Friend class and inheritance

C + + Primer has the following description: The friend relationship can not inherit. A friend of a base class has no special access to a member of a derived class
Permissions. If the base class is granted a friend relationship, only the base class has special access rights, and the derived class of the base class cannot access the class that is granted the friend relationship.

In practice, however, the VS compiler does not install the description described above, and the following rules contradict the above description but conform to the VS compiler's processing rules.

Note: The g++ compiler is needed to verify this.

1 inheritance problems with friend class

derived class C of friend B of Class A 1.1 cannot access private or protect member variables of Class A. But you can access a by using the interface provided by B. (nonsense definitely can)

#include <iostream> 
using namespace   std; 
Class B; 
Class A 
{ 
	int A; 
Public: 
	A (int x=0) {a=x} 
	Friend class B; 
Class B 
{ 
	int b; 
Public: 
	void Fun (a& ob) {cout << ob.a << Endl;} 
}; 

Class C:public B
{public 
: 
	//void fun2 (a& ob) {   cout <<ob.a <<endl;} The newly added function of a derived class cannot access a, and this sentence will be an error
}; 

void   Main () 
{ 
	a A (); 
	c C; 
	C.fun (a); C is a derived class of B that   can still be accessed through the function fun of base class B 

Friends of 1.2. base can access the Private,protect member variable of base through the derived class drived base, but cannot access drived private,protect member variables. (This seems to be a bit of a conflict with "C + + Primer")

Personal understanding: The DriveD object itself contains the Base,base friend Frnd can naturally access the base part.

#include <iostream> 
using namespace std; 
Class Base 
{ 
	int m_a; 
Public: 
	Base (int x=0) {m_a=x} 
	Friend class Frnd; 

Class Drived:public Base 
{ 
private:
	int m_c;
Public: 
	drived (int x): Base (x) {m_c=x;} 
};

Class Frnd 
{public
: 
	void Fun (base& ob) {cout <<ob.m_a << Endl;} 
	void Fun2 (drived& ob) 
	{
		cout << ob.m_a<<endl;
		cout <<ob.m_c<<endl; Compile error
	} 
}; 

int main () 
{ 
	drived D (1); 
	Frnd F; 
	F.fun (d); 
	F.fun2 (d);
	System ("pause");
	return 0;

3 Transfer problem of friend class

A's friend is the friend of B,b is C, that a friend is C. No, the friend class is not transitive.