您删除了a_p，*a_p=因此做错了，因为a_p指向不再分配的内存。

为什么不只是做：a_p = b_p;？

score 2 · Accepted Answer

在您给出的示例代码中，最突出的错误是您取消引用以前删除的指针。但是可以说那些是堆栈上的对象；

class Base
{
};

class A : public Base
{
};

class B : public Base
{
  int a;
};

int main()
{
  A a;
  B b;
  b = a; // This gives an error
  return 0;
}

我们试图将一个A对象复制到一个B具有不同内存布局（添加的 int 成员）的对象中。该成员将如何初始化，您认为！？出于同样的原因，您不允许有指向A的对象的指针B，它们是不同的类型。

这两种类型“相同”的唯一地方是您通过Base指针访问它们的地方。

score 1 · Accepted Answer

A 和 B 在内存中的大小可能不同。此外，delete 释放分配给 a_p 的内存。您将写入未分配的内存。这将导致“未定义的行为”

score 0 · Accepted Answer

如果您正在寻找分配指针值，我认为您是，那么您需要这样做

a_p = b_p

不是*a_p = *b_p

c++ - Subclass Overwrite c++

4 回答 4

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