Details on C++ inheritance and friends inheritance and type conversion

Examples are as follows:

#include <iostream> 

using namespace std; 

class a{
friend class pal;
private:
	int i;
protected:
	int j;
public:
	int k;
};

class b:public a{
protected:
	int l;
};

class c:protected a{};
class d:private a{};
class e:public b{};

class pal{
public:
	void f(a a1){
		cout<<a1.i;// Yes, you can access private members 
	}
	void f1(b b1){
		cout<<b1.i;// True, friends can access the base class portion of a derived class 
		cout<<b1.l;// Make a mistake, pal The class is not b A friend of 
	}
};

class deri_pal:public pal{
public:
	void f2(a a1){
		cout<<a.i;// Error, friend cannot inherit 
	}
};

int main() 
{	
	a a1;
	b b1;
	c c1;
	d d1;
	e e1;
	a1=b1;// True, only derived classes with public inheritance can be converted to base classes 
	a1=e1;// correct 
	a1=c1;// Error, only public inherited derived classes can be converted to base classes 
	a1=d1;// Error, only public inherited derived classes can be converted to base classes 
	system("pause");
	return 0; 
} 

When dynamically binding a base class pointer, the derived class must be derived from public, otherwise the base class pointer cannot be bound to a derived class object

#include <iostream> 

using namespace std; 

class a{};
class b:public a{};
class c:protected a{};
class d:private a{};
class bb:public b{};
class cc:public c{};
class dd:public d{};




int main() 
{	
	a a1;b b1;c c1;d d1;bb bb1;cc cc1;dd dd1;
	a* a1=&b1;//ok
	a* a2=&c1;//error
	a* a3=&d1;//error
	a* a4=&bb1;//ok
	a* a5=&cc1;//error
	a* a6=&dd1;//error
	system("pause");
	return 0; 
}