Introduction to the use of C ++ Zhongyou Functions

Introduction to the use of C ++ Zhongyou Functions

1. youyuan Function

class A{  friend void MyFriend(class &A);private:  int m_a;}void MyFriend(class &A){   std::cout << A.m_a << std::endl;}

2. youyuan class

class A{public:  A();  friend class B;private:  int m_a;}class B{public:  B();  void UseA(A &a);}void B::UseA(A &a){  std::cout<< a.m_a << std::endl;}

3. Membership functions

// Pay attention to the Declaration Order to ensure that class A is visible; // A is used for pre-declaration, because class B {public: B () must be used in B (); void UseA (A & a);} class A {public: A (); friend void B: UsaA (A & a); private: int m_a;} void B :: useA (A & a) {std: cout <. m_a <std: endl ;}

4. Mutual friend functions (one function is a friend of two classes)

class B;class A{public:  A();  friend void UseAB(A &a,B&b);private:  int m_a;}class B{public:  B();  friend void UsaAB(A &a,B &b);private:  int m_a;}void UseAB(A &a,B &b){  std::cout << a.m_a + b.m_a << std::endl;}

Youyuan is an extension of the class interface, because only in the class can you specify who is youyuan, so, does not violate the OOP idea!