Simple analysis of C + + friend

C + + friend, including friend function and friend//c++ friend function//c++ Friend class/* When a Class A declares another class B as its friend class, you can access the private content of a in B */#include <iostream>        Using namespace Std;class a{Private:friend class B;        Friend class C;        friend void Friend_func (A & A);        friend void Friend_func_pointer (A *a);    int i;        PUBLIC:A (int II): I (ii) {cout<< "constructor A" <<endl;        } ~a () {cout<< "Deconstructor A" <<endl;        } void Func () {cout<< "A.I = =" <<this->i<<endl;            } void set (int ii) {this->i = II;        cout<< "Set a.i =" <<this->i<<endl;        }};class b{public:b () {cout<< "constructor B" <<endl;        } ~b () {cout<< "Deconstructor B" <<endl; } void Func (a& a) {cout<< "friend of A is B, something isDone in B, A.I = "<<a.i<<endl;        Return }};//here we're going to do some evil things, what if Class C, even if A's derived class, is a friend class?//let ' s has a try//we know that even if C is a derived class, the private member of its base class cannot be accessed in C/    So what would be interesting if a class was both its derived class and its friend class? class C:public a{Private:int J;        PUBLIC:C (int II, int JJ): A (ii), J (JJ) {cout<< "constructor C" <<endl;        } ~c () {cout<< "Deconstructor C" <<endl; } void Func (a& a) {cout<< "Do something with out telling A. A.I" <<a.i<<endl        ;    }};//we define a friend function void Friend_func (a& a) {cout<< "I am A friend Func of a, A.I =" <<a.i<<endl; return;} void Friend_func_pointer (a *a) {cout<< "I am a friend func of a, which receives of pointer of a, A.I =" <<a- >i<<endl;}    int main (int argc, char** argv) {b b;    A (1); cout<< "sizeof (b) =" <<sizeof (b) <<endl;//here the size of the output B is 1Byte, isn't it strange? The Ghost knows what C + + wants to do cout<< "sizeof (a) =" <<sizeof (a) <<endl;//here the output is still 4 bytes, that is, the size of an int,    Stating that a friend is declared in a does not cause a's volume to become larger cout<< "SIZOEF (c) =" <<sizeof (c) <<endl;    B.func (a);    C C (for all);    C.func (a); C.func ();//The name hiding, which we talked about earlier, causes the compilation to fail Friend_func (a);//We declare in a that Friend_func is a friend of a function, Then this function can access the private members of A//Let's do something exciting, what happens when a friend encounters Up_cast?    Let's give it a try = =! Friend_func (c);//No problem at all, we have access to something in C.    In fact, conceptually, we still have access to something in a.//What happens if you encounter pointers to Up_cast? Try and know + + friend_func_pointer (&a); Friend_func_pointer ((A *) &c);//There is no problem at all. return 0;} /* There are some concepts to note about Friends (1) A friend relationship cannot be inherited.    B is the friend of a class,c is the subclass of a, but B is not a friend of C. This is also very well understood. From the perspective of encapsulation, friend Class B can access the private content of a, but should not see the private object of its subclass. Not even a can see, let alone its friend Class B. (2) Friend relationship is one-way and not commutative.    If Class B is a friend of Class A, class A is not necessarily a friend of Class B, it depends on whether there is a corresponding declaration in the class. This better understand, a said: B, you are my friend, all my things can show you, but B said: Well, you wishful thinking, I have no way. But I still can't show you anything.    (3) Friend relationship does not have transitive nature.    This is also better understood, if B is a friend class, a is a friend of D.    Then B can see the private,a of a can see the private of D, but B should not directly see the private of D. SimpleThe example is that my friend's friend doesn't necessarily know me. *///A friend function, a function that defines the outside of a class, but can directly access the private member of the class//We'll come up with an example of the above code. */Summary: 1. A word, disorderly 2. Personal opinion Friend class and friend Function destroys the encapsulation feature of the OOP language. 3. As you can see from the above example, when friend function encounters a up_cast, friend function destroys the contents of the derived class.  4.any, I think it is still not a last resort, do not use friend class and friend function bar. After all, it is an uncivilized phenomenon to move others ' underwear without others ' perception. */

　　

Simple analysis of C + + friend