friend 类和函数

friend 类

#include <iostream>

class FriendClass {
private:
  int private_data;

  // this means Other is my friend,
  // please let him have access to my private memeber
  friend class Other;
};

class Other {
public:
  FriendClass clz;
  Other() { clz.private_data = 2; }

  void Print() { std::cout << "PRIVATE: " << clz.private_data; }

  void RetrieveData();
};

int main() {
  Other other;
  other.Print();

  return 0;
}

friend 函数

#include <iostream>

class FriendClass;

class Another {
public:
  void member_function(const FriendClass &clz);
};

class FriendClass {
public:
  FriendClass(int private_data) : private_data(private_data) {}

  // please let him have access to my private memeber
  friend void friend_function(const FriendClass &clz);

  friend void Another::member_function(const FriendClass &clz);

private:
  int private_data;
};

void friend_function(const FriendClass &clz) {
  std::cout << clz.private_data << '\n';
}

// friend function definition
void Another::member_function(const FriendClass &clz) {
  std::cout << "Private Variable: " << clz.private_data << std::endl;
}

int main() {
  friend_function(2);

  Another another;
  another.member_function(2);
  return 0;
}

1. 友元类和函数具有对类 private 和 protected 成员的访问权限。
2. friend 函数中，friend 标识符位于函数的声明式中，而非定义中。
3. friend 函数可以在 private, public or protected 中声明。

违反私有性，建议仅在运算符重载时使用。