什么时候可以比较指向同一个对象在c ++中的指针？ [英] When can I compare pointers to the same object in c++?

问题描述

例如，我有一些类层次结构（可能，有各种继承 - public，private，public virtual，multi-inheritance等）：

For example, I have some class hierarchy (possibly, with all kinds of inheritance - public, private, public virtual, multi-inheritance, etc.):

class A {
  int a;
public:
  virtual ~A() {}
};

class B: public A { int b; };
class C: public virtual B { int c; };
class E: public virtual B { int e; };
class F: public C, public E { int f; };

使用casts获取指向主big对象的每个子对象的指针：

Using casts I get pointers to every sub-object of the main "big" object:

F * f = new F;
E * e = f;
C * c = f;
B * b = f;
A * a = f;

我可以比较这些指针对的相等性（operator ==），为什么？
比较是否使用delta逻辑或其他技术？

What pairs of these pointers may I compare for equality (operator==) and why? Will the comparison use delta-logic or some other technique?

当我不能比较指向同一个复杂对象的指针时，有什么可能的情况？
它可以是什么类型的对象？

What are the possible situations, when I can't compare pointers to the same complex object? What kind of object it can be?

我希望所有指向同一对象的指针总是相等的。

I expect, that all of the pointers to the same object are always equal.

推荐答案

如果一个指针类型可以隐式转换为另一个指针，则可以比较两个指针;也就是说，如果它们都指向同一类型，或者一个指向另一个的基类。转换将对地址进行必要的调整，以便如果两个指针指向同一个对象，它们将比较相等。

You can compare two pointers if one pointer type is implicitly convertible to the other; that is, if they both point to the same type, or one points to a base class of the other's. The conversion will make the necessary adjustment to the address so that, if both pointers point to the same object, they will compare equal.

在这种情况下，对 c == e ，因为 C 和 E 是从另一个派生的。要比较这些，你需要交叉转换，或将两者转换为它们的共同基类;

In this case, you can compare any pair except c == e, since neither of C nor E is derived from the other. To compare those, you would either need to cross-cast, or to convert both to their common base class; neither of these can be done implicitly.

顺便说一下，您的代码中不需要 dynamic_cast ，因为你正在转换为基类指针，并且可以隐式完成安全转换。

By the way, there's no need for dynamic_cast in your code, since you're casting to base class pointers and that safe conversion can be done implicitly.

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