c++ - 访问说明符对 friend 功能重要吗？

Question

在一个类中，如果函数在不同的说明符(如 private、protected 或 public)中被声明为 friend，那么有什么区别吗？据我了解， friend 功能不是成员。因此，这应该无关紧要。但是，如果我看到 static - 它也不是成员，但访问说明符很重要。所以，我有点困惑。所有这些代码如何正常工作？下面的类有区别吗？

/** Private friend function **/

class frienddemoFunction
{
  private:
      unsigned int m_fanSpeed;
      unsigned int m_dutyCycle;
      /** This function is not a member of class frienddemo **/
      friend void printValues(frienddemoFunction &d);

  public:
      void setFanSpeed(unsigned int fanSpeed);
      unsigned int getFanSpeed();

};


/** Protected -- Friend Function **/
class frienddemoFunction
{
  private:
      unsigned int m_fanSpeed;
      unsigned int m_dutyCycle;

  public:
      void setFanSpeed(unsigned int fanSpeed);
      unsigned int getFanSpeed();

 protected:

 /** This function is not a member of class frienddemo **/
      friend void printValues(frienddemoFunction &d);


};

class frienddemoFunction
{
  private:
      unsigned int m_fanSpeed;
      unsigned int m_dutyCycle;

  public:
      void setFanSpeed(unsigned int fanSpeed);
      unsigned int getFanSpeed();

 /** This function is not a member of class frienddemo **/
      friend void printValues(frienddemoFunction &d);


};


 /** This function is not a member of class frienddemo **/
  friend void printValues(frienddemoFunction &d);

Answer 1

不，没关系。

C++ 标准，第 11.3/9 节 [friend.class]

The meaning of the friend declaration is the same whether the friend declaration appears in the private, protected or public (9.2) portion of the class member-specification.

注意:

在类中声明的静态函数仍然是类成员。友元函数不是。