Why does explicit typecasting allow upcasting for private inheritance?

Question

#include<iostream> using namespace std;  class A {     public:     void f(){cout<<"A"<<endl;} };  class B : private A {     public:     void f(){cout<<"B"<<endl;} };  int main (){ 

As Class B is inheriting Class A privately, this upcasting is not supposed to work:

    A* a = new B; 

But explicit typecasting is allowing it. Why?

    A* a1 = (A*)new B;     a1->f();     return 0; } 

Answer 1

The cast in

A* a1 = (A*)new B; 

is a cast to inaccessible base class.

It can only be expressed as a C style cast. It is equivalent to what a static_cast would do if a static_cast could be used in this situation, and it is not equivalent to a reinterpret_cast. In particular the result address is not necessarily the same as the argument address.

C++11 §5.4/4:

” The same semantic restrictions and behaviors [as for a static_cast] apply [for a C style cast], with the exception that in performing a static_cast in the following situations the conversion is valid even if the base class is inaccessible:

— a pointer to an object of derived class type or an lvalue or rvalue of derived class type may be explicitly converted to a pointer or reference to an unambiguous base class type, respectively;