c++中访问权限的问题

Question

1. 描述你的问题
   如下面的这段代码所示，为什么在Base这个类里面，它的拷贝函数，有个Base类的参数，为什么在拷贝函数里面，Base这个类的引用对象tmp，可以直接访问私有数据成员num，不是访问权限标志位private的，就只能被友元和成员函数访问吗？为什么可以直接在拷贝函数里面写tmp.num。这样的语句啊！如果放在main函数里面，声明一个Base类的对象，是不能访问私有数据成员的啊！

2. 贴上相关代码

   #include <iostream>

   class Base
   {
   private:

      int num;

   public:

      Base(int tmp = 0) : num(tmp) {}
      const Base& operator=(const Base& tmp)
      {
          num = tmp.num;
          return *this;
      } 

   };

   int main()
   {

      return 0;

   }

3. 贴上报错信息

4. 贴上相关截图

5. 已经尝试过哪些方法仍然没解决（附上相关链接）

Answer 1

Of course your own members and their own functions can access

Answer 2

You said it yourself,,,

If the access permission flag is private, it can only be accessed by friends and member functions

This const Base& operator=(const Base& tmp) function is a member function of the Base class. . .

Answer 3

Members within a class can naturally be accessed, mainly to distinguish between classes and objects.

Answer 4

Personally, I think it can be understood this way: a class is its own friend class.

Specifically, Base a class is its own friend, and friends can access the private members of the class. In the copy assignment operator, a member of the Base class, the type of the tmp object is Base, and Base is a friend of Base, so you can access the tmp class through the Base object Private members num, in the same way, can also implicitly access this class private members Base through the num pointer.

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In addition, the return value of the copy assignment operator should generally not be a constantreference type.